Physics demonstration experiment

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By: ______________
Class: ____________
Roll No: __________

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ACKNOWLEDGEMENT
I acknowledge the valuable contribution of Mr. Umesh
Tyagi in providing me the proper guidance to complete
these demonstration experiments. The experiments
would not have been completed without his support
and kind help. I would also be thankful of Mr. Satpal
Singh (Physics Laboratory Assistant).

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CERTIFICATE
This is to certify that ___________________, Roll
number _______ of Class _____ has successfully
completed the Demonstration Experiment under
my supervision according to the guidelines laid down
by CBSE.
Teacher Incharge Vice Principal Principal

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CONTENTS
1) Demonstration Experiment1_____________________________5
1.1) Aim
1.2) Apparatus
1.3) Principle
1.4) Circuit Diagram
1.5) Construction
1.6) Working
1.7) Description of parts
1.7.1) Step-Down Transformer
1.7.2) p-n junction diode
1.7.3) Capacitor
1.7.4) Load Resistance
2) Demonstration Experiment 2____________________________15
2.1) Aim
2.2) Apparatus
2.3) Theory
2.3.1) Diffraction
2.3.2) Diffraction through single slit (Graph)
2.3.3) Diffraction through single slit (Pattern Observed)
2.3.4) Single Slit Experiment
2.3.5) Condition for Secondary Minima
2.3.6) Condition for Secondary Maxima
2.3.7) Width of Central Minima
2.3.8) Width of Central Maxima
2.3.9) Angular Width of Central Maxima
2.3.10) Factors affecting width of Central Maxima
2.4) Observations
2.5) Result

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To construct a Full Wave Rectifier

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It is based on the principle that the diode offers low resistance when it is
forward biased and offers high resistance when it is reverse biased.
The a.c. supply is fed across the primary coil P of a step down
transformer. Two two ends of the secondary coil S of the transformer are
connected to the p- regions of the junction diodes D1 and D2 . A load
resistance RL is connected beteen the n-regions of the two diodes and
the ncentral tapping of the secondary coil. The out put d.c. is obtained
across the load reistance.
T
Output d.c.
Voltage
Input a.c.
voltage
RL
P S
C
D
B
A
D1
D2

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Suppose that during first half of
the input, the upper end A of the
secondary is at + ve pot. and
lower end B is at (–) ve pot. So
the diode D1 gets forward bias
and D2 gets reverse bias hence
current flows through D1 in load
resistance from C to D. During
the next half cycle A becomes –
ve and B becomes +ve and
hence D1 gets reverse bias and
D2 gets forward bias. Thus the
current flows through D2 from C
to D in load resistance.
Hence the full wave rectifier, rectifies the both halves of a.c. The output
d.c. is continuous but pulsating. To reduce the fluctuations, filter circits
are used in output circits. Electrolytic condenser and zener diodes are
use to reduce the fluctuations of d.c.
1. Step-Down Transformer
2. p-n Junction Diode
3. Capacitor
4. Load Resistance
A.C.
Input
Voltage
D.C.
0utput
Voltage

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1. Step-Down Transformer
It is used to decrease the alternating voltage with increase in current. It
works on the principle of mutual indication. It consists of soft iron core
over which two coils are wound. One of them is connected with A/C
input source is called primary coil and the other one is connected with
the output called secondary coil. The primary coil consists of large no. of
turns of thin insulated copper wire and secondary coil consists less no.
of turns of thick insulated copper wire.
When alternating e.m.f. is applied across the primary coil of transformer
then induced e.m.f. is developed in the primary coil due to self induction.
e = - N d/dt …………………………. (I)
N is the no. of turns in primary coil and d/dt is the change in magnetic
flux with each turns of primary coil.
Because both primary coil & secondary coil are wound on a same core,
so mutual induction takes place between them and induced e.m.f. is
developed in the secondary coil.
es = - Ns d/dt …………………………. (II)
Ns is the no. of turns in secondary coil.
Es/ep = Ns/Np = Ip/Is = k
k is constant and is called transformation ratio.

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2. p-n Junction Diode
When p-type semiconductor is brought into close contact with n-type
semiconductor then p-n junction is formed.
When p-n junction is formed, diffusion of
majority of charge carrier take place
across the junction, the holes move from p
to n leave their counter –ve charge in p-
region and the elctrons move from n to p
leaving their +ve charge in n-region.
These +ve & -ve charge accumulate near
junction and form a layer called depletion
layer in which no free charge carrier is
available.
Due to accumulation of +ve & -ve
charge at the junction, a potential
differenceis developedand is called
potential barrier as it stops further
diffusion.
Biaising of p-n junction
Connection of battery with p-n junction is called biaising.

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Forward Biaising
When p-type is connected with +ve terminal and n-type is connected
with –ve terminal of the battery then p-n junction is said to be in forward
bias.
Forward bias batteries push the majority charge carriers towards the
junction and oppose the formation of depletion layer. When an electron
combines with a hole, they neutralize each other at the same instant.
One electron leaves the –ve terminal of the battery & enters n-region tp
compensate the electron. Simultaneously one covalent bond breaks in
p-region. The electron leaves p-region & enters into the +ve terminal of
the battery. So the current flows in the circuit. This current increases
the forward bias voltage and p-n junction offers very low resistance in
forward bias.

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Reverse Biaising
When p-type is connected with –ve terminal and n-type is connected
with +ve terminal of the battery then p-n junction is said to be in reverse
bias.
Reverse bias batteries push the majority charge carriers away from the
junction but support the minority charge carriers move towards the
junction. It also supports the formation of depletion layer. As a result
depletion layer increases. In this connection very small current flows at
high reverse bias voltage due to minority charge carriers and p-n
junction forms high resistance in reverse bias.

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3. Capacitor
A capacitor or condenseris a passive electricalcomponent consisting of
an insulating or dielectric layer between two conductors. When a
voltage potential difference occurs between the conductors, an electric
field occurs in the insulator. This field can be used to store energy, to
resonate with a signal, or to link electrical and mechanical forces.
Capacitors are manufactured as electronic components for use in
electrical circuits, but any two conductors linked by an electric field also
display this property. The effect is greatest between wide, flat, parallel,
narrowly separated conductors.
An ideal capacitor is wholly characterized by a constant capacitance C,
defined as the ratio of charge +Q on each conductor to the voltage V
between them:
C = Q / V
The unit of capacitance is thus coulombs per volt, or farads. Higher
capacitance indicates that more charge may be stored at a given energy
level, or voltage. In actual capacitors, the insulator allows a small
amount of current through, called leakage current, the conductors add
an additional series resistance, and the insulator has an electric field
strength limit resulting in a breakdown voltage.

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4. Load Resistance
A resistor is a two-terminal electronic component that produces a
voltage across its terminals that is proportional to the electric current
through it in accordance with Ohm's law:
V = IR
*****************************

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To observe the effects of changing the
distance between screen and slit on the width
of central maxima in diffraction

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1. Laser Source
2. Single Slit
3. Optical Bench
4. Screen
5. Scale & Pencil

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Diffraction
Diffractionof light is the phenomenon of bending of light around corners
of an obstacle or aperture in the path of light. On account of this
bending, light penetrates into the geometricalshadow of the obstacle.
The light thus deviates from its linear path. Or in other words, Diffraction
is normally taken to refer to various phenomena which occurwhen a
wave encounters an obstacle.It is described as the apparent bending of
waves around small obstacles and the spreading out of waves past
small openings.
Diffraction of light through single slit
(Graph)

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Diffraction of light through single slit
(Pattern Observed)
Single Slit Experiment
In this experiment, light coming from a monochromatic source, falls on a
convex lens and a parallel beam of light is obtained. This parallel beam
of light falls on the single slit. The rays of light bend through the edge
and superimposein the differentphase on a differentpoint in the screen.
As a result alternate dark and bright bands are obtained. These are
called secondary bands i.e. secondary maxima or secondary minima.
The central point has the maximum intensity and maximum width called
Central Maxima.
Condition for Secondary Minima
Path difference = nλ (n=1, 2, 3…)
= asinθ
Where,
a = width of slit
Therefore,
It is because path difference betweenthe waves coming from two paths
of same wave front is λ/2 each. Hence, minima is obtained when path
differenceis a multiple of λ.
asinθ = nλ

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Phase Difference
Thus
Condition for Secondary Maxima
Path Difference =
Phase Difference=
Thus,
Width of Central Minima
It is the distance between two first secondary minima on both sides of
central point.
For secondary minima : asinθ = n
For n=1
asinθ = λ
sinθ =
λ
a
……( 1 )
For small angle : sinθ = θ =
Xn
D
……( 2 )
Φ = 2nπ
asinθ = ( 2n+1 )
𝜆
2
( 2n +1 )π = φ
Xn =
( 2𝑛 +1 )𝜆𝐷
2𝑎
xn =
𝑛𝜆𝐷
𝑎

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From 1 and 2:
λ
a
=
Xn
D
Thus,
Width of Central Maxima
β = 2Xn =
2𝜆𝐷
𝑎
If lens are very close to the slit , then D ≈ f
β =
2𝜆𝑓
𝑎
Angular Width of Central Maxima
Sinθ =
𝜆
𝑎
=
Xn
𝐷
θ =
𝜆
𝑎
=
Xn
𝐷
Angular width = 2θ =
2𝜆
𝑎
=
2Xn
𝐷
Factors affecting Width of Central Maxima
Considering the expression xn =
𝜆𝐷
𝑎
Width of central maxima varies with the following parameters:
1. It varies directly with the wavelength of light.
2. It varies directly with the distance between the screenand the slit.
3. It is inversely proportional to the width of slit.
Xn =
𝜆𝐷
𝑎

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S.no Distance of screen
( cm )
Width of central maxima
( cm )
1.
2.
3.
The observations obtained show that width of central maxima increases
with increase in the distance between the slit and the screen.

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1. Wikipedia – The Free Encyclopedia
2. Physics NCERT Class XII
3. Textbook of Physics – Pradeep’s
4. Encarta Encyclopedia
5. Britannica Encyclopedia

Physics demonstration experiment

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