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Question:

A silver sphere of radius 1 cm and work function 4.7 eV is suspended from an insulating thread in free-space. It is under continuous illumination of 200 nm wavelength light. As photoelectrons are emitted, the sphere gets charged and acquires a potential. The maximum number of photoelectrons emitted from the sphere is A × 10z (where 1 < A < 10). The value of 'Z' is

8

9

6

7

Solution:

Stopping potential = hc/λ - W = (6.2 × 10-34 × 3 × 108)/(200 × 10-9) eV - 4.7 eV = 1.5 eV
V = kq/r = 1.5
n = 1.5 × 10-19/(9 × 109 × 1.6 × 10-19 × 10-2) = 1.07 × 107
∴ Z = 7
So, the answer is option (B).