An electrostatic separator has three removable metal collection cups. During servicing, the cups are placed in an uncharged metal holder on an insulating stand, so the cups and the holder form a single conductor.
The charges on the cups before they are placed in the holder are shown in the table.
| Cup | Initial charge |
|---|---|
| A | +8 nC+8\ \mathrm{nC}+8 nC |
| B | −5 nC-5\ \mathrm{nC}−5 nC |
| C | unknown |
The holder is then connected to Earth until the assembly is neutral. A coulombmeter in the earth lead shows that electrons with a total charge of −6 nC-6\ \mathrm{nC}−6 nC flowed from Earth onto the assembly.
Calculate the initial charge on cup C.
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.