To plot the voltage current characteristics of photo cell

Photo Cell Characteristics

Aim: To plot the voltage current characteristics of photo cell.

Apparatus: Micro ammeter, voltmeter, photo cell, lamp, power supply etc.

Theory: Certain metals like cesium and other alkali metals emit electrons called photo electrons, when illuminated by light. This is called photo electric effect.

Einstein explained photoelectric effect on the basis of quantum theory.

When a photon of energy hע is incident on a metal surface a part of photon energy say W0 is utilized in ejecting an electron from the metal and rest of the energy appears as the kinetic energy of the emitted photo electrons.

hע= W0 + ½ mv2

This is Einstein photoelectric equation where W0 is the work function of the metal and is equal to the energy necessary to eject electron from the metal, m is the mass while v is the velocity of the emitted photo electron,  ע is the frequency of incident radiation and h is the Plank constant.

If the incident photon has a frequency less than a certain threshold frequency then the ejection of an electron is not possible, how so ever large the intensity may be. The photoelectric current is directly proportional to the intensity of the incident radiation, once the threshold frequency is overcome. However the energy and hence velocity of the photo electron depends on the incident frequency.

A photocell consists of two electrodes which are fixed in an evacuated glass tube. The electrode which is coated with photo sensitive material is called cathode and other electrode is called as anode. When light of suitable frequency is made incident on the cathode, it emits photo electrons. The anode is given a positive potential with respect to the cathode to attract the emitted photo electrons. Thus photoelectric current flows through the circuit and it increases with increase of a positive potential of the anode.

If the cathode is given a positive potential with respect to the anode only fast moving electrons will reach anode and photo current will be very small. This current will decrease as the positive potential of cathode is increased. The minimum positive potential of the cathode for which the photo current becomes zero is called as stopping potential.

Procedure:

1. Make connections as shown in the figure.
2. Place the source of white light at a suitable distance from the photo cell and focus the light on the cathode of the photo cell.
3. Apply positive potential to the anode and adjust it so that the micro ammeter shows the current flowing in the circuit.
4. Gradually increase this anode potential and note the current every time till you get the saturation current (i.e. further increase in anode potential will not cause any increase in the current).
5. Repeat the experiment for five distances 10cm, 20cm, 30cm, 40cm, 50cm between the source and the photo cell i.e. for different intensities.
6. Plot the graph of current against anode potential for these distances. These graphs are called voltage-current characteristics of photo cell.
7. To study the effect of intensity of incident radiation on the photo electric current, plot the graph of current against 1/d2   at a given fixed voltage.

Observation Table:

S.No	D=0cm		D=10cm		D=20cm		D=30cm		D=40cm
	V(V)	I (µ A)	V(V)	I (µ A)	V(V)	I (µ A)	V(V)	I (µ A)		V(V)	I (µ A)

  

Diagram:

Result: From the graphs it is observed that the photo current increases with increase in intensity and anode potential.

Precautions and sources of error:

Question:

1) What is photo electric effect?
2) Give applications of photo electric effect?
3) Why does current saturate?