Capacitors
A capacitor is two or more insulated metal plates with every other plate connected to one of two wires. The insulation, usually called the dielectric, prevents a steady current flowing. But connect the capacitor to a battery, and current will flow briefly as electric charge builds up on the plates, stopping only when the increasing capacitor voltage equals the battery voltage. This process takes energy from the battery, which is stored in the dielectric in much the same way as energy is stored in the stretched rubber of a balloon. The way capacitors block steady currents but pass changing ones is often exploited in electronics to isolate parts of a circuit that must be kept at different voltages while enabling signals to travel between them. Capacitors introduce time in electronics, allowing things to happe slowly, or in sequence.