Q. 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 4.2 eV and the work function of the metal is 1.5 eV.
(a) 2.08 ×1018
(b) 1.875 × 108
(c) 2.88 × 1018
(d) 4 × 1019
Ans: (b)
Sol: Kmax = E −φ ⇒ eV0 = E −φ
⇒ V0 = (E −φ)/e
V0 = (4.2 −1.5)eV/e = 2.7 V
As , Potential V0 = q/4πε0r =ne/4πε0r
2.7 = 9× 109 × n(1.6×10-19)/0.1
n = 1.875 × 108