electricity (1).ppt

In unit 2 we will learn about
the physics of electricity and
electronics.
This includes circuits, Ohm’s
law, resistance, electrical
energy and power,
electromagnetism and
electronic components.

Key words: electrons, conductors,
insulators, charge, current
By the end of this lesson you will be able
to:
State that electrons are free to move in a
conductor
Describe the electrical current in terms of
movement of charges around a circuit
Distinguish between conductors and insulators
and give examples of each.
Carry out calculations involving Q = It

What is inside an atom?
Thomson’s Plum pudding model
Rutherford model
Rutherford Bohr model
Charge cloud model
Quantum model of the nucleus

The atom
An atom is a fundamental unit of matter
made up of
protons (with a positive charge)
neutrons (neutral – no charge)
electrons (with a negative charge)

What is electricity?
Everything is made of atoms which contain
POSITIVE particles called PROTONS and
NEGATIVE particles called ELECTRONS.
Proton (+)
Neutron
Electron (-)

An atom will usually have the same number of
positives and negatives
This makes the atom NEUTRAL.
Proton (+)
Neutron
Electron (-)

Electrical Charge
Electric charge is given the symbol
Q
Electrons are the charge carriers
that flow in an electrical circuit –
from the negative to positive
terminals.

Electrical Charge
Charge is measured in
Coulombs
which is given the symbol
C

Electrical Charge
The charge on a proton is
1.6 x 10-19C
which is the same size as the charge on an
electron.

Electrons have a negative charge
(Q) measured in coulombs (C).
Electrons move round a circuit from
negative to positive (remember like
charges repel, opposites attract)
giving rise to an electric current.

Conductors & Insulators
What makes something a good conductor?
Good conductors allow electrons to move
through them easily. Insulators do not
allow electrons to move easily.

So electricity is…
movement of charge round a
circuit.
We call this electric current.

Charge, Current & Time
Electric current is given the symbol
I
Electric current is the movement of
negative charges (electrons) in a
circuit

Current is the amount of charge flowing
per second and is given the unit
Amps (A)

If current is charge flowing per second then
t
Q
I 
time in seconds (s)
Current in Amps (A)
Charge transferred
in coulombs (C)
so a current of 1 A is 1 C of charge transferred
in 1 s.

This can be rearranged as
or
It
Q 
I
Q
t 

Key words: series, current, ammeter, voltmeter,
battery, resistor, variable resistor, fuse, switch, lamp,
voltage
to:
Draw circuit diagrams to show the correct positions of
an ammeter in a series circuit.
Draw and identify the circuit symbols for an
ammeter, voltmeter, battery, resistor, variable
resistor, fuse, switch and lamp.
State that in a series circuit, the current is the same at
all positions.

Different types of circuit
There are different ways in which you can
connect cells and components (such as
lamps) to create a circuit:
series
parallel
a mixture of both

Series Circuit
A series circuit has only one electrical
path.
You can trace from one side of the
battery to the other, through each
component, without lifting your finger
from the page.

Series Circuit
A series circuit has only one electrical
path.
You can trace from one side of the
battery to the other, through each
component, without lifting your finger
from the page.
Physics Animations – Series Circuits

Name that component
Voltmeter Lamp
Fuse
Switch
Cell
Battery
Variable resistor
Resistor Ammeter
On the back of p2 carefully draw each symbol and
label – in pencil!

Build a series circuit
On the worksheet you will find four
building circuit activities.
Follow the instructions carefully!
Answer each question as you go.
Make careful observations.
Lesson 2 build a series circuit.pub

Build a series circuit which contains a
6V battery pack, three 3.5 V lamps in
lamp holders, and a meter used for
measuring current.
What is the meter called?
Where is it positioned in the circuit?

Activity 2
Bulbs are much dimmer!

Activity 3 - Change your circuit…
Move your ammeter to different positions
in the series circuit.
Make a note of the positions each time,
and of the current at each position.
What can you say about the current in a
series circuit?

Successful Circuit Diagrams
On your worksheet you have drawn a circuit
diagram.
To be successful at circuit diagrams:
• use a ruler and pencil
• draw components carefully
• draw wires as straight lines (with corners as
• right angles!)
• make sure all components are correctly draw
• and joined in the circuit.

Your circuit diagram…
should look like this:

Notice in this circuit, current is the same at all points

Series Circuits and Current
We are measuring the current I in a series circuit.
What have we observed?
We find that the current is the same at
all points.
How can this be written mathematically?
I1 = I2 = I3 = I4 and so on
Virtual Int 2 Physics – Electricity & Electronics – Circuits – Series Circuits

Think…
How could you make use of a series circuit
to investigate which materials are
conductors and which materials are
insulators?
Which components would you need?
What would you observe?

…and learn
components and names
formulae and symbols
what is a series circuit?
current in series circuit
drawing a series circuit diagram

Key words: series, parallel, ammeter, current,
to:
Draw circuit diagrams to show the correct positions of an
ammeter in a parallel circuit.
Draw and identify the circuit symbols for an ammeter, and
lamp.
State that in a series circuit, the current is the same at
all positions.
State that in a parallel circuit, the sum of the current in
the branches adds up to the current drawn from the
supply.

Quick Quiz
What is a series circuit?
What is the symbol for current?
What are the units of current?
What is the relationship between current and
time?
What do we know about the current in a series
circuit?
How do we measure current?
Draw the symbol for this.
Describe how to measure current in a series
circuit.

Build another circuit
Build a series circuit which includes a 6V
battery, a 6V lamp and an ammeter.
Draw the circuit diagram for your circuit:

We will now take one of your series
circuits, and “add it” to someone else’s.
Another ammeter has been added.
What do you notice about the readings on
the ammeter?

We will now “add” another series circuit.
What do you notice about the readings on
the ammeter?

What sort of circuit is this?
We have constructed a parallel circuit.
What does the circuit diagram look like?
Try drawing it on Crocodile Physics.

Draw the circuit diagram below

Parallel Circuit
We have constructed a parallel circuit.
This is a circuit with different branches.
When it reaches a junction, the current
can divide and take different branches.

Parallel Circuits and Current
We are measuring the current I in a
parallel circuit.
We find that the current in each of the
branches adds up to the total current.
IT = I1 + I2 + I3 and so on

Electric Circuits
How many ways can you make
two light bulbs work?

A SIMPLE CIRCUIT
CELL
LIGHT BULB
SWITCH

A SIMPLE CIRCUIT
2A 2A
2A
2A 2A
2A

1A
1A
A Series Circuit
1A
1A
1A
1A
1A

4A
4A 2A
2A
2A
2A
2A
2A
4A
4A
4A
4A
A Parallel
Circuit

Key words: voltage, potential difference,
voltmeter, series, parallel
to:
Draw and identify the circuit symbols for a
voltmeter, battery, and lamp
State that the voltage of a supply is a measure
of the energy given to the charges in a circuit.
Draw circuit diagrams to show the correct positions of a
voltmeter in a circuit.
State that the sum of potential differences across the
components in series is equal to the voltage of the
supply.
State that the potential difference across components
in parallel is the same for each component.

What is a voltage?
What is a volt?
Discussion
Demonstration
Voltage in series and parallel

What is the energy change which
takes place in a battery?
Chemical to Electrical

When a battery is in a circuit…
The electrical energy is carried by the
electrons that move round the circuit.
It is converted into others forms of
energy.

If there is a bulb in the circuit, it is
converted from
to
http://www.members.shaw.ca/len92/current_animation.gif

The amount of electrical energy the
electrons have at any point in a circuit is
known as their “potential”.
As they move the electrons transfer energy
into other forms.
This means at any two points the electron has
different amounts of energy.

Electrons start with (for example) 6J of energy. They have “potential”.
As they pass through
the bulb, some of the
energy is converted
to light.
Electrons which have
passed through the
bulb have less
energy. Or less
“potential”.
There is a “potential” difference
in the circuit

What has “potential difference”
got to do with voltage?
It is the same thing!
The potential difference (p.d.), or voltage,
of a battery is a measure of the electrical
energy given to one coulomb of charge
passing through the battery.

Potential Difference or Voltage (V)
A 9 V battery will give how much energy
to each coulomb of charge passing
through the battery?
9 J

A 1.5 V battery will give how much energy
to each coulomb of charge passing
through the battery?
1.5 J

A battery with a p.d. of 6V will give how
much energy to each coulomb of charge
passing through the battery?
6 J

Voltage or p.d.
Voltage (or p.d.) is measured in
volts
and is given the symbol
V

Summary of Units
Quantity Symbol Units Symbol
charge Q coulombs C
time t seconds s
current I amperes A
voltage V volts V

How can we measure voltage?
Voltage (or p.d.) can be measured using a
voltmeter.
An ammeter is connected in the circuit
but a voltmeter must be connected across
the component.
V

You can’t measure voltage…
in a circuit
through a circuit
through a component
flowing

Build a series circuit which contains a
6V battery, two 6V lamps, and a meter
used for measuring potential difference
across each lamp.
What is the meter called?
Where is it positioned in the circuit?

Drawing a circuit diagram
Now draw a circuit diagram of the series
circuit which you built.
Remember to use a ruler and pencil, draw
components carefully, draw wires as
straight lines (with corners as right
angles!), and make sure all components are
correctly draw and joined in the circuit.

Series Circuits and Voltage
We are measuring the potential difference (V) in a series circuit.
We find that the

Parallel Circuit
Now use the same components to
construct a parallel circuit.
This is a circuit with different branches.

Parallel Circuits and Voltage
We are measuring the potential
differences in a parallel circuit.

Tasks & Homework
Yellow Practice Questions: 2.10, 2.11
Numerical Questions: p33-36 qu 5-14
Complete for homework for Tuesday 27th
November

Quick Quiz
What have we learned?

Key words: electrical resistance, voltage,
current, Ohm’s law, ohms, resistor,
variable power supply
By the end of this lesson you will be able to:
State that V/I for a resistor remains
approximately constant for different currents.
State that an increase in resistance of a circuit
leads to a decrease in the current in that
circuit.
draw the symbol or a variable power supply and
resistor.

Key words: electrical resistance, voltage,
current, Ohm’s law, ohms, resistor,
variable power supply
By the end of this lesson you will have
practised:
building a series circuit
using an ammeter and a voltmeter to find
current and voltage.
graphing results

The symbol for a resistor is
Resistors

Resistors
Resistors oppose (or resist) the
flow of electric current. They have a
property called resistance (R) which
is measured in ohms (Ω).

What is the relationship between
current and voltage in a resistor?
Current is measured using an ammeter.
Voltage is measured using a voltmeter.
Investigation: relationship between
current and voltage in a resistor.

Relationship between current and
voltage in a resistor
I / Amps
p.d. /
Volts
Straight line through
the origin tells us that
current is
directly proportional to
voltage
The ratio V/I is constant
and is equal to resistance
in the circuit.

R
I
V

I
V
R
resistance
is
constant
The
constant
ely
approximat
is

I
V
R  Ohm’s Law
IR
V 

Resistors
A
lamp
What do you expect
to happen to the current
if you increase the
value of the resistor in
the circuit shown?
cell
resistor Demonstration

Calculate
For a voltage of 12V, calculate the
current for a resistant of
(i) 1 Ω
(ii) 2 Ω
(iii)4 Ω
(iv)24 Ω
(v)1 k Ω

What can you say about current and
resistance for a fixed voltage? Complete
the sentences.
As resistance increases, the current
As resistance decreases, the current

Varying Resistance
The opposition to current or resistance
of a material (measured in Ω) depends
on several things.
Think and discuss what some of these
might be.

Varying Resistance
The opposition to current or resistance of
a material (measured in Ω) depends on
- type of material (the better the conductor,
the lower the resistance)
- length of material (the longer the material,
the higher the resistance)
- thickness of material (the thinner the
material, the higher the resistance)
- temperature of material (the higher the
temperature, the higher the resistance)

Varying Resistance
The relationship between length of the
material and resistance allows us to make
a
variable resistor (or rheostat).

Variable Resistor
Outgoing
current
Incoming
current
A B
Demonstration

Variable Resistors
In the above diagram, if the
slider is moved in the direction
A→B the resistance will
increase because the length of
wire through which the current
passes increases.

Uses of Variable Resistors?
Variable resistors can be used
• as volume or brightness controls on
televisions
• volume control on MP3 players
• light dimmer switches.

Key words: resistance, series, parallel,
ohms, ohmmeter
to:
State the relationships between total
resistance and individual resistances in
series and parallel circuits
Carry out calculations involving the
relationships between resistors in series
and in parallel

Key words: resistance, series, parallel,
ohms, ohmmeter
By the end of this lesson you will have
practised:
building a series circuit
building a parallel circuit
drawing circuit diagrams
using an ohmmeter to measure resistance
in a circuit

Variation of Resistance and
Current for a Lamp Filament
Look at the circuit diagram below:
Handout

Name each of the components
Is this a series or parallel circuit?
As the voltage across the lamp increases, what do
you expect to happen to the current?
Sketch a graph of your prediction of the
relationship between current and voltage.

In the resistor, current and voltage are
directly proportional.
But in a filament lamp, heat is generated.
We know that resistance increases as
temperature increases. So we see that as
voltage increases, temperature increases,
resistance increases and current
increases – but more slowly than we might
predict.

Measuring Resistance
We can find the resistance of a
component by measuring
voltage across the component using
a voltmeter
current through the component using
an ammeter

Measuring Resistance
or we can measure it directly using an
ohmmeter
Ω
Demonstration & experiment

Series and Parallel Circuits
Voltage, Current and Resistance
Vs
I3
I2
I1
R3
+
-
R2
R1
V1 V2 V3
What type of circuit is this?

Vs
I3
I2
I1
R3
+
-
R2
R1
V1 V2 V3
One electrical path from negative
to positive therefore series.

Vs
I3
I2
I1
R3
+
-
R2
R1
V1 V2 V3
What is the relationship between the three currents?
3
2
1 I
I
I 

The current is the same at each point.

Vs
I3
I2
I1
R3
+
-
R2
R1
V1 V2 V3
What is the relationship between the four voltages?
3
2
1 V
V
V
Vs 


They add to equal the supply voltage.

Disadvantages of Series Circuits?
When one component fails the whole circuit
fails.
The current is the same at all points and the
voltage is divided between the bulbs. The
more bulbs added the dimmer each one is.

Vs
I3
I2
I1
R3
+
-
R2
R1
V1 V2 V3
How do you find total resistance in series?
3
2
1 R
R
R
Rtotal 


Add each resistance together.

IT
IT
Vs
I3
I2
I1
R3
+
-
R2
R1
V1
V2
V3

More than one electrical path – components connected on different
branches therefore parallel.
IT
IT
Vs
I3
I2
I1
R3
+
-
R2
R1
V1
V2
V3

What is the
relationship
between the
four
currents?
3
2
1 I
I
I
IT 


The four currents
add to give the
total current.
IT
IT
Vs
I3
I2
I1
R3
+
-
R
2
R1
V1
V2
V3

What is the
relationship
between the
four
voltages?
3
2
1 V
V
V
VS 


Each voltage is
equal to the
supply voltage.
IT
IT
Vs
I3
I2
I1
R3
+
-
R
2
R1
V1
V2
V3

3
2
1
1
1
1
1
R
R
R
RT



IT
IT
Vs
I3
I2
I1
R3
+
-
R
2
R1
V1
V2
V3
The
resistance
in parallel?

If more resistors are connected in
parallel the total resistance will
always
decrease
This is because there are more
branches through which the
electricity can flow.

Advantages of the Parallel
Circuit?
When one bulb fails the rest of the circuit
continues to work.
The more components, the lower the
resistance. The total current drawn
increases. Voltage in each branch is the same as
the supply voltage therefore bulbs in parallel
will each be as bright as a single bulb.What have
you learned today?

Key words: resistor, resistance, series,
potential, potential divider
to:
State that a potential divider circuit
consists of a number of resistors, or a
variable resistor, connected across a
power supply.
Carry out calculations involving potential
differences and resistance in a potential
divider.
Handout 3

Name each component.
V V

The supply voltage is 6V. What is voltage V1? V2?
V1 V2
10Ω 10Ω

V1 V2
10Ω 10Ω

V1 V2
5Ω 10Ω

A series circuit with two resistor and a power
supply is known as a potential divider.
V1 V2
Why is it
called a
potential
divider?

The potential difference of the supply is
divided between the two resistors.
When the two resistors are identical (i.e.
have the same value of resistance), the
potential difference is split equally.

Investigating Potential Dividers

Potential Divider Circuits
A voltage divider consists of two devices,
usually resistors, connected in series.
V1
V2
R1=100 Ω
6V
R2=100 Ω
V1
V2
R1=4.5 kΩ
6V
R2=9 kΩ

The current in each resistor is calculated
using Ohm’s Law:
R
V
I 

What can we say about the current in a
series circuit?
It stays the same throughout the circuit.
I2 =
V2
R2
I1 = =
V1
R1

In a voltage divider circuit
I2 =
V2
R2
I1 = =
V1
R1
V2
R2
=

This can also be written
V2
R2
V1
R1
=

If the resistance of one resistor
is increased, the voltage across this
resistor will
This means the other voltage must

Potential Dividers
S
T
S
T
S
S
V
R
R
V
V
R
R
V
V
R
R
R
V
V
R
R
R
V
2
2
1
1
2
1
2
2
2
1
1
1






V1 is the voltage across resistor R1
V2 is the voltage across resistor R2
VS is the supply voltage
RT is the total resistance
What do the symbols mean?

Potential Dividers
S
T
S
T
S
S
V
R
R
V
V
R
R
V
V
R
R
R
V
V
R
R
R
V
2
2
1
1
2
1
2
2
2
1
1
1





 Look again at the worksheet.
Use the formula to calculate
V1 and V2 for each circuit.
The answers found using the
formula match the values
measured using the
voltmeter.

Potentiometer
The potentiometer is a special type of
voltage divider.
It is a variable resistor with a sliding
contact.

What range of output is it possible to
obtain from a potentiometer?
Range of output voltages 0V to supply
voltage.

Key words: electrical energy, power,
voltage, current, resistance
to:
State that when there is an electrical current
in a component there is an energy
transformation and give some examples.
State the relationship between energy and
power.
Carry out calculations using E = Pt
State that in a lamp electrical energy is
transformed into heat and light.
State that the energy transformation in an
electrical heater occurs in the resistance wire.

What is a voltage?
What is a volt?

What is “potential difference” ?
What is voltage?
It is the same thing!
The potential difference (p.d.), or voltage,
of a battery is a measure of the electrical
energy given to one coulomb of charge
passing through the battery.

If there is a bulb in the circuit, it is
converted from
to
Virtual Int 2 Physics -> Electricity ->Electrical Energy & Power ->Energy Transformation in a Lamp

Filament lamps
Filament of
tungsten wire
Glass
How does it work?

Filament Lamp
Tungsten (metal) filament becomes
so hot it glows.
Why isn’t oxygen used
inside the bulb?

Filament lamps
Electric current
passes through the
resistance wire which
is made of tungsten.
Electrical energy is
changed into heat
energy and the
wire glows white hot.
Filament lamps
produce both heat and
light.

In an electric fire, energy is converted
from
to

Resistance in a wire
We have learned that when a
voltage is applied across a
lamp, the resistance
increases.
What happens to the
temperature?

Resistance in a wire
As current passes through a resistance wire,
the wire gets hot.
This is how electric fires and filament lights
work.
The filament becomes hot enough to glow and
emit light. The bar of the electric fire is a
length of wire which also glows when hot.

Electrical appliances change
electrical energy into other
forms.
What are the energy changes
taking place in these appliances?

Power and Energy
Electrical energy has the symbol
and is measured in

Power
The power rating of an appliance or a
component is defined as
the amount of energy used by the
component / appliance in one
second

Power
The power rating tells us the rate at
which energy is transformed, that is the
energy transformed each second.

Power
For example, an appliance with a power
rating of 250 W converts 250 Joules of
electrical energy into another form each
second.

Power
How can this be written as a formula?
Power in Watts (W)
Energy in Joules (J)
time in seconds (s)
t
E
P 
Demonstration / experiment

Connect the joule meter to the voltage supply and a ray box bulb to the
joule meter.
Set the supply voltage at 6V and switch on. You’ll see the counter on the
joule meter increasing (note each time the counter increases by 1, this is
100J of energy).
Record the number of joules used in 50s and 100s. Calculate the number of
joules used per second.
Power is energy used per second, in watts. Write the formula:
If the supply voltage was increased to 12V, what would you expect to
happen?
Increase supply voltage to 12V and repeat the experiment.
Investigating Energy and Power
Worksheet / experiment

Power and Energy
Ray box bulb, 6V supply Ray box bulb, 12V supply
Number of joules used in 50 s? Number of joules used in 50
s?
Number of joules used in 100 s? Number of joules used in 100
s?
Number of joules used each
second?
Number of joules used each
second?
Power (W) Power (W)
Were your results as expected?
1 watt is equivalent to the transfer of 1 joule per second.

Power & Energy Example
If an electric fire uses 1.8
MJ of energy in a time of
10 minutes, calculate the
power output of the fire.

Power & Energy Example
P = ?
E = 1.8 MJ = 1.8x106 J
t=10 minutes = 600 s

Power Ratings of Appliances
Different appliances have different
power ratings.
What is meant by power?

Watt’s my power rating?
500 W, 150 W, 1200 W,
100 W, 3000 W, 300 W,
800 W, 1500 W, 30 W, 60 W,
11 W

Watt’s my power rating?
11 W
60 W,
3000 W
1200 W
30 W
800 W
150 W
1500 W
500 W
100 W
300 W

Key words: electrical energy, power,
voltage, current, resistance
to:
State that the electrical energy
transformed each second = VI
Carry out calculations using P=IV and E=Pt
Explain the equivalence between VI, I2R
and V2/R.
Carry out calculations involving the
relationships between power, current,
voltage and resistance.

Current through Appliances
Different appliances have different
power ratings.
P = IV
For appliances which use the mains supply
V =

Current through Appliances
As power increases for a fixed voltage,
what happens to the current?
As power increases the current
increases

Red flag
indicates
9V.
Live Neutral

Live Neutral
Even with
the switch
open and
zero current
the lamp is
still at 9V.

Live Neutral
This time,
when the
switch is
open, the
lamp is at
0V and is
safe to
touch.

The red flags indicate that
voltage at these points is 9V.

Closing the third switch results in a
current greater than 1A, blowing the
fuse.

Inserting a voltmeter across a bulb shows that
the bulbs are at zero volts.
If you touch them, you won’t receive an electric
shock as they are isolated from the voltage
supply.

The red flags indicate that
voltage at these points is 9V.
The fuse is now in the neutral
wire.

Closing the third switch results in a current
greater than 1A, blowing the fuse.
The red flags show that at these points the
voltage is still at 9V.
If you touch this now, you’ll complete the circuit
and receive an electric shock – you become the
“neutral wire” and allow electricity to flow
through you.

Why can a bird
sit safely on this
high voltage
power line?
What will
happen if the
bird spreads its
wings and
touches the
pylon?

Which fuse to use?
How would you calculate which fuse is
required for an appliance?
An appliance operating from the mains
supply has a supply voltage of 230V.
The rating plate gives you information
on the power of the appliance.

The formula which links voltage, power
and current:
P = VI

The general rule for fuses
If the appliance
has a power rating
of:
Fuse value should
be:
Less than 700W 3A
More than 700W 13A
The fuse value needs to be just above the
normal operating current

Example
What is the appropriate choice of fuse for a
mains appliance with a power rating of
330 W?
?



I
W
P
V
V
330
230
V
P
I
IV
P



A
I
I
44
1
230
330
.


Example
What is the appropriate choice of fuse for a
mains appliance with a power rating of
330 W?
?



I
W
P
V
V
330
230

Which type of appliances tend to have the
highest power ratings?
Generally, appliances which produce heat.

Which type of appliances draw the
highest current?

Which type of appliances need the largest
value of fuse?

Investigating…
power, voltage, current and
resistance.
What do you notice about
R
V
R
I
IV
2
2
,
,
Worksheet / experiment

Power can be calculated from the
voltage across the appliance and the
current flowing through it. Written as
an equation:
P = IV

Relationship between power,
current, voltage and resistance
Our experiments showed that
R
V
R
I
IV
2
2



Relationship between power,
current, voltage and resistance
R
V
R
I
IV
P
2
2




Equations for Power
R
I
P
IxRxI
P
ng
Substituti
IR
V
VI
P
2
and





Equations for Power
R
V
P
R
V
Vx
P
ng
Substituti
R
V
I
IR
V
VI
P
2
and






Key words: alternating current, direct
current, mains supply, frequency
to:
Explain in terms of current the terms a.c. and d.c.
State that the frequency of the mains supply is 50Hz.
State that the quoted value of an alternating voltage is
less than its peak value.
State that a d.c. supply and an a.c. supply of the
same quoted value will supply the same power to
a given resistor.

Direct Current (d.c.)
• The voltage drives a steady
or direct current.
• The electrons move in one
direction.
• The current (or voltage) does
not change with time.

Alternating Current (AC)
•An alternating
current is continually
changing direction
•The alternating
voltage and current
has a distinctive
waveform

Alternating Current
Using the oscilloscope, we can measure
the peak voltage of the a.c. supply.
The declared, quoted or
“effective”, voltage is always less
than the peak voltage.

Calculating Declared Voltage
The declared (or effective) voltage can be
calculated from the peak voltage.
The quoted voltage is ~ 0.7 x
peak voltage.
The declared voltage is the value of a.c.
voltage which gives the same heating or
lighting effect as d.c. voltage.

Mains Supply
What is the frequency of the mains
supply?
50 Hz

Mains Supply
What is meant by the frequency of the
supply?
Alternating current flows one way then
the other. It is continually changing
direction. The rate of the changing
direction is called the frequency and it is
measured in Hertz (Hz) which is the
number of forward-backward cycles in
one second.

Mains Supply
Why does the current change
direction?
Voltage pushes the current. The voltage
changes polarity causing the current to
change direction.

Mains Supply
What is the declared value of the mains
supply voltage?
What is meant by the voltage of the
supply?
230V
The voltage of a power supply or battery
is a measure of how much “push” it can
provide and how much energy it can give
to the electrical charge.

Measuring effective voltage /
current in an a.c. circuit
The effective voltage or current in an a.c.
circuit can be measured using a.c.
voltmeter or ammeter.

Measuring peak a.c. voltage using
an oscilloscope
1. Adjust the position so the trace is
central on the screen.
2. Adjust the volts/div so the trace fills
the screen.
3. Count the number of boxes from the
axis to the peak.
4. Multiply the number of boxes by the
volts / div.

Key words: electromagnetism, induced
voltage, field strength, turns.
State that a magnetic field exists around
a current carrying wire.
Identify circumstances in which a voltage
will be induced in a conductor.
State the factors which affect the size
of the induced voltage i.e. field strength,
number of turns on a coil, relative
movement.

Permanent Magnets
A magnetic field is the region around a
magnet in which a magnetic force can be
detected.

Magnetic Field Around a Current
Carry Wire
What happens when the direction of the
current is reversed?
The direction of the magnetic field is
reversed.

Electromagnets
When an electric current passes through a
wire which is coiled around an iron core, the
core becomes magnetised and an
electromagnet is produced.
When an a.c. current is used, the current
changed direction and so the magnetic field
changes direction.
e-m demo

Electromagnets
Strength of electromagnet with/without iron
core?
Effect of increasing current through the coil?
Effect of increasing number of turns in the
coil (while keeping current constant)?

How is an electromagnet
constructed?
A current through a wire can be used to
create an electromagnet.
http://micro.magnet.fsu.edu/electromag/java/compass/index.html

How is an electromagnet
constructed?
A conducting wire is wound round an iron core.
When a current passes through the
conductor there is a magnetic field around
the conductor. By wrapping it round a soft
iron core, the magnetic field is concentrated.

How can the strength of an
electromagnet be increased?
By increasing the current through the
coil.
By increasing the number of turns on the
coil of wire.

What are the advantages of an
electromagnet over a permanent
magnet?
The electromagnet can be switched off.
The magnetic field strength can be varied
(how?)
The electromagnet provides a much stronger
magnet field for the same size than a
permanent magnet.

Electromagnetic Induction
What happens when a wire is moved in a
magnetic field?
A voltage is created – or induced. For this
reason we call this electromagnetic
induction.

Electromagnetic Induction
http://micro.magnet.fsu.edu/electromag/java/faraday2/
What happens when a permanent magnet
is moved towards or away from a coil of
wire?

What do we know so far?
When a current passes through a coil of
wire, there is a magnetic field around the
wire.
Changing direction of the current changes
the direction of the magnetic field.

What do we know so far?
When we move a wire in a magnetic field,
voltage is induced.
When we move a magnet in a coil of wire,
a voltage is induced.
What do we have in common? Changing
magnetic field leading to electricity!

I
FARADAY’S EXPERIMENT 1832
A
B

I
FARADAY’S EXPERIMENT 1832
A current in B is
only present when
the current in A is
changing.
A
B

I HAVE DISCOVERED
ELECTROMAGNETIC INDUCTION

Now I understand!
VOLTAGE IS ONLY
INDUCED WHEN
THERE IS RELATIVE
MOTION BETWEEN A
CONDUCTOR AND A
MAGNETIC FIELD

THE INDUCED VOLTAGE IS
DIRECTLY PROPORTIONAL TO
THE RATE OF CHANGE OF MAGNETIC
FIELD

What is observed when…
the magnet is stationary next to the coil?
Nothing! No voltage is induced.
The magnet is moved in the opposite
direction (towards the coil instead of
away from it)?
The voltage produced has opposite
polarity.

What is observed when…
the magnet is moved backwards and
forwards?
Voltage induced which has a changing
polarity.
What does this mean for the current?
The current will change direction – it is
a.c.!

Generating Electricity
A voltage can be induced in a coil of wire
if a magnet is moved towards (or away
from the coil).
This effect is known as induction.
What does the induced voltage depend
on?

Induced voltage depends on:
strength of the magnetic field (the stronger
the greater the induced voltage)
speed of movement (the faster the greater the
induced voltage).
number of turns in the coil (the more turns of
wire on the coil the greater the induced
voltage).
Virtual Int 2 Physics – Electricity & Electronics – em induction

To summarise:
A voltage is induced across the ends of a wire
coil is the coil experiences a changing magnetic
field.

How do we “create” electricity?

A Simple Generator
A current can be passed through a wire to
result in movement (a motor!).
Electrical energy was changed to kinetic
energy.

A Simple Generator
The motor can work “in reverse”.
Kinetic energy can be used to create
electricity in a dynamo or simple
generator.

Transformers
What is a transformer?
Demonstration.

Transformers
A transformer consists of two separate coils of
wire wound on the same iron core.
The first coil, the primary, is connected to an a.c.
voltage supply. There is therefore a changing
magnetic field around the core.
This changing field induces a voltage across the
other coil, the secondary. A current flows as a
result of the induced voltage.

Transformer Terms
We talk about
Primary coil (the first one – connected to
a.c. voltage)
Secondary coil (the second one – voltage
is induced)
Number of turns – number of “loops” of
wire in coil

Transformer Terms
We talk about
Np – the number of turns on the primary coil
Ns – the number of turns on the secondary coil
Vp – the voltage applied to the primary coil
Vs – the voltage induced across secondary coil
Ip – the current in the primary coil
Is – the current in the secondary coil

PRIMARY Np Turns
AC input VP Volts
SECONDARY NS Turns
AC output VS Volts
THE TRANSFORMER
Laminated soft
iron core
P
S
P
S
N
N
V
V


Equipment
2 coils
1 x a.c. voltmeter
Four wires
A variable power supply.
Set your power supply to 2V. YOU
MUST NOT EXCEED 2V as the
primary voltage.

Measure the output voltage for a 2V input
for each of the combinations of number
of turns in the primary and secondary.
Record your results in your table.

Calculate and
and record your results in your table.
p
S
V
V
p
S
N
N

Investigating Transformers
2V 125 125
2V 125 500
2V 125 625
2V 500 125
2V 500 500
2V 500 625
s
V
P
N s
N
P
V p
s
V
V
p
s
N
N

Transformers
2 V 125 2 V 125 1 1
2 V 125 8 V 500 4 4
2 V 125 10 V 625 5 5
2 V 500 0.5 V 125 0.25 0.25
2 V 500 2 V 500 1 1
2 V 500 2.5 V 625 1.25 1.25
s
V
P
N s
N
P
V p
s
V
V
p
s
N
N

Transformers
A step-up transformer is one in which the
secondary voltage is greater than the primary.
A step-up transformer has more turns on the
secondary coil than the primary coil.
Which of the transformers are step-up?

Transformers
A step-down transformer is one in which the
secondary voltage is less than the primary.
A step-down transformer has fewer turns on
the secondary coil than the primary coil.
Which are step-down transformers?

What would happen if a d.c. supply
was connected to a transformer?
At the moment of switching on, there is a
changing magnetic field which would induce
a voltage in the secondary coil. The same at
the moment of switching off.
Once switched on, no changing magnetic
field (since steady current) and therefore no
induced voltage.

Step-Up Transformer
What is a step up transformer?
What can you say about the relationship
between the number of turns in the secondary
and primary?
and the voltage in the secondary and
primary?
P
N

s
N
P
V

s
V

Step-Down Transformer
What is a step down transformer?
What can you say about the relationship
between the number of turns in the secondary
and primary?
and the voltage in the secondary and
primary?
P
N

s
N
P
V

s
V

Energy Losses in Transformers
For calculations, we often assume that
the transformer is 100% efficient.
however in reality they are about 95%
efficient.
What causes the energy losses?

Energy Losses in Transformers
- Heating effect of current in coils (coils
are long length of wire with resistance
hence electrical energy changed to
heat)
- Iron core being magnetised and
demagnetised
- Transformer vibrating -> sound
- Magnetic field “leakage”

Voltage and Current in Transformers
Assuming an ideal transformer with no
energy losses total energy input must
equal total energy output.
Since rate of energy input is power:
power input = power output

Voltage and Current in
Transformers
Power is given as
P = V I
so
which can be rearranged as
S
S
p
p I
V
I
V 
S
P
P
s
I
I
V
V


S
P
P
s
I
I
V
V

In a step-up transformer, the
voltage in the secondary is
greater than the primary.
What happens to the current?

S
P
P
s
I
I
V
V

The current in the coils is in
the reverse ratio to the
voltage therefore as voltage
increases, current decreases.

S
P
P
s
I
I
V
V

In a step-down transformer,
the voltage in the secondary is
less than the primary. What
happens to the current?

S
P
P
s
I
I
V
V

The current in the coils is in
the reverse ratio to the
voltage therefore as voltage
decreases, current increases.

Transformers
p
s
s
p
s
p
I
I
V
V
n
n


np = number of turns on primary coil
ns = number of turns on secondary coil
Vp = voltage across primary coil
Vs = voltage across secondary coil
Ip = current in primary coil
Is = current in secondary coil

Type of
transformer
Turns
ratio?
Effect on
VOLTAGE?
Effect on
CURRENT?
Step-up
Step-down

Key words: electromagnetism, induced
voltage, field strength, turns.
State that high voltages are used in the
transmission of electricity to reduce
power loss.
Carry out calculations involving power loss
in transmission lines.

Transmitting Electrical Energy
Transformers are used by the National
Grid system through which electrical
energy is transmitted.
Demonstration

Electricity Transmission
Electrical energy is transferred from the power station to
the consumer via the National Grid.
• Electricity is sent for many kilometres along transmission
lines on pylons.

Transformers in Electrical
Transmission
What happens as current flows through
the wires?
The length of the wires means large
resistance and hence heating in the wires.

Transformers in Electrical Transmission
Energy is changed from electrical to heat
resulting in large power losses in the wires.
Relationship between power, current and
resistance?
R
I
P 2


Transmission
At the power station, a step-up transformer
is used to increase the voltage.
Why?

Transmission
As voltage stepped up, current stepped down
by the same factor. And since by
reducing current the power losses due to
heating are reduced.
S
P
P
s
I
I
V
V

R
I
P 2


Transformers in Electrical Transmission
This stepping up of the
voltage and hence stepping
down of the current makes
the transfer much more
efficient. The losses due
to heating are reduced.

Transmission
At the consumer end, a
step-down transformer
reduces the voltage to
230V, increasing the
current.

electricity (1).ppt

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