Capacitor

A capacitor is a device that stores energy in the form of electrical charges. A battery, in contrast, stores energy in the form of chemical potential energy. A capacitor is charged (given some amount of energy to store) by placing a voltage across its two terminals. When the capacitor is partly or fully charged, it can be removed from the circuit. To release the energy stored, the two contacts of the capacitor are connected through some electrical circuit (closed path).

Physically, capacitor consists of two electrical conductors that are not in contact. The conductors are usually separated by a layer of insulating material, a dielectric. The dielectric is not essential—the conductors could be separated by vacuum, instead—but in practice it keeps the conductors from touching and increases the amount of charge the capacitor can store. When the capacitor is charged, one conductor becomes positively charged and the other negatively charged. The conductors are not in contact, so a current cannot flow through the capacitor (unless the voltage is raised high enough to break down the device). In this state, the capacitor is storing electrical energy. The energy is released when current flows in the opposite direction, discharging the capacitor and partly or entirely equalizing the voltage difference between its two conductors.

Capacitors take many shapes. The simplest is a parallel-plate capacitor, which consists of two flat conductors (such as metal plates) placed parallel to each other. Larger plates can store more charge, hence more energy. Putting the plates close together also allows the capacitor to store more energy.

The capacitance of a capacitor is the charge on the conductor at any given time divided by the voltage between its conductors. Capacitance expresses the ability of a capacitor to store energy. The capacitance of a parallel-plate capacitor is proportional to the area of the plates divided by the distance between them. This number must then be multiplied by the permittivity of the insulator between the plates. (This is a property of the material and is expressed as a number with appropriate units.) If the permittivity is greater than 1, the dielectric increases the capacitance.

Capacitors come in a wide range of sizes. Banks of large capacitors can store and rapidly release large bursts of electrical energy. Engineers can use such devices for many purposes—for example, testing a system’s performance when struck by a bolt of lightning. A camera flash works by retrieving energy from a chemical battery, storing it up in a capacitor, and then releasing it suddenly through a gas-filled tube to cause a bright flash of light. (The capacitor is used because small batteries cannot supply such large, brief currents.) Electronic circuits use large numbers of small capacitors. For example, a typical RAM (random access memory) chip uses millions or billions of microscopic capacitors coupled with switching transistors.