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Capacitance and Capacitors

Capacitance and capacitors Capacitors are components which store static electric charge. A capacitor essentially consists of two metal plates serving as electrodes. Charge separation leads to an electric potential difference (voltage) U between the electrodes. The diagram below shows an example of a plate capacitor with a plate area A and plate spacing d carrying a charge Q. The charge separation produces an electric field (not shown here) between the plates.

CAPACITOR IN ALTERNATING CURRENT AC CIRCUITS !

Between the plates, there is usually an insulator or dielectric (not shown here). The following linear relationship exists between the charge and voltage:

CAPACITOR IN ALTERNATING CURRENT AC CIRCUITS !

The value C is termed the capacitance of the capacitor which is expressed in units called Farad (F). The larger the capacitance, the greater the quantity of stored charge needed to produce a defined potential between the capacitor's electrodes. As an analogy, take a swimming pool where the floor area corresponds to capacitance, water volume to charge quantity and water level to potential. The larger the floor area (capacitance), the more water (charge) is needed to achieve a particular depth (voltage). A capacitor's value can be considered constant, dependent only the geometric design and the dielectric material employed. The following relationship applies to plate capacitors (and also approximately to closely spaced concentric cylinders such as in the case of paper capacitors):

CAPACITOR IN ALTERNATING CURRENT AC CIRCUITS !

e0 is the electric field constant, also called the permittivity of free space, which has a value of 8.8542·10-12 AS/Vm, er is the (unitless) relative permittivity, A is the plate area and d the plate spacing.

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