Two identical conducting spheres P and Q: P has +6 nC, Q has 0. They are touched together and then separated. What is the charge on each sphere after separation?

• A. +3 nC on each
• B. +6 nC on each
• C. 0 on each
• D. -3 nC on each

Answer: (A)

When two conductors are touched, charges flow between them until their electric potentials are the same. For identical conducting spheres, the potentials become equal with equal sharing of charge, because their capacitances are equal. The total charge is conserved: initially one sphere has +6 nC and the other has 0, so the total is +6 nC. After they touch, this +6 nC is split equally, giving +3 nC on each sphere. When they separate, each keeps its +3 nC. So the charge on each sphere is +3 nC.