State the three basic properties of electric charge. Explain quantisation of charge and show that a body cannot carry a charge of 2.4 × 10⁻¹⁹ C.

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

1 · Name the three properties. Electric charge is additive, conserved and quantised.
2 · Explain additivity. Charge is a scalar, so the total charge is the algebraic sum: +2 C, +5 C and −3 C give +4 C.
3 · Explain conservation. The total charge of an isolated system stays constant. Rubbing gives +q to the glass and −q to the silk.
4 · Explain quantisation. Every charge is q = ne, where n is an integer and e = 1.6 × 10⁻¹⁹ C.
5 · Test the given charge. n = 2.4 × 10⁻¹⁹ ÷ 1.6 × 10⁻¹⁹ = 1.5. This is not an integer, so the charge is not possible.
6 · Real life. A petrol tanker's metal chain lets its charge leak into the ground; the charge is moved, never destroyed.
7 · Close in one line. Charge can be moved and added, but only in whole units of e.

Scoring tip: Always write q = ne with 'n is an integer'. The examiner gives the mark for the word integer, not just the formula.