The work done in carrying a charge Q once round the circle of radius r with charge Q at the centre of the circle is

Two equally charged small balls A and B, placed at a fixed distance experience a force F. A smilar uncharged ball C after touching one of them is placed at a middle point between the two balls. The force experienced by ball A, is

A capacitor of capacitance 2 μF is charged to a potential difference of 200 volts. After disconnecting from the battery, it is connected in parallel with another uncharged capacitor. The common potential is 40 volts. The capacitance of the second capacitor is