The graphs I've seen for the photocurrent vs. retarding potential have generally looked like this:


enter image description here

I'm wondering why the photoelectric current doesn't immediately reach the saturation current, once the anode potential is positive. It seems like it should, since all the emitted electrons would be attracted to the anode. Can anyone explain?


1 Answer 1


The reason is the build-up of a negative space charge (electron cloud)of emitted electron near the cathode which has a repulsive effect on the further emission of electrons. This shielding the anode field there at low anode voltages is overcome at higher anode voltages. A similar effect occurs in thermal electron emission cathodes in vacuum tubes.

  • $\begingroup$ If there's a current generated won't these electrons be siphoned away from the anode? $\endgroup$
    – Allure
    Commented May 4 at 22:50
  • $\begingroup$ @Allure Yes, the electrons are eventually carried away by the anode field at high voltages so that current saturation sets in. But at low voltages, due to the negative space charge, some emitted electrons return to the cathode so that the current is lower than expected. $\endgroup$
    – freecharly
    Commented May 5 at 2:03
  • $\begingroup$ Does this mean that if the intensity of the light is very low (such that only a few electrons are ejected), then we will indeed see the photoelectric current reach the saturation current for any positive anode potential? $\endgroup$
    – Allure
    Commented May 7 at 2:26
  • $\begingroup$ @Allure In principle, yes! $\endgroup$
    – freecharly
    Commented May 7 at 16:56

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