Question

When a point source of monochromatic light is at a distance of $0.2m$ from a photoelectric cell, the cut off voltage and the saturation current are $0.6V$ and $18mA$ respectively. If the same source is placed $0.6m$ away from the photoelectric cell then :

• A. the stopping potential will be $0.2V.$
• B. the stopping potential will be $0.6V.$
• C. the saturation current will be $6mA.$
• D. the saturation current will be $18mA.$

Answer 1

The correct option is B the stopping potential will be $0.6V.$

Current in photo electric cell is inversely proportional to square of the distance.

So, $i\propto \frac{1}{d^2}$

$\Rightarrow \frac{i_1}{i_2}=\frac{d_2^2}{d_1^2}$

$\Rightarrow \frac{18mA}{i_2}=(\frac{0.6}{0.2}{)}^2$

$\Rightarrow i_2=18\times (\frac{1}{3}{)}^2$$=18\times \frac{1}{9}$$=2mA$

So, saturation current is $2mA$.

Maximum kinetic energy only depend on the energy of incident light energy and work function of the material, which is not changing in the problem. So, kinetic energy remains same. Therefore stopping potential remains same.