Define electrostatic potential and potential difference, and give the SI unit of each. Find the work done in moving a charge of 5 C between two points whose potential difference is 220 V.

1. 1 · Define potential. The work done in bringing a unit positive charge from infinity to a point is the potential at that point.
2. 2 · Define potential difference. The work done in moving a unit positive charge from one point to another is the potential difference between them.
3. 3 · Give the unit. Both are measured in volts: 1 V = 1 J/C.
4. 4 · Write the formula. W = qV
5. 5 · Substitute and answer. W = 5 C × 220 V = 1100 J.

Scoring tip: Say 'unit positive charge' in both definitions, and 'from infinity' for potential. These are the words the marks are given for.