What do we mean by conservation and quantisation of charge? Describe how a metal sphere can be charged by induction.

1. 1 · Conservation in one line. The total charge of an isolated system stays constant: rubbing glass with silk gives +q to the glass and −q to the silk.
2. 2 · Quantisation in one line. Every charge is a whole-number multiple of e: q = Ne, with e = 1.6 × 10⁻¹⁹ C (Millikan, 1909).
3. 3 · Bring the charged rod near. A negative rod near an insulated metal sphere pushes free electrons to the far side; the near side is left positive.
4. 4 · Draw the figure. [Sphere on an insulating stand; rod with − signs on the left; + signs on the near half and − signs on the far half; earth wire on the far side]
5. 5 · Earth, then remove in order. Earth the sphere: the electrons flow to the ground. Remove the earth wire first, then the rod.
6. 6 · Close in one line. The sphere is left positive, opposite to the rod, and the rod loses none of its charge.

Scoring tip: Say the order clearly: earth wire off first, rod away second. If the rod goes first, the electrons flow back and the sphere ends up uncharged.