Question

When a monochromatic point source of light is at a distance $0.2m$ from a photoelectric cell, the saturation current and cut-off voltage are $18.0mA$ and $0.6V$. If the same source is placed $0.6m$ away from the photoelectric cell, then the saturation current and the stopping potential respectively are :

• A. $4mAand1V$
• B. $2mAand0.6V$
• C. $3mAand0.5V$
• D. $12mAand0.5V$

Answer 1

Answer: (B)

$2mAand0.6V$

The voltage and current depend on number of photons coming to the photo electrical cell. Now, number of photon will vary as $\frac{1}{r^2}$

So, Voltage will vary like this and we can write

$\frac{V_1}{V_2}=\frac{r_2^2}{r_1^2}$

then $V_2=\frac{r_1^2\times V_1}{r_2^2}$

$V_2={0.2}^2\times \frac{0.6}{{0.6}^2}$

$V_2=\frac{0.6}{9}=0.06V$

$\frac{I_1}{I_2}=\frac{V_1}{V_2}$ then $\frac{I_2}{V_2}\times \frac{I^1}{V^1}=\frac{0.06\times 18}{0.6}=2mA$