A simple circuit consists of a battery(emf=V), a switch and a capacitor(C). At the steady state, the energy stored in the capacitor is 0.5CV^2. However, the work done by the battery is QV=CV^2 (move a total charge of Q across a potential difference of V). If the energy dissipated in the wire is negligible, according to the law of conservation of energy the work done by the battery=the energy stored in the capacitor. But 0.5CV^2!=CV^2. What's wrong with my reasoning above? Thanks! BTW, assuming the battery is without inner resistence(r=0). PS. When an resistor is connected in series with the capacitor, things are much easier. the rest of the work done by the battery (0.5CV^2) dissipates in the resistor.