Page 124 - Physics - XII

Principle/Theory

A light dependent resistor or a photoresistor is prepared from cadmium sulphide. Its resistance depends
upon the intensity and duration of light incident on it.

Procedure
Study the current in the circuit due to variati on in the power of lamps

1. Using the components, make the circuit as shown in Fig. 2.1.
2. Adjust the source of light, i.e. lamp so that light falls normally on LDR by fi xing the source of light in
a stand and keeping LDR vertically below it.
3. Now, set the knob of multimeter on the point marked D.C. current.
4. Keeping the lamp at a fi xed distance from LDR, place a thick piece of black paper on the LDR, so that
no light falls on LDR, now record the current in multimeter and determine the resistance of LDR.
5. Now, remove the black paper from LDR and switch on the lamp of 25 W over the LDR and again

record the current from the multimeter. Now, determine the resistance of LDR.
6. Now, replace the 25 W lamp by 60 W, 100 W, and 200 W and everytime measure the current with the
help of multimeter. Also, determine the resistance of LDR.
7. While taking the observation with the diff erent lamps, remember that the distance between lamp and
LDR must be same for all the lamps, and time duration (say one minute) also remains same for all the

observations.
8. Note all the observations.

Study the current in the circuit due to variati on in the distance of the lamp and LDR

1. Repeat the steps 1 to 3.
2. Now, take 100 W lamp and fi x the lamp at a distance of 5 cm from LDR and switch on the lamp so that
light falls on the lamp normally. Record the current in multimeter.
3. Now, move the 100 W lamp at a distance of 10 cm from LDR and record the current in multimeter.
4. Move the lamp of 100 W to 15 cm, 20 cm, 25 cm from LDR and each time, record the current in
multimeter.

5. Note all the observations.

Observations
E.M.F. of battery = _____ V
Table for current and power of lamps

S. No. Power of lamps Current (mA) Resistance R of LDR (Ω)
1. With black paper

2. 25 W

3. 60 W

4. 100 W

5. 200 W

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