Question

A source of light is placed above a sphere of radius $10cm$. How many photoelectrons must be emitted by the sphere before emission of photoelectrons stops? The energy of incident photon is $4.2eV$ and the work function of the metal is $1.5eV$.

• A. $2.08\times {10}^8$
• B. $1.875\times {10}^8$
• C. $2.88\times {10}^8$
• D. $4\times {10}^8$

Answer 1

Answer: (B)

$1.875\times {10}^8$

Stopping potential $=4.2-1.5=2.7V$

Photoelectron emission will stop, if the potential of the sphere reaches the stopping potential.

Let the charge on sphere be q when photoelectron emission stops

so $2.7=\frac{kq}{0.1}\to q=3\times {10}^{-11}$C

Now $n=q/e$ , $n=1.875\times {10}^8$