Question

What will be the curve between current $I$ and potential difference $V$ for a photocell?

Answer 1

Photocell:

1. A photocell is a device that converts light energy to electric energy.
2. It works on the principle of the photoelectric effect.
3. The photoelectric effect is the emission of electrons of suitable frequency from a metal when electromagnetic waves like a light hit the metal surface.

Photoelectric current:

1. As there are electrons emitted due to the photoelectric effect, they give rise to a current known as photoelectric current.
2. The maximum value of the photoelectric current is known as saturation current.
3. The current cannot be increased further than this value and on increasing the potential it remains constant at the saturation value.

The graph between photoelectric current and potential difference:

1. In the photoelectric effect, when the potential is applied, negative current still flows because electrons gaining kinetic energy from the incident light reach the negative terminal.
2. The negative potential shown in the diagram is the stopping potential.
3. Stopping potential or cut-off potential is defined as the required potential for stopping the removal of an electron from a metal surface when the incident light energy is greater than the work potential of the metal on which the incident light is focused.
4. As the potential applied is positive more electrons flow towards the positive terminal and the current increases.
5. But after a certain potential, the current reaches its saturation value and will become constant.
6. On increasing the potential further, it does not change.
7. In photocell, at a particular negative potential (stopping potential $V_0$) of the anode, the photoelectric current is zero.
8. As the potential difference between cathode and anode increases the current through the circuit increases but after some time constant current
   (saturation current) flows through the circuit even if the potential difference still increasing.
9. When a negative potential is applied, electrons will still gain kinetic energy from the light incident on the metal and will be emitted.
10. Thus, there will still be a current when the potential is negative.
11. As the potential is increased further, the current increases, since $I\propto V$, where $I$ is current and $V$ is potential.
12. Thus, a graph between the current and voltage of a photoelectric cell is,