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Capacitance

Capacitance is the property of an electric conductor that characterizes its ability to store an electric charge. An electronic device called a capacitor is designed to provide capacitance in an electric circuit by providing a means for storing energy in an electric field between two conducting bodies.

Around 1745, Ewald Christian von Kliest and Pieter van Musschenbroek independently discovered capacitance in an electric circuit. While engaged in separate studies of electrostatics, they discovered that an electric charge could be stored for a period of time. They were using a device now called a Leyden jar that consisted of a stoppered glass jar filled with water with a nail piercing the stopper and dipping into the water. They connected the nail to an electrostatic charge. After disconnecting the nail from the source of the charge they found that a shock could be felt by touching the nail. This demonstrated that the device had stored the charge.

A capacitor in its simplest form consists of two conducting plates separated by an insulating layer called a dielectric. When a capacitor is connected in a circuit across a voltage source, the voltage forces electrons onto the surface of one plate and pulls electrons off the surface of the other plate resulting in a potential difference between the plates. Capacitors are charged and discharged as needed by its application. Capacitors differ in size and arrangements of plates and the type of dielectric materials used. Paper, ceramic, air, mica, and electrolytic materials can be used, depending on the type of dielectric needed. The capacitance of a capacitor may be fixed or adjustable (as in a radio tuner).

The surge of electric current to the capacitor induces a counter electromotive force in the conductor and the plates. This counter electromotive force is called reactance in this context. When reactance has reached a level equal to the voltage of the battery, the capacitor is fully charged. There is no further flow of current. When the capacitor is fully charged, a switch may be opened and the capacitor will retain its charge (for a time). Because of the difference of charges on the plates there is a source of potential energy in the capacitor. The energy stored is the energy that was required to charge the capacitor. The reactance and associated time it takes a particular capacitor to charge and discharge can be used to create some interesting and important electrical anomalies in a circuit. These lead to an ability to act on electrical signals in a frequency dependent manner, which obviously has far reaching implications in the field of audio, not to mention virtually any electronic circuit.

Capacitance is measured in farads, which is named after Michael Faraday (1791-1867). The symbol for farads is F. If a charge of 1 coulomb is placed on the plates of a capacitor and the potential difference between them is 1 volt, the capacitance is then defined to be 1 farad. One coulomb is equal to the charge of 6.25 x 1018 electrons. One farad is an extremely large quantity of capacitance. Microfarads (10-6 F) and picofarads (10-12 F) are more commonly used.

The capacitance of a capacitor is proportional to the quantity of charge that can be stored in it for each volt difference in potential between its plates. Mathematically this relationship is written as:

C = Q/V

Where C is capacitance in farads, Q is the quantity of stored electrical charge in coulombs, and V is the difference in potential in volts.

Therefore, stored electric charge can be calculated using the formula:

Q = CV

The difference in potential or voltage of the capacitor can be calculated using the formula:

V = Q/C

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