For a fixed metal in the standard photoelectric model, increasing light intensity at the same frequency leaves the stopping voltage unchanged. Increasing the frequency above the threshold raises the maximum kinetic energy of the emitted electrons and increases the magnitude of the stopping voltage.
This H2 Physics 9478 practice separates two measurements from a photocurrent graph. The stopping voltage gives information about the fastest electrons. The saturation current gives information about how many electrons are collected per second.
- Check that the frequency is above threshold.
- Use photon energy minus work function for maximum kinetic energy.
- Use the magnitude of the stopping voltage.
- Keep track of which light property changes.
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Read the stopping point and the plateau
A reverse potential difference opposes the motion of emitted electrons. Increase its magnitude until even the most energetic electrons cannot reach the collector. At that point the photocurrent falls to zero. The maximum kinetic energy is e times the magnitude of this stopping potential difference.
At a sufficiently positive collecting voltage, the photocurrent can reach a plateau. In the ideal comparison, increasing intensity at fixed frequency increases the incident photon rate and raises this plateau while leaving the stopping point in the same place.
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Worked example | One metal, two photon energies
Suppose a metal has a work function of 2.0 eV. Light with photon energy 3.2 eV gives a maximum electron kinetic energy of 3.2 − 2.0 = 1.2 eV. A stopping voltage of magnitude 1.2 V is required.
Doubling intensity while keeping that photon energy fixed leaves the stopping magnitude at 1.2 V. Change the photon energy to 4.0 eV and the maximum kinetic energy becomes 2.0 eV. The stopping magnitude becomes 2.0 V. The change in photon energy alone does not establish how the saturation current changes.
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Try a frequency below threshold
If each photon has energy 1.8 eV, it has less energy than the 2.0 eV work function. In the standard single-photon model, increasing intensity supplies more photons of that same energy. Photoemission still does not occur.
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Frequently asked questions
Why might the graph show a negative stopping voltage?
The negative sign can indicate the reverse polarity used to stop the electrons. Use the magnitude when calculating maximum kinetic energy.
How do electronvolts relate to volts here?
An electron gains or loses 1 eV of energy across a potential difference of magnitude 1 V. This makes the numbers match when kinetic energy is in eV and stopping magnitude is in V.