Chapter 1-7 شرح مبسط. مراجعة اللغات فى مادة الفيزياء 3 ثانوى لغات وتجريبى revision

MR.AHMED HEKAL
PHYSICS 3RD SECONDARY
CHAPTERS (1-7)

Mr. Ahmed Hekal
1
Chapter 1
A. Definitions
1- Electrical current:
- Electrons movement through a conductor from negative pole to positive pole in the presence of electric
source
2- Traditional Electrical current:
- Movement of positive charges through a conductor from positive pole to negative pole
3- Current intensity:
- Quantity of electrical charges through a section of a conductor in 1 second
4- Ampere:
- Current intensity results in passing quantity of charges of 1 Coulomb through a conductor in 1 second
5- Coulomb
- The quantity of electrical charges that when passes through a conductor in time of 1 second it generates
current of 1 Ampere
6- Potential difference:
- The work done in joule used to transfer quantity of charges of 1 C between 2 points
7- Volt:
- The potential difference bet. 2 points when work done of 1 J used to transfer quantity of charges of 1 C
8- Electromotive Force:
- The whole work done outside and inside the battery to transfer 1 coulomb of charges in the electrical circuit
- Potential difference between the poles of the battery in case of open circuit
9- Electrical Resistance:
- The opposition ‫الممانعة‬ to the flow of the current
- The ratio between potential difference in volt across the terminals of conductor and current intensity that
flows through this conductor in ampere
10- Ohm’s Law:
- At constant temperature, current intensity is directly proportional with potential difference
11- Ohm:
- The resistance of conductor that allows passage current of 1 Ampere when potential difference across
terminals of this conductor is 1 volt
12- Resistivity:
- It’s the resistance of conductor of length 1 m and cross sectional area of 1 m2
at constant temperature
13- Conductivity:
- The reciprocal of resistivity
- Reciprocal of resistance of conductor of length 1 m and cross sectional area of 1 m2
14- Kirchhoff’s first low:
- In a closed circuit, summation of currents entering a point is equal to summation of currents exiting this
point
- The algebraic summation of currents in a specific point in closed circuit equals zero
15- Kirchhoff’s second law:
- Algebraic summation of e.m.f. in a closed circuit equals the algebraic summation of potential differences in
this circuit
- Algebraic summation of potential differences in a closed branch equal zero

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B. What’s meant by?
1- Current intensity passes through a conductor is 5 A
- The quantity of electrical charges that passes through a section of this conductor in 1second equals 5
coulombs
2- Potential difference between terminals of a conductor is 20 V
- The work done to transfer 1 C of charges bet. terminals of this conductor is 20 Joule
3- Electromotive force of a battery = 1.5 V
- The whole work done outside and inside the battery to transfer 1 coulomb of charges in the circuit = 1.5
joule
4- Electrical Resistance of a conductor 100 Ohm
- The ratio between potential difference across the terminals of conductor and current intensity that flows
through this conductor is 100 V/A
5- Resistivity of a conductor = 6 *10-6
ohm.m
- Resistance of a conductor of this material of length 1 m and cross sectional area of 1 m2
at a constant temp.
is 6 *10-6
ohm
6- Conductivity of material is 5.6 *107
ohm-1
.m-1
- Reciprocal of resistance of conductor of length 1 m and cross sectional area of 1 m2
Is 5.6 *107
ohm-1
C. Deductions
1- Resultant resistance of group of resistances connected in series
- Current intensity is constant through all resistances
- Potential difference is divided between them
V = V1 + V2 + V3
V = I R
I R` = I R1 + I R2 + I R3
Then
2- Resultant resistance of group of resistances connected in parallel
- Potential difference is constant across all resistances
- Current intensity is divided between resistances
I = I1 + I2 + I3
I = V / R
V / R` = V / R1 + V / R2 + V / R3
Then
R` = R1 + R2 + R3
1 / R` = 1 / R1 + 1/ R2 + 1 / R3

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D. Factors that depends on
Physical quantity Factors that this quantity depends on
Resistance of a conductor
R = Ꝭe L/A
1- Length of conductor (directly proportional)
2- Its cross sectional area A (inversely
Proportional)
3- Material type of this conductor
4- Temperature of this conductor
Resistivity of conductor 1- Material type of this conductor
Conductivity of a conductor 1- Material type of this conductor
F. Comparisons
resistances connected in series resistances connected in parallel
Shape of
connection
Target To obtain a large resultant resistance
from a group of small resistances
To obtain a small resultant resistance
from a group of large resistances
Current intensity Equal or constant in all resistances The whole current equals the summation
of all currents in all resistances
Potential difference The whole P.D. equals the summation of
all P.D.s in all resistances
V = V1 + V2 + V3
Equal or constant across all resistances
Law of resultant
If all resistances are
equal
R` = N R
Where N is number of them
R` = R / N
R` = R1 + R2 + R3 1 / R` = 1 / R1 + 1/ R2 + 1 / R3

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E. What Happens in these cases
1- Increasing potential difference to double value for current intensity and power consumed?
- Current intensity will increase to double as I = V / R
- Power will increase 4 times as Pw = V2
/ R
2- Current intensity increases to double for the resistance value
- Resistance remains constant as it doesn’t depends on current it depends on
- Length of conductor (directly proportional)
- Cross sectional area A (inversely Proportional)
- Material type
- Temperature
3- Increasing cross sectional area of a conductor to double and decreasing its length to half value for the
resistance
- L1 = 2 L , L2 = L , A1= A , A2 = 2 A
- R1 / R2 = L1 A2 / L2 A1 = 2 L * 2 A / L * A
- R2 = ¼ R1
4- Connecting two resistance in parallel on of them has a value of 1 ohm to the resultant resistance
- Resultant resistance will be less than 1 ohm
5- Current doesn’t flow from an electric source to potential difference between the terminals of this electric
source
- Potential difference between the terminals of this electric source will be equal to electromotive force of the
electric source according to this relation
(V = VB – I r) and I = 0 then V = VB
F. Give reason
1- Work should be done to transfer charges from point to point
- To get rid of resistance bet. the two points and current can flow
2- Some materials can conduct electricity but others cannot
- As some materials have a plenty of free electrons so it allows flow of current while other materials don’t
have free electrons or their electrons are strongly correlated to their atoms
3- Increasing radius of a wire of copper leads to decreasing its resistance to quarter value
- According to this relation
R = Ꝭe l/r2
Resistance is inversely proportional with square of radius
4- When a conductor is shaped to be parallelogram its ribs resistances are different while if shaped as a
cube its ribs resistances are equal
- As ribs of parallelogram are different in length so their resistance differs according to relation
R = Ꝭe l/A but cube ribs are equal in length and equal in resistance
5- Resistance increase when increasing temperature
- When raising temperature this increase the speed of vibrating its molecules and increase the rate of
collisions between electrons of current and conductor molecules so the opposition ‫الممانعة‬ of current
increases

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6- Resistivity of a conductor is a physical property
- As it depends on the type of the conductor material at constant temperature
7- Conductivity of a conductor is a physical property
- Because conductivity is the reciprocal of resistivity which depends on the conductor material at constant
temperature
8- Conductivity factor of copper is large
- As resistivity of copper is very small in cause of plenty of free electrons in copper
9- It’s preferred to use wires of copper in the electrical connections
- As resistivity of copper is very small so its resistance is low and this prevent wire from consuming electrical
energy
10- Home devices are connected in parallel
- All devices will work on the same potential difference of the source so each device can work alone and if
one of them damaged it doesn’t affect the others and also to decrease their total resistance which doesn’t
affect the main current
11- Home devices aren’t connected in series
- Because potential difference will be divided across them which leads to a probability of insufficient voltage
on a device that cannot operate and one device cannot work alone also their total resistance will be huge
which prevents the current from passing through the circuit
12- The electrical power increases in case of connecting two resistance on parallel
- When connecting resistances in parallel their total resistance decreases and current will increase and Power
also will increase according to this law Pw= IV
13- In a circuit of parallel connection resistances we use thick wires at the terminals of battery and thin
wires at the terminals of each resistance
- As current intensity should be maximum at the input and output of the battery so we use thick wires of low
resistance while current is divided through each resistance so we can use thin wires in terminals of each
resistance
14- Potential difference between battery poles increases when increasing resistance of its circuit
- According to this relation (V = VB – I r) when increasing resistance current will decrease and the internal
potential difference (I r) will decrease and because VB is constant then potential difference across the
battery will increase
15- E.M.F of a battery is larger than potential difference between its outer terminals when closing the
switch
- Because the internal resistance of the battery consumes power to allow current to flow inside it so
(VB = V + I r) so VB > V

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Laws
To compare between Power consumed in two
resistances
When V is constant
(Pw)1 / (Pw)2 = R2 / R1
When current is constant
(Pw)1 / (Pw)2 = R1 / R2
Resistance
R = V / I
R = Pw / I2
R= V2
/Pw
R = Ꝭe L/A = Ꝭe L/r2
Ꝭ (density) = m (mass) / v (volume)
Ꝭ = m / v
And v = L * A and L = v / A
Then Ꝭ = m / L A then
A = m / L Ꝭ and
L = m / A Ꝭ
R = Ꝭe L2 Ꝭ / m
R = Ꝭe v (volume) / A2
R = Ꝭe L2/ v (volume)
Compare between resistances
R = Ꝭe m / Ꝭ A2
R1/R2= Ꝭe1 L1 A2 / Ꝭe2 L2 A1
R1/R2 = Ꝭe1 Ꝭ1 L1
2 m2 / Ꝭe2 Ꝭ2 L2
2 m1
R1/R2 = Ꝭe1 L1 r2
2 / Ꝭe2 L2 r1
2
Conductivity
σ = 1 / Ꝭe= L / R A
Laws
Quantity of charges
Q = I t
Q = n qe
Q = W / V
Q: total quantity of charges (electrons)
n: number of electrons
qe: charge of one electron
W: work done
V: potential difference
I: current intensity
t: time
Potential difference V
V = W / Q
V = W / nqe
V = I R
V = Pw / I
Current Intensity
I = Q / t
I = n qe / t
I = V / R
I = Pw / V
Electrical Power
Pw = W / t
Pw = V I
Pw = I2
R
Pw = V2
/ R

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To Solve Kirchhoff’s Problems follow the following:
1- Divide the circuit to no. of loops (2 or 3 loops)
2- Find a point (junction) in which all currents are entering or leaving it
3- Write the first equation using Kirchhoff’s first law (sum of currents entering a point equals to sum o
currents leaving this point
4- Specify my direction as follow
A. If there is one battery in the loop, specify the direction to be from +ve pole to –ve pole of this
battery
B. If there are two batteries in the loop, specify the direction to be from +ve pole to –ve pole of
the largest battery
5- Write the 2nd
and 3rd
equations using Kirchhoff’s second law (sum of potential difference inside a
loop equals zero), we have two choices:
A. If there is one battery in the loop, then write its value direct V= IR + IR
B. If there are two batteries in the loop, then we have two choices
 If they are connected parallel ( -ve connected to –ve and +ve connected to +ve) then
subtract their values V1-V2 = IR +IR
 If they are connected in series ( -ve connected to +ve and +ve connected to negative)
Then add their values to each other V1+V2= IR +IR
C. The sign of IR depends on the direction of the current
 If the current passes in the same direction of my direction then put it in +ve (+IR)
 If the current passes in the opposite direction of my direction then put it in –ve (-IR)
6- Current that leaves the battery is the same current enters the battery
7- Potential difference between two points is equal to the potential (or voltage) at the point of higher
potential the potential (or voltage) at the point of lower potential
8- Solve the 3 equations using calculato

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Chapter 2
G. Definitions
16- Magnetic Flux:
- Number of magnetic field lines of a magnet from north pole to south pole
17- Magnetic flux density at a point:
- It’s the magnetic flux for unit area which is normal to magnetic lines around this point
Or
- The magnetic force affecting a wire of 1 m length placed normal to magnetic flux and a current of 1 A
passes through it
18- Tesla
- The magnetic flux density that generates a force of 1 Newton on a wire of 1 m length and a current of 1A
passes through it when this wire is normal to the flux lines
19- Permeability coefficient of a medium:
- The ability of medium to permit the magnetic flux through it
20- Dipole Moment :
- It’s the magnetic torque affecting a coil placed parallel to magnetic field of 1 tesla when an electrical current
passes through it.
21- Moving Coil Galvanometer (sensitive Galvanometer):
- A device used to detect a very weak current to measure its intensity and determine its direction
22- Galvanometer sensitivity:
- The deviation angle of its pointer from zero position when a current of 1 A passes through it
23- Shunt Resistance
- A small resistance connected in parallel with galvanometer to convert it to an ammeter to measure higher
currents
24- Ammeter Sensitivity
- Ratio between maximum current measured by galvanometer to maximum current measured after
converting it to ammeter
25- Multiplier Resistance:
- Large resistance connected in series with galvanometer to convert it to voltmeter to measure higher
voltages
H. What’s meant by?
7- Flux density at a point is 0.4 Tesla
- It means that a magnetic force of 1 N is affecting a wire of 1m length placed normal to the magnetic flux at
this point and a current of 1 A passes through it
8- Dipole Moment is 0.7 N.m/T
- It means that magnetic torque of 0.7 N.m is affecting a coil when a current passes through it and this coil is
placed parallel to a magnetic flux of 1 tesla

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I. Figures
J. Conditions needed to :
Attraction force bet. two wires having currents pass
through them
The two currents should be in the same direction
Repulsion force bet. two wires having currents pass
through them
The two currents should be in opposite directions
Flux density vanishes at a point between two parallel
wires having currents pass through them
The two currents should be in the same direction
Neutral point exists between two straight parallel wires
in a mid-point between them
The two currents should be equal values and in the same
direction
Impossibility of existence of a neutral point for two
straight parallel wires having currents pass through
them
The two currents are equal values and are in opposite
directions
Vanishing the Force affecting a wire have a current pass
through it inside a magnetic field
Wire is parallel two to the magnetic flux
Vanishing the torque affecting a coil in which a current
passes and is placed in a magnetic field
When the plane of the coil is perpendicular to the
magnetic flux
K. Devices
Device Usage Scientific Idea and explanation
Moving Coil Galvanometer to detect very low DC currents and
measures its values and determines
its direction
Idea:
The torque affecting a rotating coil having
a current passes through it inside a
magnetic field
Explanation:
When current passes through the coil, two
equal parallel forces are generated in
opposite directions on two ribs ‫ضلعين‬ of
the coil which causes rotation torque on
the coil
Shunt resistance in Ammeter Converts the galvanometer to
Ammeter to measure higher
currents
When a small resistance is connected
in parallel with the galvanometer coil
this leads to decreasing the total
resistance of the ammeter which avoid
affecting the current needed to be
measured

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Multiplier resistance in Voltmeter Converts the galvanometer to
Voltmeter to measure higher
potential differences
Connecting high resistance in series
with a galvanometer leads to
increasing the total resistance of it and
when connecting this Galvanometer in
parallel with the circuit it consumes a
very small current then it doesn’t
affect the potential difference needed
to be measured
L. Usages
Pair of spiral springs in Galvanometer 1- It’s used as connectors to current
2- Control the pointer movement using reverse torque to
indicate the right value
3- Return the pointer to its zero position
The concave poles in Galvanometer They remain the flux density constant in the space in which coil moves,
this way ensures that flux lines are always in a radius form and they
are parallel to the coil plane (normal to two certain rips)
Iron core inside Galvanometer Collecting and concentrating the flux lines inside the coil
Jeweled bearings in Galvanometer Coil stands on them and they facilitates its rotation
The standards and variable resistance in
Ohmmeter
Controls the current intensity to be maximum value which moves the
pointer to the end of reading (zero ohm) before adding the resistance
needed to be measured
M.Deductions
1- Magnetic force that affects a wire through which electrical current passes in a magnetic field:
-
- F α B (Flux Density) And F α I (current) And F α L (length affected in wire)
- F α BIL
-
Where  is the angle between the wire and the flux
Newton = Tesla.ampere.m, Tesla = Newton /ampere.m
F = BIL sinNewton

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2- Torque affecting a rectangle coil through which current passes and is placed inside a magnetic field
- when a coil is placed in a magnetic field and a current passes through it and the plane of the coil is parallel
to the plane of the field
- there are 2 rips are parallel to flux so force is zero, and the other 2 rips are perpendicular to the field so they
are affected by 2 equal and opposite forces not on the same line, this will cause rotation
- rotation happened in cause of torque
- Torque = force * distance
- Force = BIL sin 
- Distance = L (the normal distance between the pole and the affected rip) this distance is the wide of the
rectangle
- Torque = B I (Length* width) sin 
- Ʈ = B I A sin 
- If we have number of turns N in the coil so :
And
= B |md| sin 
Where |md| is the dipole moment |md|= I A N
3- Shunt Resistance
- Rs and Rg are connected in parallel so
- Vg = Vs
- Then Ig Rg = Is Rs
- Rs= Ig Rg / Is but Is= I - Is
- Then
4- Multiplier Resistance Rm
- Rm and Rg are connected in series so
- V = Vg + Vm = Ig Rg + Ig Rm
-
-
= B I A N sin 
[Cite your source here.]
Rs= Ig Rg / (I - Ig)
Rm= (V – Ig Rg) / Ig

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N. Factors that depends on
Magnetic Flux Density
generated from wire at a point
B = d )
1- Current Intensity (directly proportional)
2- Distance bet. the point and wire (inversely
proportional)
3- Permeability coefficient of the medium (directly
proportional)
generated from coil at the
center point
B = r)
3- Number of turns (directly proportional)
5- Radius of coil (inversely proportional)
proportional)
generated from Solenoid at a
point on its axis
B = L
5- Axis length (inversely proportional)
proportional)
Magnetic Force
F = BIL sin
1- Magnetic flux density B (directly proportional)
2- Length of affected part of the wire (directly
proportional)
3- Angle between the field and wire (directly
proportional)
Torque affecting a coil
= B I A N sin 
3- Magnetic flux density B (directly proportional)
4- Angle between the field and wire (directly
proportional)
5- Area of the rectangular coil (directly proportional)
Dipole Moment
|md|= I A N
3- Area of the rectangular coil (directly proportional)

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G. Comparisons
Two wires having a current passes through them
In the same direction In opposite directions
Resultant of flux density at a
point in between
B total = B1- B2 where B1> B2 B total = B1+ B2
Resultant of flux density at a
point outside them
B total = B1+ B2 B total = B1- B2 where B1> B2
Neutral Point B1 = B2 Falls between the two wires
I1 / (x-d) = I2 /d
Where x is the distance bet. Wires
and d is the distance bet. the point
and wire of lower current
Falls outside the wires
I1 / ( x + d ) = I2 /d
Where x is the distance bet. Wires and
d is the distance bet. the point and
wire of lower current
Force between the two wires Attraction Repulsion
- Rules
Ampere Right Hand Rule Right Screw Clock Rule Fleming Left Hand rule
Figure Straight wire Solenoid
Usage Specifying the
direction of
magnetic flux
generated by a
current passes
through a wire
Specifying the
polarity of the
field
Specifying the
direction of
magnetic flux at
center of a coil
or solenoid axis
Specifying the
pole type
(North-South)
at the face of a
coil or solenoid
Specifying the direction of
magnetic force affecting a
wire which placed
perpendicular to a
magnetic field and current
passes through it
Working
Method
Thumb finger
refers to current
and other
fingers rounding
the wire will
refer to
magnetic flux
Thumb refers
to magnetic
flux and other
fingers
rounding the
solenoid will
refer to the
current
When screw
rotates in clock-
wise direction
its rotation
refers to the
current and its
movement
direction refers
to magnetic flux
If the direction
of current in a
specific face is
clock-wise then
this face is
South pole and
if anti-clock
then it will be
North pole
Middle finger refers to
current ,index finger refers
to magnetic flux then the
thumb refers to the force
Notice: magnetic flux lines is directed from north to south outside the coil and from south to north inside the coil

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Ammeter, Voltmeter, Ohmmeter
Ammeter Voltmeter Ohmmeter
Function Measuring high currents Measuring potential
difference bet. two points
Measuring resistance
Resistance connected on
the galvanometer coil
Galvanometer coil is
connected in parallel to
small resistance called
Shunt resistance Rs
connected in series to large
resistance called Multiplier
Resistance Rm
connected in series to
standard resistance Rc and
variable resistance Rv and a
battery
Idea Of Work Torque affecting a coil that
has a current passes
through it, this coil is
rotating inside magnetic
field
Torque affecting a coil that
has a current passes
through it, this coil is
rotating inside magnetic
field
Depends on the inverse
relation bet. current and
resistance at constant
potential difference
Law Rs= Ig Rg / (I - Ig) Rm= (V – Ig Rg) / Ig I = V / (Rg+ Rc+ Rv+Rx+r)
How it’s connected in
circuit
In series In parallel Device terminals is
connected to terminals of
the external resistance
Graduation Regular as I α  Regular as V α  Not regular as I α R`+Rx)
Shunt and Multiplier Resistances
Shunt Resistance Multiplier Resistance
Method of connection In parallel with galvanometer coil In series with galvanometer coil
Function Convert the galvanometer to Ammeter to
measure higher currents
Convert the galvanometer to Voltmeter to
measure higher potential difference
Idea of work By connecting shunt resistance in parallel
with the galvanometer this leads to
reduce the total resistance of the
device(ammeter) which avoid affecting
the current needed to be measured
Connecting high resistance in series
with a galvanometer leads to increasing
the total resistance of it and when
connecting this Galvanometer in
parallel with the circuit it consumes a
very small current then it doesn’t affect
the potential difference needed to be
measured
Analog Measuring devices Digital Measuring devices
 Its idea depends on Torque affecting a coil that
has a current passes through it, this coil is
rotating inside magnetic field
 Values are appeared on a graduation on which
pointer is moving
 As Galvanometer, Ammeter ,Voltmeter
 Depends on digital electronics
 Values are appeared as digits displayed on the
screen of the device
 As devices of measuring DC and AC currents

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O. What Happens in these cases
1- Current flows in the same direction in two parallel wires
- The resultant of flux densities outside the wires will be larger than it between the wires so attraction force is
generated between them
2- Current flows in opposite directions in two parallel wires
- The resultant of flux densities between the wires will be larger than it outside them so repulsion force is
generated between them
3- Cutting a solenoid of length L and number of turns N at a middle point on its axis and connecting one half
to the same battery
- The solenoid resistance is reduced to half and current intensity will increase to double (and number of turns
remain constant in unit length) so flux density will increase to double
4- Placing wire having a current perpendicularly to a magnetic field
- Wire is being affected by a magnetic force which is perpendicular to current direction and flux lines
5- Passage of dc current of high intensity (bigger than Ig) through the galvanometer coil
- high torque is generated in the coil which is higher than the moment of the ability of the two bearing coils so
they are destroyed and the device is broken down
6- Passage of high frequency (AC) current inside galvanometer
- The pointer is oscillating at the zero reading according to inertia as the pointer cannot react with the change
of direction
7- Reducing the value of shunt resistance
- Ammeter sensitivity is reduced and the graduation of reading is increased
8- Increasing the value of multiplier resistance
- Voltmeter sensitivity is reduced and it will measure higher voltage
9- Nonexistence of standard resistance
- Galvanometer coil will be damaged if current is high and ohmmeter pointer will not be accurate

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P. Give reason
1- It’s recommended in building to be far away the higher voltage towers
- To reduce the effect of the magnetic field which is harmful to the health and environment as the magnetic
flux density is inversely proportional to the distance
2- The neutral point is positioned between two wires having currents pass through them in the
same direction
- Wires will generate two opposite magnetic fields at all points between them so neutral point exists between
them when they vanish each other
3- The neutral point is positioned outside two wires having currents pass through them in opposite
directions
- Because of generating two opposite magnetic fields outside the wires so neutral point exists outside them
when they vanish each other
4- Parallel wires are attracted when currents pass through them in the same direction
- As the resultant flux density between them is lower than it outside them so magnetic force is generated
from the higher density region to lower density region so they are attracted
5- Flux density increases on axis of solenoid that has current passes through it when placing iron core
inside it
- Because permeability coefficient of iron is more than it in air so iron will increase and concentrate the
magnetic flux lines which leads to increase the density
6- Magnetic field in a coil or solenoid may vanish although there is a current passes through it
- As the coil or solenoid is double rounded ,so the magnetic field of one direction is opposing the magnetic
field of the second direction
7- Movement of a straight wire which has electrical current and placed normal to a magnetic field
- This happens according to the difference between the original magnetic field and the field generated by the
wire so wire will move from high density to low density
8- wire having current may not move although it’s placed inside magnetic field
- because it’s placed parallel to magnetic field so (= 0) and F = B I L sin which leads to zero
9- when current passes through a solenoid and a wire placed on its axis, the wire is affected by
magnetic field
- as the wires is placed parallel to magnetic field that is generated by passage of electrical current in the solenoid
And F = B I L sin 
10- torque may not be generated on a rectangular coil which has a current and placed into a magnetic
field
- As the coil should be placed parallel to the magnetic field in order to be affected by magnetic force on two
rips which generate the torque, so if it’s placed perpendicular to the field torque will be 0
11- Torque is reduced on the rectangular coil through its rotation starting from parallel position
- At the parallel position the angle between the coil and the normal plane to field equal 90 so torque is
maximum, while rotating the angle is decreased until it reaches 0 at which angle is zero
sin 
12- Concave poles in Galvanometer
- To keep the magnetic flux lines always parallel to the coils so at any position magnetic field density will be
constant and the deviation angle will always proportional to current intensity

Mr. Ahmed Hekal
19
13- Coil of galvanometer is connected to couple of bearings coils
- They are used to
A. As connectors of current entering and exiting galvanometer
B. They generate opposite torque used to
. Stop pointer at the right value of current
. Return pointer to zero value after the current disconnected
14- Coil of Galvanometer is placed on jewels bearings
- To reduce frictions and maintain the equilibrium of coil to facilitate the rotation
15- Existence of iron core cylinder inside the coil of galvanometer
- To concentrate and increase the magnetic flux inside the coil which increase the galvanometer sensitivity
16- Graduation of galvanometer is regular and zero reading is positioned in the middle
- As the deviation angle is directly proportional with the current intensity and zero is positioned in the middle
to specify the direction of current
17- Galvanometer cannot measure AC current
- As the magnetic field generated by AC is also alternating so the direction of torque changes each half cycle
so inertia will prevent the reaction to this change
18- Galvanometer doesn’t measure high current intensities
- As its coil cannot bear high currents because part of this current is converted to thermal energy which leads
to melting the coil wire and also the torque generated by higher current is very strong which may damage
the jewels bearings
19- Ammeter is connected in series in the circuit
- To measure the total current of the circuit
20- In Ammeter, a very small shunt resistance is connected in parallel with galvanometer coil
- To reduce the total resistance of device which avoid current decrease and the majority of current will pass
through the shunt resistance which protect the coil from damage so we can use Ammeter in measuring high
currents
21- Voltmeter is connected in parallel in the circuit
- To make the potential difference across the voltmeter equal to the potential difference needed to be
measured
22- In Voltmeter, a very high multiplier resistance is connected in series with galvanometer coil
- To enlarge the resistance of the device so a very small part of current will pass through it and the majority of
current will not affected so potential difference needed to be measured also will not affected
23- Electromotive force of the battery in Ohmmeter circuit should be constant
- To maintain Ohm’s law and make the current always inversely proportional to resistance in case of
constant e.m.f
24- High standard resistance is connected in Ohmmeter circuit
- To reduce the current passing through the circuit to protect the galvanometer coil from damage and make
its pointer deviate to the maximum reading before connecting the unknown resistance
25- Graduation of Ammeter is regular and graduation of Ohmmeter is not regular
- As in Ammeter the deviation angle is directly proportional to the current, but in Ohmmeter the current is
inversely proportional with the total resistance not only the unknown resistance
26- Ammeter graduation opposes the Ohmmeter graduation
- Because the current is inversely proportional with resistance so when adding resistance the current
decreases
-

Mr. Ahmed Hekal
20
Laws
2- Flux density in 2 coils
A. In the same level
- Currents in the same direction
BT = B1+ B2
- Currents in opposite direction
BT = B1- B2 (B1>B2)
B. Coils are normal to each other
- B = √B1
2
+ B2
2
3- Flux density between a coil and a wire
A. If the current is in the same direction
- BT = Bwire+ Bcoil
B. If the current is in opposite directions
- BT = Bbig- B small
4- To calculate number of turns in a coil
A. If a wire of length L is rounded in
form of coil
- N = L /2r
B. If the coil is less than 1 turn (part of a
turn)
- N =  / 360
5- In case of the wire is tangent to the
coil
- N I1 = I2/
6- If a coil of N1 turns is reformed to N2
turns and connected
- B1/B2 = N1r2/N2r1= N1
2
/N2
2
= r2
2
/ r1
2
Laws
1- Magnetic Flux m in straight wires
m = 0 when flux is parallel to area
m = B A when flux is normal to area
m = B A sin  when flux is making angle 
with the area
If the flux rotates with angle  from
 Parallel position m = B A sin 
 Normal position m = B A sin (90- 
- If the current is in the same direction
- Neutral Point between the wires
I1 / (x-d) = I2 /d
Outside the wires between the wires
BT = B1+ B2 BT = B1- B2 (B1>B2)
- If the current is in the opposite direction
BT = B1- B2 (B1>B2) BT = B1+ B2
- Neutral Point is outside the wires
I1 / (x+d) = I2 /d
-

Mr. Ahmed Hekal
21
Laws
7- Solenoid
- B = µ N I /L = µ n I
- If turns are tangent
So: L = N*2 r`
- Where :n is the no. of turns per unit area
and r` is the radius of solenoid
8- Solenoid and Coil
Bwire/Bsolenoid = Lsoleoid / 2 rcoil
9- Force
F = B I L Sin 
- Between two wires 
- 
F = L /2d
- Force of two wires affecting third wire
F3= BT I3 L3
- When equilibrium
F = Fg
B I L = mg
10-Torque
= B I A N sin 
is the angle between the coil and
the normal plane to the magnetic field

Laws
11- Galvanometer sensitivity =
=Galvanometer sensitivity * number
of divisions
12- Ammeter
- Shunt resistance
Rs= Ig Rg / (I - Ig)
- Ammeter Resistance
R`=Rg. Rs / (Rg +RS)
- Ammeter Sensitivity
Ig/Is = Rs/ (Rs+ Rg)
13- Voltmeter
- Multiplier Resistance
Rm= (V – Ig Rg) / Ig
- Voltmeter Resistance
R`= Rg+ Rm
14- Ohmmeter
To modulate the Ohmmeter
Imax. = V / (Rg+ Rc+ Rv+r)
Rc: is the standard resistance
After adding unknown resistance
I = V / (Rg+ Rc+ Rv+Rx+r)

Mr. Ahmed Hekal
22
Q. Experiments
1- Magnetic field due to a current passes through a straight wire
1) Use iron filings sprinkled on a horizontal cartoon boards
2) Use a wire to penetrate the board vertically
3) Use battery to pass a current in the wire
4) You will notice that iron filings are aligned in circles around a wire
2- Torque produced in a coil placed in a magnetic field
1) Rectangular coil abcd is placed parallel to a regular magnetic field
2) Ribs ad,bc are parallel to flux lines, so there is no force affecting them
3) Ribs ab,cd are normal to flux lines so they are affected by two equal and opposite magnetic forces
equal B I L
4) As a result for these two forces a torque is generated so we noticed that coil will rotate in a
direction specified by using Fleming Left Hand rule for each force
- Middle finger refers to the current
- Index (forefront) finger refers to magnetic field
- Thumb will refer to the force
5) Torque is specified by this relation
= B I Lab Lbc = B I A
When coil has N number of turns so:
= B I A N sin 
Where  is the angle between the coil and the normal plan on the magnetic field

Mr. Ahmed Hekal
23
Chapter 3
R. Definitions
1- Electromagnetic induction
- The phenomena of producing induced e.m.f and induced current in a conductor as a result of
changing the magnetic flux that’s intersected by this conductor
2- Lenz’s Rule :
- The induced current is in direction to oppose the change that is caused by it
3- Faradays Law:
- Induced e.m.f produced in a coil by electromagnetic induction is directly proportional with the
time in rate of change magnetic flux intersected by the coil and also with number of turns
4- Weber:
- It’s the magnetic flux that is normally penetrating a coil of 1 turn and when it’s vanished gradually
in 1 second an induced e.m.f of 1v is generated in the coil
5- Mutual induction
- The electromagnetic effect between 2 adjacent coils one of them has AC current which affect the
other coil so an induced current is generated in the 2nd
coil to oppose the change happened to it
6- Mutual induction coefficient:
- It’s the induced e.m.f generated in a one coil when changing the current in the other coil by rate
of 1 A/S
7- Self-Induction:
- It’s the electromagnetic effect happened in the same coil to oppose the changing of its current
8- self-induction coefficient:
- it’s the induced e.m.f generated in the same coil when its current changes by rate of 1 A.S
9- Henry:
- It’s the mutual induction coefficient between 2 coils when current of one of them changes by rate
of 1 A/S an induced e.m.f of 1v is generated in the other coil
Or
- It’s the self-induction coefficient of a coil when its current changes in rate of 1 A/S an induced
e.m.f of 1v is generated in the same coil
10- Eddy Currents:
- It’s induced currents generated in a metal core as a result of its motion inside a magnetic field or if
it’s exposed to a changing magnetic field
11- Generator (Dynamo)
- A device used to convert mechanical (motion) energy to electrical energy
12- AC current:
- It’s the electrical current that changes its value and direction periodically with time
- It’s the current that changes its value from 0 to max. and returns back to 0 in half cycle then it
reverses its direction and reaches the max. value in opposite direction then returns to 0 again in
another half cycle

Mr. Ahmed Hekal
24
13- Effective value of AC current:
- It’s the dc current intensity that generates the same amount of thermal energy that’s generated
by AC current in the same resistance in the same time
- It’s the dc current intensity that generates the same electrical power that’s generated by the AC
current in the same resistance
14- Transformer:
- A device used to step up or step down the alternating voltage
15- Transformer Efficiency:
- It’s the ratio of secondary coil power to primary coil power
- It’s the ration of the electrical energy produced in the secondary coil to the electrical energy
consumed in the primary coil in the same time
16- Electrical Motor:
- A device used to convert electrical energy to mechanical (motion) energy
S. What’s meant by
T. Figures
U. Conditions needed to :
Generating of directly induced e.m.f or directly induced
current in the secondary coil
1- Moving the primary coil away from secondary
coil
2- Decreasing the current in the primary coil
3- Open the circuit of the primary coil while it is
near to or inside the secondary coil
Generating of inversely induced e.m.f or inversely
induced current in the secondary coil
1- Moving the primary coil near to or inside
secondary coil
2- Increasing the current in the primary coil
3- Close the circuit of the primary coil while it is
near to or inside the secondary coil
Generating Eddy currents 1- Moving (rotating) a piece of metal core inside a
constant magnetic field
2- Exposing a piece of metal core to a variable
magnetic field
Generating a unified directional current but variable in
its value in Dynamo (Generator)
1- Replacing the two metal rings with one metal
cylinder cracked into two halves this cylinder
called current rectifier
Generating DC current in Dynamo (Generator) 1- Using several coils separated by small equal
angles
cylinder cracked into several parts where
No. of parts = double No. of coils

Mr. Ahmed Hekal
25
Improving the transformer efficiency 1- Fabricating the coils from metal wires which have
a very small resistance to reduce losing of
electrical energy in form of heat
2- Fabricating the iron core from insulated slices of
wrought iron as it has high resistivity used to
reduce Eddy currents
3- The wrought iron is characterized by the ease of
moving of its magnetic molecules it leads to
reduce losing energy in form of mechanical
energy
4- The secondary coil is rolled around the primary
coil to prevent leakage ‫تسرب‬ ‫منع‬ of magnetic flux
lines of primary coil away from the secondary coil
Improving efficiency of rotating the electrical motor
(improving the torque of rotation)
1- Using several coils separated by small equal
angles
cylinder cracked into several parts where
No. of parts = double No. of coils
V. Applications of electromagnetic Induction
Fluorescent Lamp Lighting Idea:
Self-induction in coil
Explanation:
the magnetic energy stored in the coil will
be transferred to a vacuum tube which
has inert gas ‫خامل‬ ‫غاز‬ this energy causes
collisions between gas atoms so they are
ionized and collide with the inner surface
of lamp which is plated with fluorescent
material so visible light is released
Induction furnaces Melting metals Idea:
Eddy Currents
Explanation:
When changing magnetic that’s
penetrated by an iron core, induced
currents are produced in this core
which leads to increase its
temperature until melting degree

Mr. Ahmed Hekal
26
Dynamo (Generator) Convert mechanical (motion)
energy to electrical energy
Idea:
Electromagnetic Induction
Explanation:
When rotating the coil bet. magnet
poles, it intersects variable number of
magnetic lines so an induced e.m.f and
induced AC currents is generated
inside the coil
AC current changes its value and
direction gradually with time
Electrical Transformer 1- step up or step down the
alternating voltage
2- reduce losing energy
during its transfer from
generators to consumption
places through far
distances
3- In some home devices
Idea:
Mutual Induction bet. 2 coils
Explanation:
When primary coil is connected to AC
source, so the changing in magnetic
field will generate induced e.m.f in the
secondary coil that will be bigger than
or smaller than e.m.f of source
according to number of turns in the
two coils
It enlarges the e.m.f when NP > Ns
It reduces the e.m.f when NP < Ns
Electrical Motor Convert electrical energy to
mechanical (motion) energy
Idea:
Torque results in passage of electrical
current in a coil inside a magnetic field
Explanation:
When electrical current passes
through a coil, two equal and opposite
directions forces will affect the two
ribs normal to magnetic field so torque
is generated which rotates the coil in
one direction around its axis
W.Usages
Fleming’s Right Hand Rule  Specifies the direction of induced current in a straight wire
moves normally to magnetic field
Unified direction and variable intensity
electrical current
Preparing some metals by electrical analysis of its compounds
Unified direction and unified intensity
electrical current
Mobile phones chargers

Mr. Ahmed Hekal
27
Deductions
5- Faraday’s Law:
Induced e.m.f produced in a coil by electromagnetic induction is directly proportional with the time rate
of change in magnetic flux intersected by the coil and also with number of turns
e.m.f m / t and e.m.f N
So
-
m / t: change in magnetic flux with time, N: Number of turns of coil
6- Induced e.m.f in a straight wire
- when a wire of length L moves with velocity V normally on a regular magnetic field of density B
An induced e.m.f is produced in the wire
e.m.finduced =  m / t =  B t
Where is the change in the area the wire moved through
- if it moves a distance of x so L x
e.m.finduced = B L x / t
And x / t = V
e.m.finduced =  B L V
- And if wire makes angle with field so
7- E.m.f generated by mutual induction
- When current intensity changes in the primary coil of rate 1 /t , and induced (e.m.f)2 is generated in
the secondary coil which is directly proportional with the rate of change in magnetic flux m /t
- So (e.m.f)2 m /t
- And m /t 1 /t
- Then (e.m.f)2  /t
- Where is the mutual induction coefficient
e.m.finduced =  N m / t
e.m.finduced = B L V sin 
(e.m.f)2 =  /t

Mr. Ahmed Hekal
28
8- E.m.f generated by self-induction
- Induced e.m.f produced is directly proportional with the time rate of change in magnetic flux so
e.m.f m /t
- The time rate of changing in magnetic flux is directly proportional with time range of changing in current
m /t /t
So e.m.f /t
e.m.f = - L /t
- Where Lis the self-induction coefficient
9- The induced instantaneous e.m.f. generated in Dynamo (Generator)
- When coil rotates with a linear velocity V, where the longitudinal ribs intersect the magnetic flux lines, so
if the angle between the direction of linear velocity and the plane of magnetic field is then induced e.m.f
generated in the each rib is
e.m.finduced = B L V sin 
- And e.m.f in one turn (two ribs)
 e.m.finduced = 2 B L V sin 
V = distance / time = (circumference of circle for 1 rotation) / time for 1 rotation
V = 2 r / t and frequency f = 1 / t
 V = 2 r f take 2 f = 
 V = r where  is the angular velocity, r is radius of rotation and V is linear velocity
- e.m.finduced = 2 B L r sin 
- We can take ( 2 r L = A ) where A is the Area of coil (L is the length and 2r is the width)
- e.m.finduced = B A  sin 
- And if coil has number of turns = N
- e.m.finduced = N A B  sin 
- = 2 =  t =2 f t
= 2 f = t
e.m.finduced = N A B  sin 
e.m.finduced = N A B  sin  t

Mr. Ahmed Hekal
29
10- Relations for the electrical transformer
A. Relation between the two e.m.f. of the two coils of ideal transformer and their number of turns
- When primary coil is connected to source and circuit of the secondary coil is open, an induced e.m.f is
generated in the primary coil by self-induction which is
- VP =  NP m / t 1)
- When closing the secondary coil circuit, an induced e.m.f. is generated in the secondary coil by mutual
inductions because secondary coil intersects the flux lines of primary coil
- VS =  NS m / t 2)
- We suppose there is no lose in magnetic flux then
Divide 2) by 1)
B. Relation between the two currents of the two coils of ideal transformer and their number of turns
- We suppose that there is no energy lose in the transformer so
Energy consumed in the primary coil in a specific time = energy generated in the secondary coil in the same
time
VP P t = VS S t
VP P = VS S
- Power input (primary coil) = Power output (secondary coil)
VS / VP = P /S
And VS / VP = NS / NP

This means that the current intensity in any coil is inversely proportional with its number of turns
VS / VP = NS / NP
P /S = NS / NP

Mr. Ahmed Hekal
30
C. Factors that depends on
Induced e.m.f generated in a
coil
1- Time in rate of change magnetic flux intersected by the coil
2- Number of turns
Induced e.m.f in a straight
wire
1- Flux Density
2- Length of the wire
3- Velocity of wire intersecting to flux
4- Sin of angle between the wire and the field
Mutual coefficient between
two coils
1- Permeability coefficient of medium
2- Volume of the coils (length and are of each turn)
3- Number of turns of the coils
4- The distance between them
Self-induction of a coil 5- Geometric shape of the coil
6- Number of turns
7- Length of the coil
8- Permeability coefficient of medium
The induced instantaneous
e.m.f. generated in Dynamo
(Generator)
1- Number of turns
2- Magnetic Flux Density
3- Area of the coil
4- Angular velocity that coil rotates with, or frequency of
rotation
5- Sin of angle between the coil and the normal plane of
the magnetic field
Consumed energy in a wire E = I V t
1- Current intensity
2- Potential difference
3- Time of current passage
e.m.finduced = B L V sin 
e.m.finduced = N A B  sin  t

Mr. Ahmed Hekal
31
H. Comparisons
AC current DC current
How it’s obtained AC Generator - DC generators
- Batteries and cells
Characteristics 1- it changes its value and direction with
time
2- can be transferred for long distances
without losing energy, by stepping up its
voltage using transformers
3- can be converted to DC current
1- it’s constant in value and direction
2- cannot be transferred because it lose
its energy in form of heat
3- cannot be converted to AC current
Usage 1- Lighting
2- Heating
3- Operating machines
1- Lighting
2- Heating
3- Electrical plating
4- Charging batteries
Step-up Transformer Step-down Transformer
Usage Raising voltage at generating stations Step-down voltage at consumption places
Number of turns NS > NP NP > NS
Electromotive Force Vs > VP VP > VS
Current intensity IP > IS IS > IP

Mr. Ahmed Hekal
32
Generator (Dynamo) Motor Transformer
Usage convert mechanical (motion)
energy to electrical energy
A device used to convert
electrical energy to
mechanical (motion) energy
1- step up or step down
the alternating voltage
2- reduce losing energy
during its transfer from
generators to
consumption places
through far distances
3- In some home devices
Structure Rectangular coil of copper
wire rolled around core of
wrought iron, the coil and
core can be rotated easily and
placed inside a magnetic field
Rectangular coil of copper
wire rolled around core of
wrought iron, the coil and
core can be rotated easily
and placed inside a magnetic
field
Two coils (primary, secondary)
are rolled around a core of
wrought iron formed of
insulated slices to avoid eddy
currents
Operation Terminals of coil are
connected to two split rings
rotate with the coil and each
ring touches a fixed graphite
brush which connect the
induced current of coil to
external circuit
Terminals of coil are
connected to two halves of a
cracked cylinder, each half is
connected to a fixed
graphite brush, these
brushes are connected to DC
source (Cell)
Primary coil is connected to the
source of AC current needed to
raise or reduce its voltage VP
and secondary coil is
connected to the external
circuit that needs a specific
value of voltage Vsf

Mr. Ahmed Hekal
33
D. Experiments
Experiment 1 Faraday’s Experiment
OR
- Experiment to generate induced current in a coil
OR
- Experiment to convert mechanical energy to electrical energy
Steps and Notices
1- Connect terminals of coil of copper wire to a sensitive galvanometer which has zero in middle grade
2- Place a magnet inside the coil
Notice
- Galvanometer pointer moves in a specific direction
3- Pull the magnet outside the coil
Notice
- Galvanometer pointer moves in opposite direction
4- Fix the magnet and move the coil near and far the magnet
Notice
- Galvanometer pointer moves in the two directions according to movement of the coil
Deduction - Results
An induced e.m.f and induced current is generated in a coil as a result of changing the magnetic flux around this
coil (when intersecting the magnetic lines by the coil), the direction of this induced current depends on the
movement direction of the magnet near or far the coil
- Experiment 2 Mutual induction between two coils
Steps and Notices
1- Connect a coil to a circuit has (battery ,key and rheostat) to be the primary coil and connect another coil
(secondary coil) to a galvanometer has zero in its middle graduation
2- Close the circuit of the primary coil and move it near to secondary coil
Notice
3- Move the primary coil far away the secondary coil
Notice
4- Fix the primary coil inside the secondary coil and increase current intensity in the primary coil
Notice

Mr. Ahmed Hekal
34
5- Decrease current intensity in the primary coil
Notice
Deduction
We can generate induced e.m.f and induced current in a secondary coil by effect of a primary coil where
- Reverse induced e.m.f and reverse induced current : by increasing the magnetic field of primary coil so the
induced current in the secondary coil is in direction to oppose the change causing it (in the primary coil) to
resist the increase of magnetic field
- Directed induced e.m.f and directed induced current : by decreasing the he magnetic field of primary coil so
the induced current in the secondary coil is in direction to oppose the change causing it (in the primary coil)
to resist the decrease in the magnetic field
-
Experiment 3: self- induction
Steps and Notices
1- connect a coil of a strong magnet (of high number of turns) in series with (a battery of 6 volts, key) and in
parallel with a lamp which works under voltage of 180 v
2- close the circuit
Notice
- Lamp doesn’t work
3- Open the circuit
Notice
- An electrical spark between the terminals of the key and lamp is lightening for a very short time
Deduction
1- When closing the circuit a reverse induced e.m.f and a reverse induced current is generated in the coil which
delays the main current to reach its max. value and a strong magnetic field is generated in the coil because each
turn is considered a small magnet
2- When opening the circuit the current is attenuated ‫يضمحل‬ (decreases) so an induced e.m.f and induced current
is generated in the coil by self-induction this induced e.m.f is high because number of turns is big and the time
rate of changing the current is also big e.m.f /t and induced current is high so it generates an electrical
spark

Mr. Ahmed Hekal
35
E. Some Explanations
1- Specifying the direction of induced current using Lenz’s rule:
- When moving the north pole of a magnet near to a coil which has a current passes through it, the face of the
coil which is near to the magnet will be also a north pole and an induced current will pass through the coil in
the anti-clockwise direction to resist the change causing it N  N
- When moving the north pole of a magnet far from a coil which has a current passes through it, the face of
the coil which is near to the magnet will be a south pole and an induced current will pass through the coil in
a clock-wise direction to resist the change causing it N  S
2- Fleming’s Right Hand Rule
Usage:
- Specifying the direction of induced current passing in a straight wire which moves perpendicular to a
magnetic field
How it’s used
- make the thumb, index (forefront) and middle fingers are perpendicular, so
- Thumb: refers to direction of movement
- Index (forefront) : refers to magnetic field
- Middle: refers to the induced current direction
3- Eddy currents disadvantages and how to avoid them :
- Disadvantages: large part of electrical current is lost in thermal energy
- How to avoid them? Made the iron core from thin insulated slices of silicon-wrought iron which has high
resistivity
4- Transformer Operation:
- The primary coil is connected to an AC source (needed to be transformed) and the secondary coil is
connected to the external circuit which will use the transformed current
- When closing the circuit of the secondary coil and AC current passes through the primary coil, an alternating
magnetic field is generated and the iron core will collect and concentrate it in the secondary coil turns
- An induced e.m.f and induced current is generated in the secondary coil which is larger or less than the
source according to the ratio between number of turns in both coils
5- How does Motor works along complete cycle
- In the first half cycle
 When the coil is parallel to the magnetic field its terminals (the two halves of cracked cylinder) touch the
graphite brushes, so current will pass through coil and two opposite forces acting on two ribs so they produce a
torque causing rotation for the coil
 Torque decreases gradually with the rotation of the coil until it vanishes when its plane is perpendicular to
magnetic flux, but the coil continue rotating according to inertia until it returns back to its original position
(parallel to the field)
- In the second half cycle
 Coil is parallel to the field so torque will be generated in the same direction so coil will continue rotation in the
same direction
 Torque decreases gradually until it vanishes , but the coil continue rotating according to inertia until it returns
back to its original position (parallel to the field) this will happen each cycle

Mr. Ahmed Hekal
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Current flows in the same direction in two parallel wires
F. What Happens in these cases
1- Moving a coil has electrical current near to another coil connected to a galvanometer
- A reverse induced e.m.f is generated in the second coil by the mutual induction
2- Open an electrical circuit contains a magnetic coil connected in parallel with a battery
- An electrical spark happens across the key terminals
3- Open the primary coil circuit when it’s inside the secondary coil
- A direct induced e.m.f. is generates in the secondary coil to resist the shortage in the primary coil current
4- Increasing value of current in the primary coil placed inside a secondary coil which is connected to a
galvanometer
- Galvanometer pointer deflects in one directions because of generating reverse induced e.m.f. on the
secondary coil by mutual induction
5- A high frequency current passes through a coil rolled around a piece of metal
- Its temperature will increase according to eddy currents
6- Growth of current in a coil rolled on a wrought iron core inside it to the time of current growth
- The time of current growth will increase in the coil as a result of generating a big reverse induced e.m.f
because permeability coefficient of wrought iron is high and self-induction coefficient of it will be also high
7- Wires of electrical resistance are doubled rolled
- Self-induction will vanish so the main current will be only affected by Ohmic resistance, because the
magnetic field produced by on turn will cancel the magnetic field produced by the next turn
8- Increasing number of turns of dynamo to double and number of cycles in 1 second to double
- Induced e.m.f. will increase 4-times
9- Increasing number of turns of Dynamo coil to double and decreasing its angular velocity 4 times
- Instantaneous induced e.m.f. will be reduced to half
10- Replacing the two split rings of Dynamo with cylinder cracked to 2 halves
- AC current will be converted to unified direction current of alternating values
11- Dividing the cracked cylinder in Dynamo to number of pieces equal double number of coils
- This will completely convert the AC current to DC current which is unified in direction and constant value
12- Connecting primary coil in a transformer with a cell (battery)
- The magnetic flux resulting on it will be constant so, no mutual induction will happen between the coils and
the transformer will not work
13- Opening circuit of secondary coil in the transformer and connecting its primary coil with an AC source
- A reverse induced e.m.f. is generated in the primary coil, it’s equal to e.m.f. source so current is approximately
zero
14- Transferring AC current for long distances without raising voltage before transfer
- A lot of energy is lost in the wires in form of thermal energy

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Give reason
1- An induced e.m.f. is generated in a wire moves normally on magnetic flux lines
- Because the magnetic flux affects the free electrons of the moving wire so these electrons will release from
one terminal of the wire(+ve terminal) to another terminal (-ve terminal) so a potential difference is
produced between the two terminals of wire
2- Induced e.m.f may not be generated in a moving wire in a magnetic field
- Because the direction of wire movement is parallel to the magnetic flux lines, so according to law
e.m.finduced = B L V sin angle between field and wire will be 0 and e.m.f. will be zero
3- Induced e.m.f. produced in a coil is higher if the core of the coil is made of wrought iron
- As the wrought iron has a higher permeability coefficient and this will increase concentration of magnetic
flux lines that are intersected by the coil which leads to increase the induced e.m.f
4- Wires of standard resistance are double rolled
- To avoid the self-induction because the magnetic field produced by a turn is cancelled by the magnetic field
produced by the next turn, so current will only affected by the Ohmic resistance
5- A piece of wrought iron doesn’t magnetized if it’s caught by double rolled wire has a current passing through
it
- Because the direction of current in the first wire opposes the direction of current in the second wire so
magnetic field generated by one of them will cancel the other, so resultant magnetic field equals zero
6- The direct e.m.f. induced in a coil by self-induction always bigger than the reverse induced e.m.f
- Because the collapse rate of current is always bigger than the growth rate of current
7- Current intensity doesn’t reach maximum value in coil in the moment closing the circuit, and doesn’t vanish
also in the moment of opening the circuit, it takes time
- This happens because of generating a reverse e.m.f. in the moment of closing and opening the circuit this
reverse e.m.f will oppose the main current whether when it increases or when it decreases
8- Current is growing in a straight wire faster than a coil
- Because the magnetic field produced around the wire is not intersected by the wire itself so there is no
reverse induced current produced in it, but in case of coil the magnetic field produced in coil is intersected
by the coil itself so a reverse induced e.m.f is generated in coil by self-induction and also a reverse induced
current so it resist the main current in the coil
9- Vanishing the induced current in a straight wire is faster than it in a coil of air core which is faster than it in a
coil rolled on an iron core
- There is no reverse induced e.m.f generates in the wire because the wire doesn’t intersect its magnetic field
- In the coil of air core, there is a reverse induced current is generated in coil to oppose the shortage in
current its value is high
- In the coil of iron core, the reverse induced current is higher because the magnetic field is bigger than it in
the coil of air core according to the permeability coefficient
10- When opening circuit of electrical magnet there is a spark appears in the position of cutting the current
- Because the rate of vanishing the current is very high so the rate of changing the magnetic flux is very high
which leads to generating high induced current in the same direction of the main current to oppose its
shortage

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11- When AC current passes in a coil rolled on a piece of metal its temperature increases
- Because of the eddy currents produced in it which leads to melt the metal
12- Eddy currents don’t produced in a fixed piece of metal otherwise the magnetic field around it is variable
- As in the variable magnetic field, the magnetic flux lines intersected by the metal is changing so eddy
currents are generated
13- Temperature of an iron cylinder is raised if it’s rolled by a coil connected to AC source
- As the AC current changes its value and direction periodically, so the magnetic field resulting by it is also
changing, so eddy currents are produced in the cylinder
14- Induced e.m.f in the Dynamo (generator) coil is max. value when its plane is parallel to the magnetic field
- According to this relation
e.m.finduced = N A B  sin 
is the angle between the coil and the normal plane to the magnetic field so in this case  equals 90 so
Value of e.m.f will be maximum = N A B 
15- E.m.f (average) in Dynamo through ¼ cycle = E.m.f (average) in Dynamo through ½ cycle
In ¼ cycle:
m = BA , t = ¼ T
e.m.f = 4 N B A F
In ½ cycle:
m = 2 BA , t = ½ T
e.m.f = 4 N B A F
16- E.m.f (average) in Dynamo coil through a complete cycle = 0
- As the average value of e.m.f in one direction( ½ cycle) is equal to average value of e.m.f in the opposite
direction (in the second ½ cycle) so the resultant equal zero
17- the cracked cylinder in dynamo produce a unified direction current
- when coil is rotated in half cycle their terminals rotate with it and the current will pass in the same direction
in the external circuit when touching the graphite brushes
18- Terminals of Dynamo coils are connected to number of cracked pieces which is double number of coils
- To guarantee that brushes are always touched to the coil which is parallel to the magnetic field to finally
produce a current of constant value DC current
19- The core of transformer is made of slices of wrought iron which are insulated
- Permeability coefficient of wrought iron is high so it helps in concentrating the magnetic flux lines
- Resistivity of wrought iron is high and when it’s made of insulated slices, this will increase its resistance to
resist the eddy currents and reduce the energy loss in form of heat
20- In Ammeter or Galvanometer the iron core is NOT divided into insulated slices
- Because Ammeter and Galvanometer used to measure dc currents, so there is no eddy currents are
produced in the core , no change is happened in the magnetic flux
21- Transformer coils are made of copper wires
- Because its resistivity is very low so the coils resistance is very low which avoid energy loss in form of heat

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22- There is no transformer with 100% efficiency
- Because there is energy loss in forms of:
A. Loss in magnetic flux lines from primary coil to secondary coil
B. Loss in heat through the wires
C. Mechanical energy according to the movement of the iron core magnetic molecules
23- Transformers are not used in step-up or step-down a DC e.m.f.
Or Transformers don’t work if the primary coil is connected to DC source
- Because the magnetic flux produced by the DC current is constant so no induced e.m.f is produced in the
secondary coil by mutual induction (there is no mutual induction happens)
24- Transformer doesn’t consume power when opening circuit of secondary coil, although its primary coil is
connected to electrical source
- When opening the secondary coil circuit a reverse induced e.m.f. is produced in the primary coil (by self-
induction) and it’s equal to e.m.f (source) so there is not potential difference and there is no current passes
in the primary coil and no power is consumed
25- Transformer works when closing its secondary coil circuit
- When closing secondary coil circuit, current passes through it, the magnetic flux resulting by it will be
intersected by the primary coil and it vanishes the reverse induced current in it, so the current of source will
pass through the primary coil and continue working
26- Electrical energy is transferred from generators stations to consumers under a high voltage and low current
- To reduce the consumed energy in the wires because power is directly proportional with square of current
intensity
27- Use of step-up transformers at the generating stations
- Because the step-up transformers raise the voltage at the generating stations which leads to reducing the
current intensity in the transformer and this is useful in avoiding loss in energy consumed in transferring
wires
28- The step-up transformers are current step-down and vice versa
- Because the power is constant and this makes the voltage inversely proportional with current
V = Pw/ I
29- Motor continue in rotation although it passes through the position which is normal to magnetic field
Or
The coil of electrical motor doesn’t stop when the graphite brushes touches the insulation part of the cracked
cylinder halves
- Because the inertia makes the coil to continue rotating and the two halves exchange their positions and also
current exchange its direction so torque will be in the same direction
30- To increase Motor power, we use several coils separated by small angles
- To increase the torque by guarantee that the coil is always parallel to the magnetic flux so the torque is
always maximum value an coils rotates with high angular velocity, this leads to enhance the motor efficiency

Mr. Ahmed Hekal
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Laws
E.m.f in Dynamo (Generator)
A. Instantaneous e.m.f.
e.m.f = e.m.fmax. Sin 
e.m.f = N A B  sin  t
V / r = t = 2T = 2  f
inside (sin) = 180
outside (sin) = 22/7
B. Maximum e.m.f.
E.m.f max. = N A B 
C. Effective e.m.f.
E.m.f effective = e.m.fmax. Sin 45
D. Average e.m.f.
In ¼ cycle and ½ cycle
e.m.f = 4 N B A F
In ¾ cycle
e.m.f = 4/3 N B A F
In 1 cycle
e.m.f = 0
Instantaneous induced current
Instantaneous = IMAX sin 
Number of reaching the AC current to maximum value
in 1 second= 2 f
Number of reaching the AC current to zero in 1 second
= 2 f +1
Laws
Faraday’s Law
- If area changed
e.m.finduced =  N B / t
- If flux density changed
e.m.finduced =  N   B / t
- If coil rotates
A. ¼ cycle (90)
e.m.finduced =  N  B / t
B. ½ cycle (180)
e.m.finduced =  2 N  B / t
C. 1 complete cycle
e.m.finduced = because m=0
E.m.f. induced in a coil
Self-induction:
e.m.f = - L /t =  N m / t
(Self-induction coefficient)
L = 
/ L (length)
Mutual Induction
(e.m.f) 2 =  /t =  N2 (m )2/ t

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Laws
Electrical Transformer
A. Ideal Transformer:
VP P t = VS S t
VS / VP = P /S
P /S = NS / NP
In case of 2 secondary coils:
(Pw)p = (Pw)s1 + (Pw)s2
B. Non-Ideal Transformer:
ɳ = (Vs Is/VP Ip) * 100
ɳ = (Vs Np/Vp Ns) * 100
(Pw) p > (Pw) s
Pw (consumer) = Pw (station) - Pw (loss in wires)
1- Power at generating stations = I V
2- Power consumed in wires = I2
R
3- Shortage in Volt = I R
Laws
Electrical Motor

A. Current Intensity
- Before operation
I = e.m.f.(source)/ R
- During Running
I = e.m.f.(motor)/ R
B. Electromotive force
E.m.f motor = e.m.fsource – e.m.freverse

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42
Chapter 4
X. Definitions
1- AC current
- It’s the current that changes its magnitude gradually from 0 to maximum after quarter cycle and
changes its direction after half cycle
2- AC frequency :
- It’s the number of complete cycles of AC current in 1 second
3- Periodic Time of AC:
- It’s the time taken by AC current to make a complete cycle
4- Hot Wire Ammeter:
- Device used to measure AC or DC current and it depends on the expansion of a wire made of alloy of
platinum and iridium by the thermal effect of electrical current
5- Inductive Reactance:
- It’s the resistance of a coil caused by its self-induction Xl
6- Electrical Capacitor:
- Two parallel insulated metal plates, used to store electrical energy in form of electrical field
7- Capacitance:
- The ratio between the charge placed on one plate and the potential difference between the two plates
8- Capacitive Reactance:
- It’s the resistance of a capacitor caused by its capacitance Xc
9- Farad:
- It’s the capacitance of a capacitor that ,if it’s charged by a charge of 1 coulomb the potential difference
between its plates is 1 volt
10- Impedance:
- It’s the equivalent for the ohmic resistance, inductive reactance and the capacitive reactance for an AC
circuit
11- Oscillator Circuit:
- It’s an electrical circuit in which there is an exchange for energy stored in inductive coil in form of
magnetic field and the energy stored in a capacitor in form of electrical field
12- Resonant Circuit :
- It’s an oscillator circuit contains a resistance, inductive coil, capacitor and AC source and it only allows
passage of AC current has frequency equal or very close to its frequency

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Y. What’s meant by
1- AC frequency is 50 Hz
- it means that number of complete cycles that made by AC current in one second is 50 cycles
2- Periodic Time of AC current is 0.02 second
- This means that time taken by AC current to complete 1 cycle is 0.02 second
3- Inductive reactance of coil is 100 ohm
- It means that the resistance for the coil resulting in its self-induction is 100 ohm
4- Capacitance for a capacitor is 5 micro-farad
- It means that the quantity of charges placed on one plate is 5 *10-6
coulombs when the potential
difference between is plates is 1 volt
5- Capacitive reactance is 100 ohm
- It means that the resistance of the capacitor according to its capacity is 100 ohm
6- Impedance is 50 ohm
- it means that the equivalent resistance for (ohmic resistance. Inductive reactance, capacitive reactance)
is 50 ohm
7- Phase angle for a circuit has inductive coil and resistance is 45o
- It means that the total voltage leads current by angle 45o
Tan  = VL/VR = XL/R = 1 , XL =R, VL=VR
8- Phase angle for a circuit has capacitor and resistance is 45o
- It means that the total voltage lags current by angle 45o
- Tan  = - VC/VR = -XC/R = - 1 , XC =R, VC=VR
9- Resonant circuit frequency is 104
Hz
- It means that the oscillator circuit frequency equals source frequency = 104
Hz and it only allows the
current of this frequency to pass through it and inductive reactance equals the capacitive reactance only
at frequency 104
Hz
Z. Devices
1- Hot Wire Ammeter Measures the effective value of AC
current and it also measures DC
current
Idea:
Thermal effect of electrical current
Explanation:
When current (DC or AC) passes through
an ohmic resistance it generates a
quantity of heat which depends on the
effective value of this current
2- Antennas Radio channels Idea:
Resonance circuit
Explanation:
When we change the channel on radio
device, the frequency of the resonance
circuit changes to a specific value
which equals the frequency of the
desired channel current (because
electromagnetic wave of the channel is
converted to AC current)

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44
AA. Usages
1- Platinum-Iridium Wire  It’s heated up and expands when electrical current passes
through it so we can measure the effective value of current
2- Silk thread in Hot-Wire Ammeter  It is pulled by the platinum-iridium wire so the roller will
rotates and pointer will move and stops on the effective value
of the current
3- The board on which the platinum
wire is tensed
 Get rid of zero error
4- Roller in Hot-Wire Ammeter  It rotates when it’s pulled by the silk thread, so pointer will
deviate until it reaches the effective value
5- Coil in Hot-Wire Ammeter  It pulls the silk wire to rotate the roller and move the pointer
to the effective value of the current
6- The resistance connected to the
platinum-Iridium wire
 It divides the total current to allow a suitable current to pass
through the wire
7- The variable-capacity capacitor in
RLC circuit which works as resonant
circuit
 when changing the capacitor capacitance its capacitive
reactance will change until it equals the inductive reactance of
coil which means that the impedance will equal the ohmic
resistance only (minimum impedance) and current will be
maximum value (it’s used in receiver devices)
8- Resonance Circuit  Used in receivers to receive a specific wave
BB. Figures
CC. Deductions
1- Frequency of current in resonant circuit
- In resonant circuit, current will be maximum when inductive reactance equal capacitive reactance
- XL= XC
- 2fL = 1 / 2fC
- f 2
=1/4
LC
f = 1/2√ LC

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45
DD. Factors dependent on :
4- Angle of deviation in Hot-Wire Ammeter - Square of current intensity 
5- Inductive reactance of a coil XL= 2 f L
4- Self-induction coefficient
5- Frequency of current
6- Capacitive Reactance of a capacitor XC= 1/ 2 f C
1- Capacity of capacitor
2- Frequency of current
7- Total impedance Z = √ R2
+(XL-XC)2
1- Ohmic Resistance
2- Inductive reactance
3- Capacitive reactance
8- Frequency of resonant circuit f = 1/2√ LC
1- Square root of capacity
2- Square root of self-induction coefficient
I. Comparisons
Hot-Wire Ammeter Moving Coil Ammeter
Idea Of Work Expansion resulting on thermal effect of
electrical current
Torque resulting on the magnetic effect of
electrical current
Usage Measuring intensity of DC current and effective
value of AC current
Measuring DC current only
Scale (graduation) Non-regular Regular
Effect of room
temperature
It’s affected by room temperature It doesn’t affected
Pointer move It moves slowly when passing the current or cut-
off
It moves faster when passing the current or
cut-off the current

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46
Circuit of R and XL (RL)
circuit
Circuit of R and XC
(RC) circuit
Circuit of XL, XC,R
(RCL) Circuit
Resonant Circuit
Circuit
Total Voltage V = √ VR
2
+ VL
2
V = √ VR
2
+ VC
2
V = √ VR
2
+ (VL- VC)2
VL=VC
V = VR
Impedance Z = √ R2
+XL
2
Z = √ R2
+XC
2
Z = √ R2
+(Xl-XC)2
XL= XC
Z = R
Phase Angle Tan = VL/VR = XL/XR
 is Positive
Tan = - VC/VR
= -XC/XR
 is Negative
Tan = (VL-VC) / VR
=(XL-XC)/R
When XL> XC  is Positive
When XL< XC  is Negative
Tan = 0


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47
J. Important Explanations
1- Disadvantages of Hot-Wire Ammeter
A. Its pointer moves slowly until stop at effective value and also it returns back to zero slowly
B. Platinum-Iridium wire is affected by the room temperature which causes some errors in readings
called zero error
2- AC circuit contains non-inductive resistance
A. V = Vmax. sin ω t
B. I = V / R So I = Imax. sin ω t
C. From A. and B. we found that in this circuit voltage and current are in phase it means they reach 0
together and maximum value together
3- AC circuit contains an inductive (non-resistive) coil
A. When closing the circuit, voltage between terminals of coil reaches Vmax. ,current grows gradually and
voltage decreases gradually according to the reverse induced e.m.f. until current reaches maximum at
the moment in which voltage is zero
B. Induced current is generated in the coil and it resist the change causing it this is the cause of lag1-ging
current in reaching maximum with voltage
C. Vinduced = - L /t
D. Current lags ‫عن‬ ‫يتأخر‬ voltage by 90o
or ¼ cycle
E. XL = 2πfL

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48
4- AC circuit contains a capacitor
A. In 1st
quarter, current reaches maximum, where –ve charges are transferred on plate A and its potential is
decreased, these charges affect plate B and repulsed with –ve charges on plate B so plate B has only +ve charges, at
this moment capacitor is charged and current stops (equal 0)because voltage on capacitor = voltage of source =
maximum value
B. In 2nd
quarter e.m.f.source decreases so potential difference across the capacitor is higher than source so it discharges
in source, current will reach maximum and voltage of capacitor reaches 0
C. In 3rd
quarter, capacitor will charge again but in the opposite direction (plate B is –ve and Plate A is +ve) until its
voltage reaches e.m.f.source so current stops =0 and voltage is maximum
D. In 4th
quarter, capacitor discharges and its voltage will be 0 and current is maximum
I = C V/t
E. Current leads ‫يسبق‬ voltage by 90o
or ¼ cycle
F. XC= 1 / 2πfC
5- RL Circuit
A. In the coil V leads  by 90o
or ¼ cycle
B. In Ohmic resistance V and  are in phase
C. Current is the same because they are connected in series
D. Voltage of coil leads voltage of resistance by 90o
E. V = √ VR
2
+ VL
2
F. Z = √ R2
+XL
2
6- RC Circuit
A. In capacitor V lags  by 90o
or ¼ cycle
C. Current is the same because they are connected in series
D. Voltage of resistance leads voltage of capacitor by 90o
E. V = √ VR
2
+ VC
2
F. Z = √ R2
+XC
2
7- RCL circuit
A. In the coil V leads  by 90o
or ¼ cycle
C. In capacitor V lags  by 90o
or ¼ cycle
D. Current is the same because they are connected in series
E. V = √ VR
2
+ (VL- VC)2
F. Z = √ R2
+(Xl-XC)2

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49
8- Oscillator Circuit
A. Structure
I. Inductive coil of a very small resistance
II. Capacitor
III. Battery and all are connected in parallel
B. Operation
I. When close key “A”
- Current passes in the capacitor
- One plate (connected to positive pole) is charged with
positive charge, and the other plate is charged with
negative charge
- Current stops when potential difference across capacitor
is equal to VBattery1
- Energy is stored in capacitor in form of electrical field
- Open key “A” ,now capacitor is charged
II. When closing key “B”
- Capacitor discharges through coil and current flows from the positive plate to negative plate, potential
difference between plates will decrease until it vanishes and the electrical field disappears
- The coil stores energy in form of magnetic field resulting on the current passes through it
- In the beginning, the potential difference through capacitor is high so current passing through the coil is
high, after an interval of time P.D. in capacitor decreases and current also decreases in the coil
- This shortage in current leads to generate a direct induced e.m.f in the coil by its self-induction, this
induced e.m.f. attracts positive charges from the positive plate to negative plate, so positive plate will be
charged with negative charges and negative plate will be charged with positive charges
- Capacitor is now charged in opposite direction and this means that magnetic energy is converted again
to electrical energy
- Capacitor will start discharging in opposite direction in the coil and electrical field is converted to a
magnetic field and so on , which causes many oscillations in the circuit according to this exchange
9- Relation between frequency and (XL XC R Z)
- Impedance Z decreases until it reaches minimum Z =R when XL=XC and it increases with frequency
- Current increases with frequency until it reaches maximum when XL=XC then it decreases with frequency
increase, this is because current is inversely proportional with impedance
- Circuit is resonant (in resonance state) when XL=XC
f = 1/2√ LC

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G. What Happens in these cases
1- Flow of AC current in an Ohm resistance to its temperature
- Its temperature increases because of energy loss in form of thermal energy
2- AC or DC current pass through the Hot-Wire Ammeter
- Thermal energy is generated in the Platinum-Iridium wire so it expands and allows to the silk thread to
make the roller to rotate and pointer to deviate so it gives the value of effective current
3- Cutting-off current passes in Hot-Wire Ammeter
- The wire is cooled and attracts the silk thread which rotate the roller to returns the pointer to zero
4- The silk thread is cut
- The expansion of platinum-iridium wire will not affect the roller or the pointer so, Ammeter will not give
a reading
5- Passage of AC current in an inductive coil to the phase angle between current and voltage
- Voltage leads current by 90O
or ¼ cycle
6- if frequency of AC current is highly increased
- the inductive reactance will increases also by relation XL= 2 f L until it prevents the flow of current
7- Capacitor is connected to DC source
- Current flows in the circuit in a small interval of time then it decreases until it vanished when P.D. of
capacitor = VBattery
8- Passage of AC current in a circuit contains capacitor to the phase angle between current and volt
- Current leads voltage by 90O
or ¼ cycle
9- High Increase of frequency of electrical current passes through a capacitor
- Capacitive reactance will decrease according to relation XC= 1/ 2 f C and circuit is considered short-
circuit
10- Connecting a resistance with an inductive coil and AC source to the phase angle between current and
total voltage
- Voltage leads current by angle  where tan  = VL / VR = XL / R
11- Connecting a resistance with an inductive coil and AC source to the phase angle between current and
total voltage
- Current leads voltage by angle  where tan  = - VC/ VR = - XC / R
12- Connecting a charged capacitor to inductive coil
- Capacitor discharges in the coil and an instantaneous current flows ,so a reverse induced e.m.f. is
generated in the coil in opposite direction of the main current, this operation is reversed several times
causing oscillations so it’s called Oscillator circuit
13- The inductive reactance equals the capacitive reactance in RCL circuit
- The circuit is resonant, and total impedance is minimum (Z =R) and current is maximum , voltage and
current are in phase = 0 and frequency f = 1/2√ LC
14- Replacing AC source with DC source of the same effective value in RL circuit to the current intensity
- Current will increase because total impedance is decreased when AC source is replaced by DC source
- In case of AC : current has frequency f and coil has inductive reactance XL= 2fL so Z is high Z = √ R2
+XL
2
- In case of DC : frequency =0 and XL=0 so Z is low Z = R only , current will increase

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51
K. Give reason
31- Hot-Wire Ammeter is used to measure AC and DC currents
- As it generally depends on the thermal effect of any electrical current, it measures the effective value of
current according to the expansion of Iridium-Platinum wire by heat caused by current passage in the
wire.
32- An alloy of Platinum-Iridium used in the Hot-Wire Ammeter
- As it expands easily by heat when current flows through it
33- Platinum-Iridium wire is connected in parallel with a resistance R
- To work as a shunt resistance (it divides the current) to allow a suitable part of current to pass through
the wire
34- Hot-Wire Ammeter is connected in series in the electrical circuit
- To measure the current needed to be measured not part of it
35- Hot-Wire Ammeter divisions are not regular
- Because the quantity of heat generated in the wire is directly proportional with the square of effective
current not the current only
36- There is an error in Hot-Wire Ammeter called zero error
- Because the platinum-iridium wire is affected by the room temperature
37- The platinum-iridium wire is tensed on a board made of a material has the same expansion coefficient of
the wire and insulated from it
- To overcome the error caused by affecting the wire by the room temperature
38- Current and Volt are in phase in ohmic resistance circuit
- Because V = Vmax sin t and  = V / R ,  = (Vmax sin t) / R
So  = max sin t
They have the same phase angle so they vanishes together and reaches maximum together
39- At very high frequencies AC current may not pass through the inductive coil
- Because inductive reactance is very high XL= 2 f L and circuit is considered open circuit
40- Passage of AC in an inductive coil (non-resistive) don’t loss energy
- Because the only existent resistance is the inductive reactance that results in generating reversed
induced e.m.f in the coil so it maintains the energy in form of magnetic field
41- When increasing number of turns for a coil the inductive reactance increases if AC current of a constant
frequency passes
- Because inductive reactance XL is directly proportional with self-induction coefficient L when the
frequency is constant XL= 2 f L
And self- induction is directly proportional with square of number of turns L = µ N2
A / L (length)
42- Inductive reactance increases when put wrought iron core inside the coil and passing the same AC current
- Because inductive reactance XL is directly proportional with self-induction coefficient L when the
frequency is constant XL= 2 f L
- And self- induction is directly proportional with permeability coefficient L = µ N2
A / L (length)
- And permeability coefficient of wrought iron is higher than Air
43- When connecting group of inductive coils in parallel, the resultant inductive reactance is lower than the
smallest one
- Because the reciprocal of the resultant inductive reactance is equal to the sum of the reciprocal of all of
them (1/XL = 1/XL1+1/XL2 +1/XL3 +…….)

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52
44- When connecting a capacitor with DC source, current flows in a short time then it vanishes
- Current passes from battery to capacitor and +ve charges are placed on one plate and negative charges
placed on the other plate so a reverse potential difference is generated on the capacitor and increases
with time and current decreases until Vcapacitor = VBattery , at this moment current stops because there is no
difference in potential between battery and capacitor
45- The capacitive reactance doesn’t cause loss in energy
- Because capacitor store electrical energy in form of electrical field
46- When an AC current of high frequency passes through a capacitor, the circuit is considered closed circuit
- Because XC= 1/ 2 f C , this means that the capacitive reactance is inversely proportional with frequency
so at high frequency the capacitive reactance is very low and current passes in a closed circuit(no
resistance)
47- When connecting a group of capacitors in parallel the capacitive reactance for the group is less than the
lowest capacitive reactance for each capacitor
Because the total capacitance of a group connected in parallel equals the summation of them
(CT = C1 + C2+ C3+ ….) so it will be higher than any of them, and the capacitive reactance is inversely
proportional with the capacitance XC= 1/ 2 f C
48- The inductive reactance of a coil passes through it a dc current equals 0
- Because DC current is unified in magnitude and direction so its frequency equals 0 and inductive
reactance XL= 2 f L so it will be XL= 0
49- It’s impossible to produce an inductive coil of resistance zero
- Because any coil made of conducting wires which should have a little value of resistance according to its
resistivity
50- If an inductive coil has ohmic resistance is connected to AC source, the total voltage leads current by angle
where 0 <  < 90
- Because current and volt are in phase in the ohmc resistance and volt leads current by angle of 900
in
the coil so the total voltage leads current by angle tan = VL / VR
51- If a capacitor is connected to an ohmic resistance and AC source of high frequency in series, the current
will lead the total voltage by angle where 0 <  < 90
- Because voltage and current are in phase in the resistance and current leads voltage by angle of 900
in
the capacitor so the current will lead the total voltage by angle  where tan = - VC / VR
52- In Oscillator circuit, the process of charging/discharging stops after an interval of time
- Because a part of electrical energy is converted gradually to thermal energy consumed in wires
according to its resistance so after time , AC current decreases and P.D. across capacitor also decreases
until it vanishes
53- To continue in charging/discharging process we should feed the capacitor with additional charges after
intervals of time
- To overcome energy loss as heat according to wires resistance
54- Ohmic resistance has a constant value regardless the value of frequency but inductive reactance and
capacitive reactance changes with frequency
- Because ohmic resistance doesn’t depend on the frequency but inductive reactance and capacitive
reactance depend on frequency according to these relations
- XC= 1/ 2 f C
- XL= 2 f L

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53
-
55- Capacitor allows the AC current to pass through its circuit
- When AC current passes, the capacitor is charged in the 1st
quarter until voltage of it is equal to VSource
then e.m.f of source decreases in the 2nd
quarter so VCapacitor > VSource so capacitor will discharge in the
source but source continue in decreasing its e.m.f until it reaches zero at the moment in which VSource
also reached zero and this process is repeated in the 3rd
and 4th
quarter but in the opposite direction
56- In resonance state current intensity is maximum
OR
In resonance state the current and total voltage are in phase
- Because the inductive reactance is equal to capacitive reactance so the total impedance Z = R so the
current is maximum value because resistance is minimum value and the voltage and current are in same
phase according to these relations
- V = Vmax sin t and  = V / R ,  = (Vmax sin t) / R
So  = max sin t
57- The average of electrical power consumed in a complete cycle of AC current in an inductive coil is 0
- Because coil stores energy in the 1st
quarter in form of magnetic field and discharges it in the 2nd
quarter and repeats this in the second half(3rd
and 4th
quarters ) so after 1 cycle the total power =0
58- The average of electrical power consumed in a complete cycle of AC current in a resistance is not 0
- Because current needs work to transfer charges in both directions and this work doesn’t depend on the
direction of the current
59- We don’t sum the voltages in RCL circuit to get the total voltage
- Because each volt has a specific direction so we deals with them as vectors
-
V = √ VR
2
+ (VL- VC)2
60- When replacing DC voltage source by an AC voltage source of the same effective e.m.f. in RL circuit, the
impedance increases
- Because an inductive reactance is generated in the coil according to its self-induction which wasn’t exist
in case of DC because frequency was zero ,
- It was Z = R and became Z = √ R2
+XL
2

Mr. Ahmed Hekal
54
AC and DC current Comparison
Hot Wire Ammeter
Used to measure the effective value of AC
Construction:
1- Thin wire of (platinum-Iridium) alloy, this wire is stretched between terminals of the device, so when it heated
up it will ex
pand
2- One end of silk thread attached to the middle of the wire, the other end rolled over a roller which is fixed on a
spring to the wall
3- A pointer is attached to roller and moves over a scale
4- The alloyed wire is connected with a shunt resistance in parallel
Direct Current DC
1- It’s a constant value current that
flows from +ve pole to –ve pole of a
battery (conventional direction)
2- Its sources are (cells, batteries and
DC generators)
3- Its e.m.f cannot be raised or
reduced
4- Measured by moving coil
Galvanometer or Ammeter
5- Used in electrolysis, electroplating
and Batteries charging
6- Transferred, but with a big loss of
energy
Alternative Current AC
1- It’s a current that changes its value
from zero to maximum each quarter
cycle and changes its direction each
half cycle
2- Its source is AC generators
3- Its e.m.f increases and decreases
using transformers
4- Measured by the Hot wire Ammeter
which measures its effective value
5- Used in most electrical devices and
lightening
6- Transferred with minimal loss of
energy

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55
Operation of Hot-Wire Ammeter:
1- It’s connected in series
2- When AC passes through the wire, temperature increases and wire will expand
3- The silk thread pulls the roller, so pointer will move on the scale
4- Pointer deflection is directly proportional with the current flow
5- When current stops the wire is cooled and roller will pull the pointer to zero
Disadvantages
- Pointer is slow in moving.
- It’s affected by the room temperature that may cause errors in readings
To overcome that we stretch the wire on a plate that has the same expanding coefficient of the wire material and
insulate it from wire
How to calibrate this device?
- Calibrating means to ensure that it’s working good and it can measure the correct effective value
- By comparing its reading with the reading of the moving coil Ammeter by connecting both in series in a dc circuit
with rheostat
Notices
-
The scale is not regular because the thermal amount generated
in the wire is directly proportional with I2
, not I
- It can measure both AC and DC currents because the thermal
effect of current doesn’t rely on the current direction

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56
Laws
Capacitor
A. Capacitive reactance
XC= 1 / 2πfC = 1 / C
B. Capacity
C = Q / V
C. Compare between 2 capacitors capacitive
reactance
XC1/ XC2= f2 C2 / f1 C1 = C 2 / C1
D. AC current intensity in a capacitor
 = VC / XC
E. Connecting Capacitors
a- In parallel
1/XC= 1/XC1+1/XC2 +1/XC3 +……
CT =C1+ C2 + C3
In case of capacitors are similar
CT = C1 *n
XC = XC1 / n
b- In series
XC= XC1+ XC2 + XC3 +……
1/CT = 1/C1 + 1/C2 + 1/C3
In case of capacitors are similar
CT = C1 /n
XC = XC1 * n
Laws
Inductive Coil
A. Inductive reactance
XL= 2 f L = L
B. Induction or (self-induction coefficient)
L = 
/ L (length)
C. Compare between 2 coils inductive
reactance
XL1/ XL2= f1 L1 / f2 L2 = L 1 / L 2
D. AC current intensity in a coil
 = VL / XL
E. Connecting inductive Coils
a- In parallel
1/XL = 1/XL1+1/XL2 +1/XL3 +……
1/LT = 1/L1+ 1/L2 + 1/L3
In case of coils are similar
L = L1 / n
XL = XL1 / n
b- In series
XL= XL1+ XL2 + XL3 +……
LT = L1+ L2 + L3
In case of coils are similar
LT = L1 * n
XL = XL1* n

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57
Laws
RL Circuit
V = √ VR
2
+ VL
2
Z = √ R2
+XL
2
Tan = VL/VR = XL/XR
 is Positive
RC Circuit
V = √ VR
2
+ VC
2
Z = √ R2
+XC
2
Tan = - VC/VR
= -XC/XR
 is Negative
Laws
RCL Circuit

V = √ VR
2
+ (VL- VC)2
Z = √ R2
+(Xl-XC)2
Tan = (VL-VC) / VR
Tan  =(XL-XC)/R
When XL> XC  is Positive
When XL< XC  is Negative
Resonance Circuit
A. Resonance circuit frequency
f = 1/2√ LC
B. To compare between two frequencies
f1 /f2 =√ (L2 C2 /L1 C1)
C. VL = VC
D. XL = XC
E. Z = R
F. I = V / R
G.  0

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58
Chapter 5
Definitions
1- Black Body
Is the body that can absorb all kinds of radiations of different wave lengths (ideal absorber) and re-emits them in
an ideal form (Ideal emitter
2- Planck’s Curve
It’s the curve that describes the relation between radiation intensity and wave length of spectrum emitted from
bodies
3- Wein’s Law:
The wavelength corresponding to maximum radiation intensity is inversely proportional with the temperature
on Kelvin scale
4- Remote sensing
It’s the technology of discovering the natural resources by imaging the earth surface using the different
spectrum regions (such as the infrared radiation)
5- Surface Potential Barrier
It’s the attraction force that used to attract electrons inside the metal and prevent it from exiting the surface
6- Thermionic emission (effect)
It’s the phenomena of emitting electrons from metal surface when heating
7- Photo-electric emission (effect)
It’s the phenomena of emitting electrons from the metal surface when a light beam of specific frequency falls on
the metal surface
8- Metal Work function
It’s the minimum energy required to free the electron from the metal surface
9- Critical (threshold) frequency
It’s the minimum frequency required to free the electron from the metal surface
10- Compton Effect:
When a high energy photon (Of X-ray or gamma-ray) collide with free electron, photon frequency will
decrease and its direction will change and electron velocity will increase and its direction will increase
11- Planck’s constant:
It’s the ratio between the photon energy and its frequency
12- Photon:
Is a quantum of energy uncharged and has a mass during its movement
13- De ’Broglie Equation:
The wave length for the wave corresponding a moving particle is equal to the ratio between the Planck’s
constant and linear momentum of the particle
What’s meant by?
1- Work function for iron equals 6.89*1014
J
The minimum energy required to free an electron from the iron metal surface is 6.89*1014
J
2- Critical wave length for a metal is 0.0002 m
It means that the highest wavelength required for the incident light to free an electron from the surface of this
metal is 0.0002 m

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59
What are conditions required to
1- Emitting an electron from metal surface
Answer:
A. Electron should gain a thermal or light energy bigger than or equal the metal work function
Or
B. The frequency of the incident light is bigger than or equal the critical frequency of the metal
2- Vision of a very small body using the electronic microscope:
Answer:
- The wave length corresponding the electron beam is smaller than the details of this body
Devices
Device Usage Scientific Idea
Remote sensing devices 1- Military applications
Like night vision devices
2- Medicine: specially in
Oncology and
embryology
3- Discovering natural
resources
Thermionic emission
Cathode Ray Tube CRT T.V. and Computers Monitors Thermionic emission
Explain: emitting electrons from
the metal surface when heating
Photo-Electric Cell Converting the light energy to
electrical energy as in
calculators and automatically
(open/close) doors
Photo electric effect
Explain: emitting electrons from
the metal surface when a light
beam with the critical frequency
falls on this surface
Electronic Microscope Enlarging the very small objects Particle-Wave Duality
Explain: it increase the speed of
electrons by increasing the
potential difference between
cathode and Anode so
,electrons will gain large Kinetic
Energy and large linear
momentum which decrease the
wave-length of the wave
corresponding the electron
beam until reaches the details
of the small object which
achieve the vision condition

Mr. Ahmed Hekal
60
Usages:
Infra-Red Radiation 1- Discovering the natural resources
2- Night Vision Devices
3- Remote Sensing
Micrometer Waves Radars
Grid in the CRT Control the electron beam intensity
The electric and magnetic fields in CRT Control the direction of the electron beams to fall
on the fluorescent screen dot by dot
Relations Diagrams
Relation between Diagram Used Law and Slope
Square of electron velocity v2
and Potential difference
between Cathode and Anode V
eV = ½ mv2
Slope = Δv2
/ΔV =2e/m
Photons Energy E and its
Frequency 
E= h
Slope=h
Kinetic Energy emitted from
metal surface K.E. and
frequency of incident light
hK.E.+ Ew
K.E.= hEw
Slope=ΔK.E/Δ
Energy E and Mass m E= mC2
Slope= ΔE/Δm= C2

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61
Force (F)Affecting by an incident
light beam on a surface and
Power of the beam Pw
F= 2 Pw/C
Slope= ΔF/ΔPw=2/c
Wave Length of wave
corresponding a moving particle
and invert of the linear
momentum (1/PL)
= h/ PL = h/mv
Slope= h
Deductions:
1- Force resulted in falling a light beam of photons on a surface
Momentum of incident beam = mc
Momentum of reflected beam=-mc
Change of momentum = mc-(-mc) =2mc = 2h/c where m= hc2
Rate of falling photons on the surface= φL
Force is the time rate of change in momentum (Newton’s Law)
Then:
F= 2hφL /c
φL=1/Δt
F=2 h/ Δt.C
Power Pw= h/ Δt
Then
F = 2 Pw/C
2- Relation between the wave length of Photon and linear momentum of it
= c/ 
= hc / h= h / (h/c)
But PL= (h/c)
= h / PL = h / mc

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62
Factors that is depends on
Physical Quantity Factors that it depends on
Wave-Length corresponding the maximum
Radiation intensity
Temperature in Kelvin
mT
/=T2/T1
Work Function of Metal The material of this metal,
Not on the intensity of light or time elapsed
facing the light or the potential difference bet.
Cathode and Anode
Generating electrical current from the photo-
electric cell
The frequency of the incident light
Not on the intensity of light or time elapsed
facing the light
Intensity of the photo-electric Current The intensity of light Provided That > c
The Wave-Length Corresponding a moving
particle
= h / PL = h / mv
1- Mass of the particle m
2- Velocity of the particle v
Comparisons
1- Sun radiation and Lamp radiation
Radiation of Sun Radiation of Lamp
- Kelvin Temperature of Sun Surface =
6000
- This makes the maximum radiation
intensity falls in the region of Visible
light where m= 500 nm
- 40% of sun radiation is visible light ,
50 % is infrared and the rest falls in
the different spectrum regions
- Kelvin Temperature of glowing lamp
= 3000
- This makes the maximum radiation
intensity falls in the region of Infrared
where m= 1000 nm
- 20% of lamp radiation is visible light
and the rest is infrared
2- Electron And Photon
Electron Photon
Nature Physical particle has a negative
charge and has a wave nature
Quantum of energy uncharged,
and has a wave and particle
nature

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63
Speed increase capability We can increase its speed by
the electrical field
We Cannot increase its speed
Speed is constant in air = 3*10
m/s
Linear Momentum Pl=mv=h/ Pl=mc= h/= h/c
Mass It has a rest mass with a
constant value
me=9.1*10-31
kg
it still constant when it stops
It has a mass only when it
moves m= h/c2
When it stops its mass will
vanished and converted to
energy E= mc2
3- Microscopic model and macroscopic model of Light
Microscopic Model Macroscopic Model
1- It’s applied when the obstacle that
collides the light photon is in size of atom
or electron
2- The interatomic spaces of surface is
larger than wavelength of incident light
3- It discusses the photon as a sphere with
radius r equal to the wave length  of
light wave that has frequency 
4- Light will penetrate the surface and deal
as a particle
1- It’s applied when the obstacle that
collides the light photon is very large (to
be larger than the wave length of the
light wave)
2- The interatomic spaces of surface is
smaller than wavelength of incident light
3- It discusses the photons as a package of
has magnetic and electric fields that are
perpendicular to each other and
perpendicular to the direction of the
wave direction
4- Light will reflects on the surface and deal
as a wave
- Electronic Microscope and Optical Microscope
Electronic Microscope Optical Microscope
Used Beam Electrons of high energy Light from light source
Used Lens Electric or magnetic lens Optical-glass lens
Limits Of usage Magnifying very small bodies of
dimensions smaller than wave
length of light(like viruses)
Magnifying small bodies but its
details are larger than the wave
length of light
Final Image Received on a fluorescent
screen or photographic plate
Received directly on eye
Magnifying Factor Very Big Limited

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64
Planck’s Interpretation for the black-Body radiation
1- Radiation is formed of units of energy called quantum which has energy E=hwhere h is Planck’s constant
and is  the frequency
2- Photons are released as a result of stimulation of the atoms of the bodies
3- The stimulated atoms energy is insulated –not continuous- and takes levels of energy where the energy
value of each level is E= n hwhere n is the number of the level
4- Atoms don’t release radiations as long as they are stable in one energy level
5- When atom transfers from a higher level of energy to a lower level it releases a photon of energy E =
hand the emitted radiation is formed of billions numbers of photons and we noticed them as a wave i.e.
we cannot notice each photon as a single unit
6- The intensity of light is depending on the energy of a single photon and number of emitted photons
7- If frequency increased the energy of each photon increases and no. of photons decrease when the total
energy is constant
Cathode Ray tube Operation:
1- Cathode is heated up by a filament, so electrons are released from the electron gun by the thermionic effect
of heat (which get rid of the surface potential barrier)
2- Potential difference is generated between this cathode and the positively charged Anode so electron beams
will directed to the fluorescent screen which is connected to the Anode
3- When these electrons collide with the screen it makes light which is different in its intensity from point to
point on the screen according to the electron beam intensity
4- Grid controls the intensity of the electron beam, and the electromagnetic fields (vertical, horizontal) controls
the direction of this beam
Einstein Interpretation for the photo-electric effect
1- If photon energy is less than the surface potential function, no electrons will emitted from the surface regardless
the intensity of the light or the time elapsed facing the light
2- If the photon energy equals the surface potential function so electron will be hardly free from the surface but it
hasn’t kinetic energy to move, hence its frequency equals the critical frequency
3- If the photon energy is larger than the surface work function, the difference in energy will gained by electron as
a kinetic energy
Photo-electric Cell Operation:
1- It formed of a metal surface called Cathode
2- When light of critical frequency falls on Cathode, some electrons are released
3- Anode Picks up these electrons which cause a current flows in the external circuit

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65
Compton Phenomena:
- When a photon of (x-ray or Gamma-ray) collide with an electron
1- Photon frequency is decreased and it scatters and changes its direction
2- Electron speed is increased and it scatters and changes its direction
- Planck put hypothesis to this phenomena
1- This light radiation consists of photons, these photons can collide electrons as billiard balls
2- The sum of momentums for photons and electrons before collision equals the sum of them after
collision
3- The sum of energies for photon and electron before collision equals sum of them after collision
4- We should deal with photons as electrons as it has mass, velocity and linear momentum
Photon Properties:
1- Photon is a quantum of energy E=hwhere h is Planck’s constant and is  the frequency
2- It has a mass m during its motion only, and a linear momentum and it move with the speed of light which is
constant regardless the value of frequency
3- When photon stops, all of its mass will be converted to energy gained by the body that stops the photon
4- It has a particle nature and a wave nature so it maintains the law of mass and energy conservation
What happens in these cases?
1- Heating-up a metal surface to a very high temperature
- Electrons will be emitted from the surface
2- Increasing the temperature of a glowing body? To the wave length corresponding the maximum intensity
- Wave-length corresponding the maximum intensity will be displaced to shortest value in the relation
mT
3- Photons falls on a surface of interatomic spaces is less than the wave length of photons
- Photons are reflected as they deals with this surface as a continuous surface and it acts with its wave nature
(reflection)
4- Photons falls on a surface of interatomic spaces is higher than the wave length of photons
- Photons will penetrate the surface and collide electrons around atoms, photons are act with its particle
nature

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66
Give reason
1- Light emitted from a glowing source is different according to difference in temperature
- As the glowing source doesn’t emits wave-lengths with the same values ,the wave length corresponding the
maximum intensity of radiation is inversely proportional with the kelvin temperature
2- The color of the radiation that result in heating the body changes from red to yellow to blue as the
temperature increased
- According to Wein’s Law the wavelength corresponding the maximum intensity is inversely proportional
with temperature so color is changed from red(which is the longest wavelength) then yellow (wavelength
less than red) then blue (very short wavelength)
3- We cannot see the radiations emitted from earth
- As earth has a small temperature then radiations have higher wavelengths which fall in the infrared region
which is invisible for the human
4- Classical physics doesn’t interpret the Planck’s curve
- As they consider Radiations as only electromagnetic waves so they think that the intensity of radiation
increases with increasing frequency , but Planck found that intensity of radiation leads to zero at very high
frequencies ( very low wavelengths)
5- Classical physics doesn’t interpret the photo-electric phenomena
- As they states that electrons will be emitted according to the intensity of light and the time elapsed facing
the light, but modern physics discovered that this emission if electrons depends on the frequency of light, it
should be higher than or equal the critical frequency
6- Electrons will be emitted from the surface of a specific metal when it receives a blue ray of light with a
lower intensity ,but it doesn’t emitted when it receives a very strong red ray
- As the blue ray has small wave length and high frequency which is higher than the critical frequency of this
metal regardless its intensity , and red is larger wavelength smaller frequency which is lower than the critical
7- Electrons may be released with a kinetic energy
- Because the energy of the incident photon is higher than the surface work function so the difference in
energy will be gained to electrons in form of kinetic energy
8- Anode in the photo electric cell is a thin wire
- To allow the incidence of light on Cathode
9- When a photon collide free electron the electron speeds up and photon frequency will decrease
- Because according to Compton phenomena the electron gained a part of photon energy so it converted to
kinetic energy ,photon will lose a part of its energy so frequency will decrease
10- Compton proved the particle nature of light
- As he proves that photon acts as electron and collide with it, and it has a mass and velocity and linear
momentum
11- When nucleus fission a huge quantity of energy is released
- Nucleus fission corresponds shortage in mass which is converted to energy E=mc2
the mass is low quantity
but the square value of light velocity= 9*1016
m2
/s2
12- Light has a dual nature (particle and wave)
- Because the photons of light has a mass in motion and a velocity and linear momentum which are particle
properties and the light beam corresponds a wave has wave-length and frequency ,it can reflect, refract
,diffract and overlap

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13- Wavelength of the wave corresponding the electron beam is decreased when the linear momentum
increases
- According to De ‘Broglie equation
= h / PL = h / mv
Wave length is inversely proportional with the linear momentum
14- Force resulting on the light beam doesn’t affect a solid wall but it affects an electron
- Force F = 2 Pw/C and because the light speed is very high then force is very low which cannot affect solid
wall but it affects electrons as it has a very small mass and increases the electron speed
15- Optical Microscope cannot be used to view viruses
- As the condition of vision stated that : the wave length of the incident beam should be lower than the body
details
- Optical microscope uses light beam which has a wavelength bet 400-700 nm which is longer than the virus
details so it‘s unable to view it
16- When increasing potential difference between anode and cathode in electronic microscope the
wavelength of wave corresponding the electron beam decreases
- When increasing the p.d. the electron energy will increase and its speed will increase and its wavelength
decrease according to De ‘Broglie equation
= h / PL = h / mv
Laws
- Wein’s Lawm1/m2 = T2/T1
- Photo-Electric Effect
E = Ew + K.E.
E = hEw= h C = h C / C , K.E. = ½ m v2
= e. V
- Energy in joule = energy in electron.volt * Electron charge
- = C/ = E/h
- E = h = m C2
= PL C
- PL= E / C = h C = h / = m C
- m = E / C2
= h C2
= h / C
- Pw= E φL = h φL
- F= 2 h/ Δt.C = 2 h φL / C = 2 Pw / C
e = 1.6 *10-19
C , h = 6.625*10-34
J.S , c =3*108
m/s , me= 9.1 * 10-31
kg
- De ‘Broglie equation
= h / PL = h / mv = h / mc

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Chapter 6
EE.Definitions
1- Spectrometer:
 A device used to obtain pure spectrum by analyzing light to its visible and invisible components
14- Emission Spectrum
It’s the emission resulting from the transfer of an excited atom from a higher energy level to lower energy level
15- Continuous Spectrum
The spectrum consisting of all wavelengths in a continuous manner
16- Line spectrum
It’s the spectrum that occurs at specified frequencies and not continuously distributed
17- Absorption spectrum
Dark lines for some wavelengths in the continuous spectrum of white light, which resulting in the absorption of
the element vapor to the spectrum lines that is distinctive to it
18- Fraunhofer Lines
It’s Absorption spectrum for elements in the outer atmosphere of sun like atoms of Helium and Hydrogen
19- X-rays
It’s invisible Electromagnetic waves of high energy, high frequencies and very short wave-lengths and it falls
between wave-lengths of gamma-rays and ultraviolet rays
FF.What are conditions required to
3- Obtain pure spectrum from Spectrometer
Answer:
 Prism should be placed in the minimum deviation position
 paralleled rays for each color should be collected in focal plane using the objective lens
4- Obtain distinctive line spectrum for the objective material
Answer:
 High potential difference should be applied between filament and objective in Coolidge tube to let
electrons gain high kinetic energy
 One accelerated electron should collide with another electron which is existing near to one of nucleuses
of the objective material

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GG. Usages:
Spectrometer 4- Analyzing light to its visible and invisible
components
5- Obtain pure spectrum
6- Estimating stars temperatures and its
gases
X-rays 1- Studying the Crystalline structure of
materials
2- Detection of structural defects in the
materials used in the metallurgical
industry
3- Bone imaging and locating fractures or
cracks
Filament in Coolidge tube 1- Electrons Source
High potential difference between
filament(Cathode) and objective in Coolidge
tube
1- Accelerating emitted electrons from
Cathode Filament
HH. Comparisons
4- Hydrogen Spectrum Series
leyman’s Balmer’s Paschen’s Bracket’s Pfund’s
Cause of emission Electron
transfer from
higher levels to
the first level
K(n=1)
Electron
transfer from
higher levels to
the second
level L ( n = 2 )
Electron
transfer from
higher levels to
the third level
M ( n = 3 )
Electron
transfer from
higher levels to
the forth level
N ( n = 4 )
Electron
transfer from
higher levels to
the fifth level
O ( n = 5 )
Region of
electromagnetic e.m.
spectrum
Ultraviolet Visible light Infra-Red Infra-Red Very Far Infra-
Red
Wavelength low wavelength high wavelength
frequency High Frequency low frequency

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Follow Comparisons
5- Continuous spectrum & Line spectrum (for X-rays)
Continuous spectrum for x-rays Line spectrum distinctive for x-rays
How energy is generated  When accelerated
electron that is emitted
from Cathode passes
near the electrons of
atoms of the objective
material its velocity and
energy will decrease as a
result of collisions and
scattering
 According to Maxwell
theory, loss in electron
energy will appear in a
form of electromagnetic
radiation that contains all
available wavelengths
because electrons lose
their energy in a
different amounts &
different time length
 When one of the
accelerated electrons that
are emitted from cathode
collides another electron
that are near to nucleus of
one of atoms for the
objective material, this
other electron gains high
energy makes it move to
higher energy level or leave
the atom and another
electron from outside atom
(which is in a higher energy
level) will take its place
 The difference in energy
between the two levels will
appear in a form of
radiation of a specific
wavelength can be specified
from this relation:
ΔE = E2 – E1 = h c/ 
h c / ΔE
Factors on which
wavelength depends on
 The least value of wave
length depends on
potential difference bet.
Filament and objective
mV
 Wavelength doesn’t
depend on objective
material
 It doesn’t depend on
potential difference bet.
Filament and objective
 Wavelength depends on
objective material
,wavelength decrease by
increasing the atomic
number
Naming Called brake –rays, soft radiation Called Intense radiation
Diagram

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II. Some Explanations
 Bhor’s Model for Hydrogen Atom (using Rutherford imaginations (
1- At the center of atom there is a positively charged nucleus
2- Negative electrons moves around nucleus in specific energy levels called shells, electrons don’t emit energy as
long as they move in its level of energy.
3- Atom is neutral as number of –ve charges (electrons) around nucleus equals number of +ve charges inside
nucleus
 And he added three Hypotheses
1- Electrical forces (Coulomb’s Law) and mechanical forces (Newton’s Law) are applied inside atom
2- Radius of electron shell can be obtained by the relation 2 𝜋r = n 
3- When electron transfers from higher energy level E2 to lower energy level E1 , a photon is released with energy
equals the difference between these two levels energy
ΔE = E2 – E1 = h 
 Bohr’s Interpretation for Linear Spectrum of Hydrogen atom
When stimulating Hydrogens atoms by gaining energy
1- They don’t stimulated with the same amount, so electrons transfer from first level K (n=1) to different
higher levels (2, 3, 4…)
2- Energy of any level En in Hydrogen atom can be calculated from relation
En= (13.6/n2
) eV
Notice: Energy in joule = energy in eV * 1.6*10-19
(charge of electrons)
3- Electrons remain in higher levels for very short time about (10-8
) and returns back to lower levels
4- When electrons return from higher levels to lower levels, it lose energy ΔE = E2 – E1 = h hc/
5- Linear Spectrum of Hydrogen atom is divided to 5 groups (5 chains)
Pfund’s
Bracket’s
Paschen’s
Balmer’s
Lyman’s

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JJ. Spectrometer
Device used to obtain pure spectrum by analyzing the white light to its visible and invisible components
Usages:
1- Obtain pure spectrum
2- Analyze white light to its visible and invisible components
3- Estimate temperature of stars and gases inside these stars
Structure:
1- White light source with a slit in a front of a convex lens
2- Turntable: with a prism place on it
3- Telescope of two lenses (objective and eye lens)
Work Operation:
1- When light beam falls on the slit the convex lens collects it to fall on the prism
2- Prism is placed in the minimum deviation position and it redirects the light beams to the telescope lenses
3- Prism will analyze white light so each color beams will be parallel to each other and not parallel to other colors
(As each color has a different deviation angle)
4- Objective lens will collect each color in a focal plane to be specified and viewed by the eye lens
KK. Coolidge tube
Structure
1- High potential difference source
2- Target of Tungsten
3- Filament used as source of electrons
4- Vacuum tube to place target and filament inside it

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How to obtain X-rays using Coolidge tube
1- When heating the filament, electrons will released towards the target under the effect of the electric field
2- These electrons will gain very high kinetic energy depending on the potential difference between Cathode
(filament) and Anode (target)
3- When electrons collide the target (Tungsten) its kinetic energy or part of it will converted to x-rays
X-rays Properties:
1- Very high ability to penetrate media
2- Very high ability to ionize gases
3- It diffracts when passing through crystals
4- It affects the accurate photographic plates
LL.What’s happened in these cases?
1- Exciting Hydrogen atoms by different amounts of energy
Answer:
- Electrons move to different higher energy levels (2,3,…) and they return back to the lower levels then it
emits energy equal to the difference between the two levels, this energy E =h is different in
wavelengths so it emits the 5 chains of spectrum for hydrogen atoms
2- Electron falls from higher energy level to lower energy level
Answer:
- it emits energy equal to the difference between the two levels E =h E2- E1
3- exciting an electron until it reaches higher level of energy
Answer:
- electron will remain for a very short time in the higher level and back again to lower level producing
energy equal to the difference between the two levels, E =h E2- E1
4- Electrons return back from higher energy levels to level M (n=3)
Answer:
- The emitted spectrum falls in the region of infrared radiation (Paschen’s)
5- Light white passing through gas or vapor and analyzing the resulting spectrum
Answer:
- Wavelengths or dark lines in continuous spectrum will appear, these wavelengths are itself the
wavelengths for the line spectrum which is distinctive to this gas
6- X-rays pass through crystals
Answer:
- Rays will diffract and then overlaps because it passes through the interatomic spaces of crystals atoms
composing dark hems and luminous hems ‫(توض‬ ‫مظلمة‬ ‫وهدب‬ ‫مضيئة‬ ‫هدب‬)‫الكريستاالت‬ ‫شكل‬ ‫ح‬ so it’s used in
studying crystalline structure

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7- X-rays pass through a gas
Answer:
- It ionizes the gas atoms as a result of high energy of x-rays
8- Electrons of high kinetic energy penetrate a target in Coolidge tube
Answer:
- Free electrons will collide another electron in the inner levels of target atoms so it’s scattered and
another electron of higher energy will replace the scattered electron generating high energy which is
X-rays
9- Replacing the target material with another material
Answer:
- Continuous spectrum will remain the same, but line spectrum will change as it depends on the material
of target
(Wave length of line spectrum will change if the material changed)
10- Replacing the target material with another one which is higher in atomic number
Answer:
- Continuous spectrum will remain the same, but line spectrum wave length will decrease and frequency
will increase so its energy will increase
11- Shedding ‫تسليط‬ low potential difference between filament and target in Coolidge tube
Answer:
- Continuous spectrum wavelength will increase and maybe the line spectrum will not appear
MM. Give reason
1- There are number of spectrum chains formed when exciting hydrogen atoms
- Because these atoms don’t be excited with the same degree, so atoms will move to different levels of
energy and return to different lower levels after a very small amount of time so it emits different
amounts of energy in different radiation regions
2- Lyman’s chain is the highest energy and pfund’s is the lowest
- In Lyman’s chain the electrons transfer from outer levels to the first level n =1 , so energy difference will
be maximum value, while in pfund’s the electrons transfer from outer levels to fifth level n = 5 so energy
difference will be minimum value
3- We can see Palmer’s series but cannot see Pfund’s
- Because Palmer’s spectrum wavelengths falls in the region of visible light but pfund’s falls in the region
of maximum Infra-Red radiation which is invisible
4- Line spectrum radiated only from material that is in state of separated atoms or in gas state under low
pressure
- Because the line spectrum results in transfer of excited atoms from high energy levels to lower energy
levels and materials cannot be excited except it was in this states (atomic states) not molecules state

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5- Appearance of dark lines when analyzing the sun spectrum, these lines called Fraunhofer lines
- As the sun spectrum is continuous and contains all available wavelengths but the outer atmosphere of
sun has some elements in gas state, these elements absorb its distinctive spectrum which appear in a
form of dark lines are the absorption spectrum of these elements
6- Using high voltage in Coolidge tube to generate X-rays
- To grant Cathode electrons high kinetic energy so after collision with target its energy converted to
radiation in form of X-rays
7- X-rays generated on Coolidge tube has a high frequency
- Because energy gained by electrons before collision with target is very high according to high potential
difference and it appears as a high frequency spectrum (with very low wavelengths)
8- Continuous spectrum appears in x-rays
- When accelerated electrons are closed to the target electrons, it loses its energy gradually on different
amounts as a result of collisions so the resulting spectrum contains continuous spectrum
9- Wave length of distinctive line spectrum of X-rays depends on the target material and not on the potential
difference between Cathode and Anode
- Because the distinctive line spectrum of X-rays results in collision of accelerated electrons with electrons
that are very near to the nucleus of target atoms which jumps to higher energy levels or leaves the atom
and another electron from other higher energy level will takes its place and the difference between the
two levels differs from material to another material so this difference appears in a form of line spectrum
of specific wavelength which is distinctive to the target material
10- X-rays has a very high ability to penetrate the atoms
- As it has a very high frequency and very low wavelength which is less than the interatomic distances of
atoms so it can penetrates
11- X-rays used in discovering the defects of materials used in Metallurgical Industries
- As it has a very high ability to penetrate the interatomic spaces of these materials as its wavelength is
very small
-
12- X-rays used in diagnosis of fractures in bones
- As x-rays penetrate bodies in different degrees so its penetration degree differs in case of fracture
bones and in case of healthy bones so it can specifies the places of fractures
13- X-rays used in studying the crystalline structure
- As it has ability to differ when collide the crystals, so it then interfere and according to the interference
waves we can specify the structure of crystals

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NN. Laws
1- Specify energy of the shell (level)
En = (- 13.6 / n2
) eV = (- 13.6 / n2
) * 1.6 *10-19
Joule
2- Radius of shells (levels)
Rn = n./ 2 = n h / 2 m v


3- ΔE = E2 – E1 = h c/  
Highest energy 
E∞ - En = hc/ = h  E∞ = 0) 
Lowest Energy
E(n+1) – En = hc/ = h 
4- X-rays emitted
E = h = hc/ 1/2 me v2
e V
5- Shortest wavelength of X-rays
=hc / E = hc / e V
6- Highest frequency of X-rays
= E / h = e V /h

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77
Chapter 7
OO. Definitions
2- Laser:
- Light Amplification by Stimulated Emission of Radiation
3- Lifetime
- The time elapsed by atom to lose its exciting energy and returns back to its original state
4- Spontaneous Emission
- It’s automatic release of radiation from excited atom when it moves from higher energy level to lower
energy level after lifetime end
5- Stimulated emission
- It occurs when an external photon stimulates excited atoms to emit new photon of energy equals the
difference between the excited level and the low level
6- inverse-square law
- the incident light intensity on a surface is inversely proportional with the square distance between that
surface and the light source
7- Optical Pumping process:
- It’s the process of exciting the active medium atoms to generate laser by light energy
8- Population Inversion State:
- It’s the state in which number of active material atoms in the higher energy levels bigger than its number in
lower energy levels
9- Meta-stable excitation level:
It is an energy level characterized by a relatively long lifetime 10-3
s
10- Hologram
- It’s a 3-D image which is formed as a result of interference of the Reference beams with the reflected beams
on the body
11- Reference Beams:
- Parallel beams used in 3-D imaging these beams has the same wave-length for the reflected beams on the
body
PP. Scientific Ideas
Laser Theory Idea: stimulated emission
Explanation:
- Atoms or molecules of medium reach
population inversion state
- Photons released from excited atoms by
the stimulated emission
- Amplifying the released beam by
stimulated emission inside the resonant
cavity

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3-D Photography (Holography) Idea: Laser
Explanation:
1- In 3-D Photography other beams with
the same wavelength of reflected
beams on body which called reference
beams meet with reflecting beams
having all information of the body at a
photographic plate
2- When photographic plate is
developed there are interference
fringes appear which is called
Hologram
3- When lighting this hologram using
Laser beam which has the same
wavelength for the reference beams
we can see a 3-D image for the body
QQ. Usages:
Radio frequencies source in Laser  Granting atoms or molecules of
active medium the required
energy for excitation
The highly frequency electric field in
(Ne-He) (Neon-Helium) Laser
 It excites Helium atoms to higher
energy levels
Helium atoms in in (Ne-He) (Neon-
Helium) Laser
 It transfers the excitation energy
to Neon atoms and this helps
reaching the population inversion
state
Neon atoms in Ne-He) (Neon-Helium)
Laser
 It’s the active medium in (Neon-
Helium) Laser as it reaches the
population inversion state this
causes the stimulated emission
to generate laser
The reflecting mirror in Laser Generating
Tube
 It reflects photons which move in
the direction of the tube axis,
during this process, some
photons collide with Neon atoms
existing in Meta-stable
excitation level and its lifetime
doesn’t end so these Neon atoms
emits coherent photons which
are doubled so this is how light is
amplified

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Reference beams  Used in the holography as it
meets the beams reflected on a
body at the photographic plate
which bears all the body
information and appear as
interference fringes
RR. Comparisons
6- Spontaneous and Stimulated emission
Spontaneous Emission Stimulated Emission
How
does it
happen?
 When excited atom transfers from
higher exciting level to lower exciting
level after the end of lifetime and
without any external effect
 When excited atoms transfers from
higher exciting level to lower exciting
level by the effect of incident photon
has the same energy of photon causing
the excitation of atoms before the end
of lifetime
Diagram
Emitted
Photons
Propertie
s
1- Emitted photons has the same
frequency of the original photon but
not the same phase or direction 
2- Emitted photons fall in large range of
frequencies in the electromagnetic
spectrum
3- Photons propagate randomly in all
directions
4- Radiation intensity decreases during
propagation as it follows the inverse-
square law
1- Two coherent photons are equal in
frequency and move in same phase and
direction
2- Emitted photons has only one
wavelength
3- Photons propagate in one direction in
form of coherent and paralleled beams
4- Intensity is constant during
propagation even for a long distance as
it doesn’t follow the inverse-square law
Examples - Ordinary light sources - Laser sources

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7- Ordinary light and laser
Ordinary light Laser
Spectral Purity 1- Emitted photons has large range of
wavelengths (it has wide spectrum range)
so when we see light white by eyes we can
see its different colors in spectrum
2- lamination intensities are different in
wavelength
1- Emitted electrons has a very small
range of wavelengths (it has
narrow spectrum range)
2- the intensity is concentrated in a
specific wavelength so it’s
considered a monochromatic light
with a single wavelength
Light beam
collimation
The light package diameter increased during
its propagation as a result of scattering
Laser beams continue parallel for long
distances and don’t scatter
Coherence Ordinary light photons are not coherent as : Laser photons are coherent as:

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81
- They are released from source
in different time
- The propagates in variable
difference in phase
- They are released from
source in the same moment
- They propagate in constant
difference in phase
Intensity Subject to ‫ل‬ ‫يخضع‬ the inverse square law, so
intensity of incident light on a surface
decreases by increasing the distance
between source and surface
Doesn’t subject to the inverse square law,
so intensity of incident laser beam
remains constant in short or long distance
as laser photons are very coherent and
propagate without scattering
8-
Ordinary photography Holography (3-D photography
Type of image 2-D image (flat) 3-D Image
Information recorded on image Photographic plate records parts
of information that are taken
from the reflected beams, this
parts is the difference in light
intensity only
Photographic plate records all
information that are taken from
the reflected beams and they
- differs in light intensity
- differs in beams path
length = path
difference * 2/
path length which results in the
difference of topology of body or
difference in phase of light waves
-
SS.Some Explanations
 Basic elements of Laser
4- Active medium: it’s the active material for producing Laser beam, it may be
A. Sold crystals: as ruby Laser
B. Semi-conductor solid material: as Silicon crystals
C. Liquid dyes: as organic dyes dissolved in water
D. Gaseous atoms: as the (Helium-Neon) gas Mixture
E. Ionized gases: as ionized Argon
F. Gaseous Molecules: as carbon dioxide gas
5- Energy sources: responsible for exciting the active medium atoms as
A. Electric energy: by (electric discharge using high direct potential difference, or using radio frequency
sources)
B. Light energy: called light pumping by ( using glowing lamps as in Sapphire Laser, or using Laser beam as in
Liquid dyes)
C. Thermal energy: using the thermal effect resulted in the pressuring gases for exciting materials that can
emit Laser
D. Chemical energy: chemical reactions produce energy that is used in producing stimulated radiation (Laser)
as used in reaction of Hydrogen and Floor or reaction of deuterium florid and carbon dioxide

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6- Resonance Cavity: which is the container that contains the active material and it’s responsible for the
amplification process, and it’s two types:
A. External resonance cavity: formed of two reflecting mirrors to hold the active material between them, this
cavity is used in Gases Laser(He, Ne) Laser
B. Internal resonance cavity: the two ends of the active material are paint to work as two reflected mirrors,
one of them is semi-transparent to allow passage of generated Laser beams, this cavity used in the Laser of
solid material like Ruby Laser
 He – Ne Laser
1- Device structure
A. Quartz tube has a mixture of Helium and Neon gases in ratio of (10:1) under low pressure of
0.6 mmHg
B. Two paralleled mirrors and normal to the tube axis on of them is reflecting (reflection
coefficient 99.5%) and the other is semi-transparent (reflection coefficient 98%)
C. High DC Voltage discharged in the gases mixture to excite the atoms
2- Operation
A. High voltage will excite the Helium atoms

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83
B. Helium Atoms will collide with the stable Neon atoms inelastic collision and according to the
near equality of their exciting levels, the exciting energy transfer from He to Ne
C. Continuous collisions between them results accumulation of neon atoms in an exciting level of
Relatively high lifetime about 10-3
s called metastable exciting level thus the condition of the
population inversion is achieved (this is the condition of Laser to take place)
D. Some neon atoms returns to lower levels of energy, then they release photons of energy equal
the difference between the two levels, these photons propagate randomly in all directions inside
the tube
E. Photos that move in the direction of the tube axis or paralleled to it collide with one of the two
mirrors and reflect internally and make several reflections between the two mirrors inside the
cavity
F. During the movement of these photons, they reflect with some Neon atoms that doesn’t finish
their lifetime in the metastable level, So Stimulated Emission takes place and each atom will
generate two photons of the same frequency, phase and direction
G. These steps repeat several times and in each time the number of stimulated photons is doubled
until we obtain an Amplified beam
H. When this radiation reaches a certain level, part of it exits the semi-transparent mirror in form
of Laser and the rest of radiation remains inside the tube to continue the process of stimulated
emission
I. Neon atoms that returned to lower level lose their energy in form of thermal energy and they
returns to the earth level and the process repeats ( they collide with Helium atoms)
J. Helium atoms that lose their energy by collision with the Neon atoms will be excited again by
the discharging DC Voltage inside the tube and so on …..
Laser Applications
1- Holography
2- Medicine: laser beams used is diagnosis, operative surgeries, far and near sightedness, and retinal detachment
3- Communication: Laser with optical fibers used in data networks
4- Manufacturing field : vaporizing the iron
5- Military field: smart bombs, Laser Radar (LADAR), missile guidance
6- Computers: CDs and Laser Printers
7- Arts and Shows
8- Surveying
9- Space Research

Mr. Ahmed Hekal
84
G. What happens if?
A. Ending the lifetime of optical excited atoms
Or the transfer of atoms from higher levels of to lower levels after the lifetime end
- Atoms return back to earth level and photon is released of the same energy of the photon causing the
excitation
B. The transfer of atoms from higher levels of to lower levels before the lifetime end
- Stimulated emission takes place and atoms transfer from higher levels to lower levels and two coherent
photons are equal in phase, energy are released (their energy equals the difference of the two levels
energy)
C. Photon of energy (E2-E1) passes through an excited atom of level E2
- Atom returns back to level E1 and two coherent photons are equal in phase, energy are released (their
energy equals the difference of the two levels energy) by stimulated emission
D. Helium atoms exist lonely in the Laser tube
- No Laser is emitted
E. Atoms of active medium reach the population inversion state
- Stimulated emission happens
F. No reflecting mirrors in the ends of the active medium
- No reflections happen for photons and no amplification will happen ,no Laser will emit
G. Hologram excited with Laser beams have the same wavelength of the reference beams
- A 3-D identical image is appeared on photographic plates

Mr. Ahmed Hekal
85
H. Give Reasons
1- Stimulated emission is happened
- when photo of energy h = E2-E1 collide with an excited atom which exists in level E2 and before the end of
lifetime, photon will stimulate the atom to emit its excitation energy in form of another photon has the
same energy, phase ,direction of the incident photon so two coherent photons are released and atom
returns to the earth level
2- although there are two photons are emitted by the effect of one photon , this is not a violation of the
energy conservation law
- because one of them is the incident photon and the other results in the transfer of atom from higher level to
lower level
3- Spectral purity of the Laser beam
- As Laser beams has the same wavelength
4- Laser beams can transfer long distances without any energy loss
- As Laser beams are released in a paralleled package and they are coherent so they doesn’t scatter
5- Laser is not subject to the inverse square law
- As Laser beams are paralleled and coherent so its intensity doesn’t change inversely with the square
distance between the source and the body
6- Existence of two reflecting mirrors, one of them is semi-transparent at the ends of the (He-Ne) Laser
tube
- To make series of repeated reflections for photons resulting from the stimulated emission process so these
photons collide with some Neon atoms which are exist in the metastable excitation level, photons will
stimulate these Neon atoms to release new photons , so number of photons are doubled with each
reflection and radiation is amplified until a certain level then it’s emitted through the semi-transparent
mirror
7- In Laser sources the active medium should derived to the population inversion state, but in ordinary
light sources this condition is not required
- As the scientific idea of Laser depends on existence of large number of atoms in the meta-stable level to
achieve the stimulated emission
8- The mixture of Helium and Neon gases is suitable to generate Laser
- Due to the near equality of their meta-stable excitation levels
9- We can obtain 3-D image only by using Laser
- To obtain 3-D image we should use coherent photons to indicate the difference in (illumination intensity and
phase) for the interference fringes of these photons and this is achieved only in Laser
10- Laser is used in the retina detachment cases
- The thermal effect of the paralleled Laser beams can be used in the retina cells fusion
11- Laser used as a missile guidance in military fields
- As Laser beams are paralleled and its intensity doesn’t changed by increasing distance so it’s suitable to
guide the rockets for very long distances

Chapter 1-7 شرح مبسط. مراجعة اللغات فى مادة الفيزياء 3 ثانوى لغات وتجريبى revision

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