Solution to 1986 Problem 31

Initially, when the voltage of the collecter is 0, there is a current measured in the ammeter assuming that the photon energy is greater than the work function W. As the voltage V is lowered, the electrons are decelerated by the electric field between the photocathode and the collector. So, when the electrons reach the collector, they have a smaller velocity and so the measured current is 0. Eventually, when the voltage V is lowered enough, the electrons reaching the collector have a velocity of exactly 0, so no current is measured. This occurs when the given equation is satisfied. Therefore, answer (A) is correct.

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