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What Is an Electrical Capacitor?

By Paul Dohrman, eHow Contributor

An electrical capacitor is a device on which a charge collects. The usual shape is two metallic plates parallel to each other but touching only two terminal ends of a circuit wire, not each other. Electrons flow from one plate through the electrical circuit and to the other plate, due to the electromotive force of a battery or other source of energy. The charges on each plate are equal and opposite in polarity. The plates are therefore attractive.

Storage of Energy

1. Because the battery, or some other source of electromotive force, displaces electrons from one plate of the capacitor to another, energy is built up and stored in the capacitor. The circuit wires could be removed, and the charge difference between the plates would remain. The energy stored in the capacitor equals the energy required to keep the two plates apart, opposing their Coulomb attraction (an attraction due to their electrical charge). If the plates were not held in place, they would attract each other, make contact, and discharge.


2. A voltage can be discharged by placing a piece of metal between the two plates, making contact with both. The excess electrons of the negatively charged plate would then rush over to the other plate, thus removing the excess negative charge on one plate, and the excess positive charge on the other.

Charge Accumulation

3. Charge accumulation is not absolute. That is, it does not stop until the capacitor is detached from the circuit. This is because getting the capacitor's voltage (due to the accumulation of charge) any proportion closer to the final voltage takes a finite amount of time. The voltage building up between the plates therefore has an asymptotic shape, when graphed against time.

In Alternating Currents

4. When the power source supplies an alternating current, the capacitor alternates charge polarity along with the circuit. Electrons falling onto one plate are then pulled off and sent to the other plate, and so on, each time the power source switches polarity.

Use in Electronic Products

5. Capacitors can be used for pulsed power, upon discharge of the charge on the capacitor plates. Pulsed power is useful when sudden bursts of energy are required, e.g. in a laser, or a linear accelerator.

Capacitors, after charging, can be used like a second battery, when the primary battery is being recharged. This is particularly important when vital memory is involved.

Capacitors can be used to smooth out fluctuations in power supply inside electronic equipment.

Capacitors are also used as noise filters, radio tuners, in amplifiers, and in numerous other devices.