Stopping potential

What is stopping potential?

Direct answer: Stopping potential is the smallest reverse voltage that reduces the photocurrent to zero. We write its positive magnitude as Vₛ, so eVₛ = Kₘₐₓ = hf − φ, where e is the positive elementary charge. If the graph plots reverse voltage as negative, its intercept is −Vₛ, not Vₛ.

How to calculate or read stopping potential

1. Identify the frequency f or wavelength λ of the incoming light and the work function φ of the metal. Use consistent energy units.
2. Find the photon energy from E = hf or E = hc/λ.
3. Apply Einstein's photoelectric equation Kₘₐₓ = hf − φ; if hf is below φ, photoemission does not occur.
4. Use eVₛ = Kₘₐₓ. When Kₘₐₓ is expressed in electron-volts, the numerical value of Vₛ in volts is the same.
5. For a graph, read the reverse voltage at which the photocurrent first reaches zero, using the magnitude unless the question asks for the signed voltage.

Worked example: 400 nm light and a 2.00 eV work function

Use hc ≈ 1240 eV·nm, so the photon energy for λ = 400 nm is 1240/400 = 3.10 eV.

The maximum photoelectron kinetic energy is 1.10 eV, so the magnitude of the stopping potential is 1.10 V.

Common stopping-potential mistakes

• Using the saturation voltage instead of the reverse voltage where the photocurrent becomes zero.
• Subtracting the photon energy from the work function instead of using Kₘₐₓ = hf − φ.
• Mixing joules and electron-volts without converting units.
• Assuming greater light intensity raises the stopping potential; at fixed frequency it mainly changes the photocurrent, not Kₘₐₓ.
• Reporting a negative magnitude when the graph uses a negative reverse-voltage axis; distinguish the signed intercept from the positive magnitude Vₛ.

Sources and review

• OpenStax University Physics Volume 3 — Photoelectric Effect
• Official CSCA exam introduction

Last reviewed: 2026-09-08. The equation and graph convention were checked against OpenStax. The worked example uses a 2.00 eV work function and gives a stopping-potential magnitude of 1.10 V.