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.