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Question:

Photons with energy 5 eV are incident on a cathode C in a photoelectric cell. The maximum energy of emitted photoelectrons is 2 eV. When photons of energy 6 eV are incident on C, no photoelectrons will reach the anode A, if the stopping potential of A relative to C is:

−3V

+3V

+4V

−4V

Solution:

Case 1 :Energy of photon E=5eV
Maximum kinetic energy of photoelectrons K=2eV
Using K=E−ϕ where ϕ is the work function
∴2=5−ϕ ⇒ϕ=3eV
Case 2 :E=6eV
∴Maximum kinetic energy of photoelectrons K=6−3=3eV
thus stopping potential of A relative to C is −3V.