<s>[INST] When the switch is in position A, the capacitor is connected to a battery with constant voltage V₀. When the switch is moved to position B, the capacitor is connected to a battery with constant voltage -V₀. The switch moves back and forth between positions A and B very quickly, so that the charge on the capacitor is always Q = C V₀ or Q = -C V₀. The energy stored in the capacitor is U = ½ C V². We choose the zero point of energy such that U = 0, if the capacitor is uncharged. Thus, at any instant, the energy of the capacitor is either U₀ = ½ C V₀² or U₀ = ½ C (-V₀)² = ½ C V₀² = U₀. The energy U₀ is constant, since V₀ is, and is periodically changed between the two values.[/INST]