17.1.5 I-V Graph

The data from a photoelectric effect experiment is often presented as an IV graph.

At zero bias, there is some photoelectric current since some (but not all) of the emitted photoelectrons do arrive at the collector. On the right half of the graph, the photoelectric current increases as the positive bias increases, but eventually saturates when all emitted photoelectrons have been successfully collected at the collector. On the left half of the graph, the photoelectric current gradually decreases[1] as the negative bias increases, eventually dropping to zero when even the most energetic photoelectron has been successfully prevented from arriving at the collector

Quantitatively, the most important information on the graph are the magnitude of the saturation current Is and the stopping potential Vs.

  1. \displaystyle {{I}_{{sat}}}=\frac{Q}{t}=\frac{N}{t}e allows us to calculate the rate of emission of photoelectrons, which is the number of photoelectrons emitted per unit time, and
  2. \displaystyle e{{V}_{s}}=K{{E}_{{\max }}} allows us to calculate the maximum kinetic energy of the photoelectrons.

These two quantities inform us of the manner in which light is delivering energy to the electrons. And as we shall see, the experimental findings were rather disturbing.

[1] It is worth noting that this also provides evidence that the photoelectrons are emitted with a range of KE. If not, the photoelectric current should plunge abruptly to zero at a particular negative bias voltage.

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