An air-cleaning device contains a metal collecting plate mounted on insulating supports. During a test, positively charged dust particles are repelled by the plate before touching it.
A metal chain is connected between the plate and Earth for a short time. The chain is then removed.
In a second test, negatively charged dust particles pass close to the plate without touching it.
Explain what the first test shows about the charge on the collecting plate, how earthing changes the plate, and the force on the negatively charged particles in the second 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.