AP Physics 2 · Exam guide
AP Physics 2 Photoelectric Effect: Photon Energy, Work Function, and Stopping Potential
The photoelectric effect is a favorite modern physics topic, and it combines an equation with a conceptual argument that classical wave theory could not make. The equation is short, and the conceptual points are the ones graders look for. This guide shows both.
The central equation
A photon of frequency \(f\) has energy \(E = hf = \dfrac{hc}{\lambda}\). A photon that ejects an electron gives up an amount equal to the work function \(\phi\) of the metal, and the rest becomes kinetic energy: \(K_{\max} = hf - \phi\). The stopping potential \(V_s\) satisfies \(eV_s = K_{\max}\).
A worked example
Light of wavelength 400 nm hits a metal with a work function of 2.3 eV. Use \(hc \approx 1240\ \mathrm{eV\cdot nm}\). (a) Find the photon energy. (b) Find the maximum kinetic energy of the electrons. (c) Find the longest wavelength that ejects electrons.
(a) \(E = \dfrac{1240}{400} = 3.1\) eV.
(b) \(K_{\max} = 3.1 - 2.3 = 0.8\) eV, so the stopping potential is 0.8 V.
(c) At the threshold, \(hf = \phi\), so \(\lambda = \dfrac{1240}{2.3} \approx 540\) nm. Light with a longer wavelength has too little energy per photon.
The conceptual points
| If you change | Then | Because |
|---|---|---|
| Intensity (more photons), same frequency | More electrons are ejected, but each has the same maximum kinetic energy. | Each electron absorbs one photon, and the photon energy does not change. |
| Frequency (higher) | The maximum kinetic energy of each electron increases. | Each photon carries more energy. |
| Frequency below the threshold | No electrons are ejected, however bright the light. | No single photon has enough energy. |
Brighter light has more photons, not more energy per photon. The maximum kinetic energy depends on frequency.
It separates the number of electrons from their energy and uses the equation.
Common mistakes
- Using wavelength where frequency is needed.
- Mixing electron volts and joules.
- Saying that brighter light raises the kinetic energy of each electron.
- Forgetting that no electrons are ejected below the threshold frequency.
Frequently asked questions
What is the work function?
The minimum energy needed to remove an electron from the surface of the metal.
What is the stopping potential?
The reverse voltage that just stops the fastest electrons, so that it equals the maximum kinetic energy divided by the electron charge.
Why did this contradict classical physics?
A classical wave would deliver energy continuously, so bright light of any frequency should eventually eject electrons, and it should increase their energy.
Where can I find the official scoring rules?
Exam formats change a little from year to year, so check the current course and exam description on the College Board site before test day.
Keep practicing
Every lesson in the AP Physics courses ends with practice problems and an Exam focus box that shows how the topic is tested, and every unit has a review with worked answers. These free lessons also build the skills the free-response section tests:
See the AP Physics 2 course Free lessons and lesson list
More AP Physics 2 guides (6)
Start here
Free-response and writing
- AP Physics 2 Experimental Design Explained: Planning an Experiment and Analyzing the Data
- AP Physics 2 Thermodynamics FRQ Explained: PV Diagrams and the First Law
- AP Physics 2 Circuits Question: Series, Parallel, Current, and Power
- AP Physics 2 Optics Question: The Lens Equation and Image Types
Multiple choice
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