During a process described by the photoelectric effect, ultraviolet light can be

Question

During a process described by the photoelectric effect, ultraviolet light can be used to charge a piece of metal.
(a) If such light is incident on a slab of conducting material and electrons are ejected with enough energy that they escape the surface of the metal, how long before the metal has a net charge of +1.48 nC if 1.05 106 electrons are ejected per second?
h

(b) If 1.17 eV is needed to eject an electron from the surface, what is the power rating of the light beam? (Assume this process is 100% efficient.)
W

Explanation / Answer

a)The required time is the ratio of the charge that accumulates to the rate at which it is delivered:

t=q/I =q/dq/dt =1.48*10-9/(1.05*106*1.602*10-19)

=8.8*103s *1h/3600s

=2.44 hours

b)The power rating of the light beam is the rate at which it delivers energy:

P=Eper electronIelectron

=1.17*1.602*10-19*1.05*106

=1.968*10-13watts

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