KVPY 2020-21 SB SX Exam Previous Papers

Question : 39 of 120

Marks: +1, -0

A zinc ball of radius, R = 1 cm charged to a potential –0.5 V. The ball is illuminated by a monochromatic ultraviolet (UV) light with a wavelength 290 nm. The photoelectric threshold for zinc is 332 nm. The potential of ball after a prolonged exposure to the UV is

–0.5 V
0 V
0.54 V
0.79 V

Validate

Solution:

\ϕ={12431}/{3320}=3.74 {eV}

\ε={{hc}}/{\λ}={12431}/{2900}=4.28 {eV}

({KE})_{\max }=4.28-3.74

=0.54 {eV}

Initially sphere is negatively charged so

{e}^{-}

will go easily then potential becomes

{O}

. After that as

{e}^{-}

will leave the potential will increase till it reaches the stopping potential value,

{V}_{0}

{eV}_{0}=0.54 {eV}

{V}_{0}=0.54 {V}

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