First slide

Photo Electric Effect

Question

A silver ball of radius 4.8 cm is suspended by a thread in the vacuum chamber. UV light of wavelength 200 nm is incident on the ball for some times during which a total energy of $1 \times 10^{- 7}$ J falls on the surface. Assuming on an average one out of 10³ photons incident is able to eject electron. The potential on sphere will be

Difficult

A

1 V
B

2 V
C

3 V
D

Zero

Solution

$\begin{array}{l} n = \frac{Eλ}{hc} = \frac{1 \times 10^{- 7} \times 200 \times 10^{- 9}}{6 .6 \times 10^{- 34} \times 3 \times 10^{8}} = 1 \times 10^{11} \\ Number of electrons ejected = \frac{10^{11}}{10^{3}} = 10^{8} \\ ∴ V = \frac{q}{4 {πε}_{o} r} = \frac{(10^{8} \times 1 .6 \times 10^{- 19}) \times 9 \times 10^{9}}{4 .8 \times 10^{- 2}} = 3 V \end{array}$

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