The below mentioned article provides a suitable formula for calculating capacitance of the capacitor.
Two metal plates separated by an insulator constitute a capacitor or condenser, namely an arrangement which has the capacity of storing electricity as an excess of electrons on one plate and a deficiency on the other. The insulator between the plates is called dielectric. It may be paper, mica, glass, air or any other insulating material.
The most common type of capacitor used in practice consists of two strips of metal foil separated by strips of waxed paper wound spirally, forming two large surfaces near to each other. A capacitor can be charged or discharged like a storage battery. If electric current is supplied to a capacitor from an external source, the current is stored in the capacitor as electrostatic charge, and the process is known as charging the capacitor.
The quantity of charge is generally denoted by Q and is measured in coulomb. A capacitor is said to have a charge of one coulomb when one ampere current is supplied to it for one second. Thus, if a current of I ampere is supplied to a capacitor for t second, the quantity of charge stored in it-
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Q = I x t coulomb.
Current can be drawn from a charged capacitor similar to a charged storage battery supplying current to load circuits. This process is known as discharging the capacitor.
The potential difference between the plates of a capacitor is nil when the capacitor is – uncharged. As the quantity of charge accumulated in the capacitor increases, the p.d. between the plates rises and the charging current decreases. Finally when the p.d. between the plates becomes equal to that of supply, the flow of current is stopped and the capacitor is considered to be fully charged at this point.
The property of a capacitor to store an electric charge when its plates are at different potentials is referred to as its capacitance. It is generally denoted by ‘C’ and is expressed in farad. A capacitance of one farad may be defined as the capacitance of a capacitor between the plates of which there appears a difference of potential of 1 volt when it is charged by 1 coulomb of electricity.
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Thus, if the quantity of charge stored in a capacitor be Q coulomb when the p.d. applied across its plates is V volt, the capacitance of the capacitor:
C = Q/V farad
or Q = CV coulomb.
Farad is considered to be very large unit, and in practice, capacitance is usually expressed in microfarad.
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1 microfarad = 10-6 farad.