Capacitance – Definition, Formula & Unit

In this topic, you study Capacitance – Definition, Formula & Unit.

Two conducting surfaces or plates separated by an insulating medium called dielectric form a capacitor (or a condenser).

The dielectric mediums commonly employed in the construction of the capacitor are air, glass, mica, paraffined paper, polystyrene, mineral oil, etc.

The property of a capacitor to store an electric charge when a potential difference is applied to it is known as its capacitance.

Capacitance of a Capacitor

The charge stored by a capacitor is always proportional to the applied voltage. Therefore, the relation between the applied voltage (V) and the charge (Q) in a capacitor can be mathematical expressed as

or Q = CV

where C is the constant of proportionality.

(3.14)

More the value of this constant, more is the ability of the capacitor to store the charge for a given applied voltage. Thus, the constant C in the above expression gives the quantitative measure of the capacitance and therefore, itself called as capacitance of a capacitor.  The unit of capacitance is farad (symbol, F).

In the Expression (1), if

Q e 1 c,v-1V, then CZIF.

Hence, a capacitor is said to have a capacitance Of one farad if it acquires a charge Of one coulomb when a potential difference Of one volt is a lied across its terminals.

In practice, the farad is found to be a very large unit. Therefore, capacitance is usually measured in microfarads (uF) or in picofarads (pF).

IF 106gF 1012 PF or ggF

Now, from Equation (3.14),

Therefore, the capacitance Of a capacitor in farads is given by the ratio Of its charge in coulombs to the volta e a lied across its terminals in volts.

It should be remembered that when a capacitor is fully charged, the potential difference established across its plates is always equal to the voltage applied across its terminals during charging.

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