The Photoelectric Effect — Stopping Potential

Learning objective: explain why the photocurrent decreases as the reverse pd increases, and why it falls to zero at the stopping potential Vs.

Circuit and photocell

Vacuum photocell Electrode Photocathode − + Monochromatic light force on e⁻ μA microammeter V voltmeter − + Battery Potential divider slider 0.00 μA
Photon Photoelectron

Energy bar chart — fastest photoelectron

Total energy of the fastest photoelectron is conserved. As it moves from the photocathode toward the negative electrode, kinetic energy converts to electric potential energy. If the reverse pd reaches Vs, all of Ek(max) has been converted to PE and the electron just fails to reach the electrode.

What's happening

Adjust the sliders to explore the photoelectric effect.

Controls

Photoelectrons are released with a range of kinetic energies, because some come from the surface and others from deeper inside the photocathode (deeper electrons lose more energy escaping the metal). As the reverse pd is increased, lower-energy electrons are stopped first; only higher-energy electrons still reach the electrode, so the current decreases.

Live calculations

Frequency, f— THz
Photon energy, hf— eV
Work function, φ— eV
Max KE, Ek(max)— eV
Reverse pd, V— V
Stopping pd, Vs— V
I = 0.00 μA
hf = φ + Ek(max)
Ek(max) = hf − φ
eVs = Ek(max)
Vs = Ek(max) / e

When energy is measured in eV, the stopping potential in volts has the same numerical value as Ek(max) in eV.