During maintenance, two detached metal parts of a ventilation unit are placed on insulating mats. The filter frame has a charge of +4 nC+4\ \mathrm{nC}+4 nC and the access panel has an unknown charge. Each part is connected separately to Earth until it is neutral. Overall, electrons carrying a charge of −7 nC-7\ \mathrm{nC}−7 nC move from the two parts to Earth. What was the initial charge on the access panel?
+11 nC+11\ \mathrm{nC}+11 nC
+3 nC+3\ \mathrm{nC}+3 nC
−3 nC-3\ \mathrm{nC}−3 nC
−11 nC-11\ \mathrm{nC}−11 nC
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.