ENERGY STORED BY A CAPACITOR When the switch is in position A the capacitor charges up. When moved to position B the capacitor discharges, the motor briefly switches on (lifting the weight) V A B

Transfer of energy: Charging: energy is transferred to the capacitor by the supply, the capacitor behaves a bit like a small rechargeable battery but it stores its energy in the electric field between the plates. Discharging: energy is transferred from the capacitor to the motor.

ENERGY STORED When a capacitor is charged energy is stored in it as electrical potential energy. When charging a capacitor, the charge on a plate is increased by a small amount Δq. The energy stored on the capacitor is the work done to force the extra charge onto the plate is given by ΔE = ΔqV

So work done charging the capacitor is the area under the line, Charge Q ΔQ V Potential difference The graph shows that at first it is easy to add charge to the capacitor (small V for a given value of Δq). However as more charge is added it repels any further charge from being added so V is larger for Δq, so more work has to be done. So work done charging the capacitor is the area under the line, W = 1/2QV or W = 1/2CV2