PHYSICS

DEMONSTRATION

PROJECT

Made by : Rahul Chauhan

Class : XII B

Roll no. : 21

Session : 2009-2010

This is to certify that Rahul Chauhan, Roll number _________ of class XII-B has successfully completed PHYSICS DEMONSTRATION EXPERIMENTS under my supervision according to the guidelines laid down by

CBSE.

TEACHER INCHARGE

VICE PRINCIPAL PRINCIPAL

CERTIFICATE

ACKNOWLEGDEMENT

I acknowledge the invaluable contribution of my physics teacher, Mr. Umesh Tyagi, in helping me to complete these demonstrations.

These experiments wouldn’t have been possible to complete without her moral support and proper guidance.

Also I’m deeply grateful to the lab assistant Mr. Satyapal Singh for his constant help.

Rahul Chauhan

CONTENTS

1) DEMONSTRATION-1

• aim• apparatus required• Diffraction• observation table• Conclusion

2) DEMONSTRATION-2

• full wave rectifier

• PN Junction (forward bias,reverse bias)

• step down transformer

• Capacitor

• biblography

DEMONSTRATION-1

To observe the variation of width of central maxima of a diffraction method with the distance of screen from the slit.

AIM: To observe the effect on the width of central maxima by changing the distance between screen and the slit in diffraction.

APPARATUS REQUIRED: laser, single slit, opticalbench, screen, scale.

THEORY:

DIFFRACTION

Diffraction is the phenomenon of bending of light at the sharp edge of the obstacle.

Condition for diffraction:

The width of the obstacle should be comparable with the wavelength of light falling on the obstacle.

Single slit experiment:

Path difference=BC=asin0

In diffraction light waves come front two different parts of same wave front superimpose and alternate dark and bright bands are obtained on this screen.The intensity of light at the centre is maximum and goes on decreasing from two sides of central maxima so other maxima and minima are called secondary maxima and secondary minima.CONDITION FOR SECONDARY MINIMA:-

1. Path difference = n

asin0 = n where n = 1,2,3,………

It is because the waves coming from two different parts of same wavefront have the path difference /2 or multiple of /2

2. Phase difference= 0 = 2Nii where n=1,2,3……

3. Position of n dark band or n secondary minima = x = n D/a

CONDITION FOR SECONDARY MAXIMA:-

1. Path difference= (2n+1) /2

asin0 = (2n+1) /2 where n =1,2,3……

It is because when the path difference between the two waves coming from two different parts is the multiple of and /2respectively and maxima is obtained.

2. Phase difference = 0 = (2n+1)ii

3. Position of n bright band or n secondary maxima x =(2n+1) D/2a

EXPRESSION FOR WIDTH OF CENTRAL MAXIMA IN SINGLE SLIT EXPERIMENT:

It is the distance between the first two secondary minima on both sides of the central point.

For secondary minima

•asin0 = n

•For n =1

•asin0 =

•So, sin0 = /a …………………….(1)

•And, sin0 = x/a …….……………(2)

•From (1) and (2)

•Sin0 = /a = x/a

•x = D/a

Hence, width of the central minima is x = D/aWIDTH OF CENTRAL MAXIMA

•B = 2x

•B = 2 D/a

OBSERVATION TABLE:-

CONCLUSION:-

Width of central maxima increases with increase in distance between the slit and the screen.

DEMONSTRATION - 2

To construct and demonstrate a full wave rectifier.

FULL WAVE RECTIFIERRectifier is a device which is used for converting alternating current/voltage into direct current/voltage. The circuit using two diodes gives output rectifier voltage corresponding to both the positive as well negative half of the cycle. Hence, it is known as a full wave rectifier.Principle:Its working is based on the fact that resistance of p-n junction becomes low when forward biased and becomes high when reversed biased.

Working:During the positive half cycle of the input AC, the upper p-n junction diode is forward biased, and the lower p-n junction diode is reversed biased. The forward current flows on account of majority carriers of upper p-n junction diode. During the negative half cycle of input AC, the upper p-n junction is diode is reversed biased and the lower one is forward biased. The forward current flows on account of majority carriers of lower p-n junction diode. We observe that during both the halves, current through R flows in the same direction. the output signal voltage is unidirectional having ripple contents i.e. DC voltage by filtering through a filter circuit, before it can be put to any use.

A single capacitor C of high value of capacitance connected across the output of rectifier, can serve the purpose of a filter circuit.

P-N JUNCTION DIODE

A semiconductor diode is basically a p-n junction with metallic contacts provided at the two ends for the application of an external voltage. it is a two terminal device.

1. FORWARD BIASED:

When an external voltage V is applied across semiconductor diode such that p-side is connected to the positive terminal of the battery and n-side to the negative terminal, it is said to be forward biased.

2. REVERSE BIASED:

When an external voltage V is applied across the semiconductor diode such that n-side is connected to the positive terminal of the battery and p-side to the negative terminal, it is said to be reversed biased.

STEP DOWN TRANSFORMERA transformer is an electric device which is used for changing the AC voltages. It is based on the principle of mutual induction i.e. whenever the amount of magnetic flux linked with coil changes, an e.m.f. is induced in the neighboring coil. Transformer which decreases the AC voltage is called a step down transformer.

The voltage Ep across the primary coil is equal to the e.m.f. induced in the primary coil, and the voltage Es across the secondary coil Is equal to the e.m.f. induced in the secondary coil. Thus,

ETURN=EP/NP=ES/NS

ES=EP/NP*NS

when NS<NP;ES<EP the device is called a step down transformer. Ns/NP= k represents the transformation ratio.

CAPACITOR

A capacitor is an arrangement for storing large

amounts of electric charge and hence electrical

energy in a small piece.

of a capacitor is defined as the capacitance C is

called capacitance of the capacitor. Capacitance e

ratio of charge on the capacitor to the potential of the

capacitor. The capacitance C is based on the principle

that the capacitance of an insulted conductor is

increased considerably by bringing near it an

uncharged earthed conductor.

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.

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

BIBLIOGRAPHY

Wikipedia – The FreeEncyclopediaPhysics NCERT Class XIITextbook of Physics – Pradeep’sEncarta EncyclopediaBritannica Encyclopedia