A copper ball of radius 1 cm and work function 4.47 eV is irradiated… | JEE Main 2013 (25 Apr, Shift 1)

Q150JEE Main 2013 · 25 APR · SHIFT 1Difficult

Photoelectric Effect: Einstein's Equation

A copper ball of radius 1 cm and work function 4.47 eV is irradiated with ultraviolet radiation of wavelength 2500 Å. The effect of irradiation results in the emission of electrons from the ball. Further the ball will acquire charge and due to this there will be a finite value of the potential on the ball. The charge acquired by the ball is:

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Answer: (a)

Photon energy $= \frac{12400}{2500} = 4.96$ eV, so the ball charges up to $V = 4.96 - 4.47 = 0.49$ V.
$Q = \frac{Vr}{k} = \frac{0.49 \times 0.01}{9 \times 10^9} \approx 5.4 \times 10^{-13}$ C
So the answer is (a) $5.5 \times 10^{-13}$ C.

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