State the electrostatic properties of a conductor, giving a reason for each. What is electrostatic shielding?
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Start from free charges. A conductor has free electrons that move under any field until they reach equilibrium.
2 · Field inside. E = 0 inside; otherwise the free charges would keep moving.
3 · Field at the surface. E is normal to the surface, with size σ/ε₀; a part along the surface would move charges.
4 · Charge and potential. Net charge lies only on the surface (Gauss's theorem with E = 0 inside). The potential is the same throughout, so the surface is equipotential.
5 · Shielding. The field inside an empty cavity of a conductor is zero, whatever the outside field.
6 · Real life. A car or bus body shields its passengers during lightning; coaxial cables shield signals.
7 · Close in one line. In electrostatics a conductor is field-free inside and has one potential throughout.
Scoring tip: Give a one-line reason after each property. A list without reasons usually earns only half the marks.
