How can separated metal plates store electrical energy?

A capacitor stores energy by separating electric charge on conducting surfaces with an insulating gap between them. A simple version has two metal plates. A connected source moves electrons onto one plate and away from the other, creating equal and opposite excess charges and an electric field in the space between.

The insulating layer prevents ordinary charge flow directly from one plate to the other. During charging, however, current can flow in the wires connected to both plates as charge is redistributed. The growing charge separation creates a voltage that opposes further charging from a fixed source. In a simple circuit with a resistor, charging is fastest initially and slows as the capacitor's voltage approaches the supply voltage.

Capacitance describes how much separated charge corresponds to a given voltage: Q = CV. Larger plate area, smaller separation and suitable insulating material can increase capacitance. More capacitance means more charge can be separated at the same voltage. Energy is associated with the electric field, not with a chemical fuel inside an ordinary capacitor. The dielectric, or insulating material, also has limits; excessive voltage can damage it.

When the charged capacitor is connected to a suitable load, charge redistributes through the external circuit and energy can be delivered. The voltage falls as discharge proceeds. A capacitor placed near an electronic chip can briefly supply current when the chip's demand changes, helping prevent a sudden supply-voltage dip. It cannot maintain that supply indefinitely because its stored energy is finite.

This charging and discharging behavior also explains why capacitors respond differently to steady and changing signals. After the initial charging of an ideal capacitor under constant DC conditions, ongoing current through that branch stops. A changing voltage can keep redistributing charge, so capacitors are useful in filters and timing circuits. Charge does not have to cross the insulating gap for changes on the two conductors to be linked. The useful action comes from electric fields and the movement of charge in the surrounding circuit.