The Photoelectric Effect: Stopping Potential

1 · Predict

A retarding voltage is applied to push ejected photoelectrons back toward the metal. What voltage is needed to just barely stop even the fastest ejected electrons?

• The stopping voltage (in volts) numerically equals the maximum kinetic energy (in eV) — it takes exactly that much reverse work to stop the fastest electrons.
• The stopping voltage doesn't depend on the photon energy.
• The stopping voltage depends on the photocell's shape, not on the light.

2 · Set Up

1. Open the photoelectric-retard preset and press Reset. The photocell's work function is fixed at 2.0 eV.
2. Enable the stopping-potential readout.
3. Set the light's wavelength for each trial and record the stopping potential needed to halt the fastest photoelectrons.

3 · Collect Data

Plot V_stop (y-axis) against photon energy E_γ (x-axis) for your three trials.

4 · Analyze

1. For one trial, compute E_γ = hc/λ, then V_stop = K_max = E_γ − φ using φ = 2.0 eV (numerically, since 1 eV of kinetic energy is stopped by exactly 1 V of retarding potential). Compare to the table.
2. Explain why measuring the stopping voltage is a clever experimental trick: it directly reads off K_max in eV without needing to measure electron speeds directly.

5 · Extend

1. If you plotted V_stop against the light's FREQUENCY (not wavelength) instead, the slope of that line would equal h/e — Planck's constant divided by the electron charge. Explain why this graph was historically used to measure Planck's constant experimentally.
2. What would the stopping potential be for light with photon energy below 2.0 eV? Would you even need a stopping potential in that case?

The Physics Behind This Experiment

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