What is meant by the work function of a metal? Describe three ways in which electrons can be made to leave a metal surface.

How to lay out the answer. Follow the frame, then write it in your own words.

1 · Define the work function. The least energy an electron needs to escape from a metal surface; written φ₀ and given in eV.
2 · Say why it is needed. Free electrons are held back by the pull of the positive ions in the metal.
3 · List the three ways.
- Thermionic emission: heating the metal
- Field emission: a very strong electric field
- Photoelectric emission: light of frequency above ν₀
4 · Link to the threshold. For light, φ₀ = hν₀: below ν₀ no electrons come out.
5 · Real life. The hot filament of an X-ray tube uses thermionic emission; a street-light sensor uses photoelectric emission.
6 · Close in one line. So, an electron leaves only when it gains at least φ₀ of energy.

Scoring tip: Give one example with each method of emission; examples pick up easy marks.