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Capacitors are devices used to store electrical energy and block DC current flow. The flow of AC current permitted by a capacitor is dependent on the frequency of the current and the capacitance of the unit. Capacitance is measured in farads and microfarads, which are units that represent an amount of stored electrons at a given voltage.

A capacitor consists of a pair of metal plates insulated from one another by a dielectric, any type of non-conductive material. As charge is sent through a circuit containing a capacitor, the metal plates begin to charge, each with an opposite charge (i.e., a positive and negative side). In small capacitors, this stored charge is relatively low, but larger units can hold more. The voltage of the stored energy is the same as the voltage of the original source of the charge. Compared to the storage capacity of a battery, a capacitor can hold very little charge per size, but unlike a battery, a capacitor can discharge its stored energy in a short burst.

This characteristic makes capacitors very useful in applications that require quick, high-energy surges, such as two way pagers that relay short bits of information, or systems that require instantaneous startup. Capacitors are also used in a wide range of power supplies, where their storage abilities smooth output voltage; energy spikes are stored and power dips are supplemented by stored charge. Other uses include the absorption of unwanted interference in certain noise filtration devices. Modern ultra capacitors are incorporated into a wide range of handheld electronic devices.

When handling capacitors and circuit attached to capacitors, one should take care to ensure that they are properly discharged, since they hold charge even after the power is gone. Because a capacitor can release its stored energy in a short burst, improper handling can result in severe problems and electrocution.