Page 109 - Physics - XII

4. It is observed that, if the resistance of the diode is high in the fi rst case, it will be low in the second case,
and vice versa. It gives the unidirectional fl ow of current in a diode.
(b) For LED

5. Now, replace the junction diode to LED and proceed as in step 3. It is observed that if the resistance of
LED is high in fi rst case, it is low in second case, and vice versa. Also, LED will glow by emitting light
when its resistance is low. It gives the unidirectional fl ow of current in LED.

(c) To check whether the given diode is in working order

6. Repeat the steps 1 and 2 to set the multimeter as an ohmmeter.
7. Now, touch the two leads to the two terminals of the diode and note whether the resistance of the diode
is low or high. Now, interchange the two leads of the multimeter and again note whether the resistance
of the diode is low or high.
8. If the resistance of the diode is high in one case and low in other case, the diode is in working order. If
the resistance of the junction diode is low in both the cases, diode is damaged one.

Precautions
1. Observations should be taken by setting the multimeter in ohm range.

2. Before measuring the resistance, the zero ohm switch should be adjusted.

ACTIVITY - 3

Aim
To study eff ect of intensity of light (by varying distance of the source) on an LDR (light dependent resistor).

Apparatus and Materials Required
A source of light, an LDR, and a multimeter

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. The intensity of light decreases inversely as square
of distance. A typical LDR has a resistance of about 1 MΩ in complete darkness and about 10 Ω in
bright light.

Procedure
1. Set the selector switch of the multimeter on R for the measurement of resistance.
2. Insert the metallic ends of black probe in the terminal marked Light from the light source
common in the multimeter and that of red in the terminal
marked as +ve (P). Short both metallic ends of the multimeter
and adjust R to get full scale defl ection reading at zero ohm in
the multimeter.
3. Now, adjust the source of light such that light falls on LDR
by fi xing the source of light in a stand and keeping LDR LDR
vertically below it (Fig. 3.1). Now, touch the metallic probes
to the two metal ends of LDR and note the value of resistance, To multimeter, which is set on ohm
when the source of light is kept at a distance of 4 cm. Fig. 3.1: LDR (Light Dependent Resistor)

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