A recycling machine contains small aluminium flakes. Before each test, a flake is neutral and its charge is evenly distributed.
In separate tests, the flake passes close to a positively charged plate and to a negatively charged plate. The flake moves towards the plate in both tests without touching it.
Which explanation is correct?
The flake has fixed positive and negative halves, and turns between the tests so that the half with the opposite sign always faces the plate.
The charges in the flake remain evenly distributed, because a neutral object is pulled towards any charged plate without its charges needing to move.
The plate creates charge of the opposite sign on the near side of the flake, so the amount of charge in the flake increases in each test.
The positive plate pulls electrons towards the near side of the flake, while the negative plate pushes them away; the near side has the opposite sign in each test.
91 exam-style questions on Edexcel GCSE Physics 11.1 Charging by friction, covering 11.1.1 Charging insulators by friction, 11.1.2 Forces between charges, 11.1.3 Electrostatic phenomena, and 11.1.4 Earthing and removing charge. Each one has a worked solution and a mark scheme showing where the marks go.