Definition and Explain of Capacitor

Capacitor

In October 1745, Ewald Georg von Kleist of Pomerania, Germany, found that charge could be stored by connecting a high-voltage electrostatic generator by a wire to a volume of water in a hand-held glass jar

Pic-Glass jar

A capacitor essentially consists of two conducting surfaces separated by a layer of an insulating medium called a dielectric. The conducting surfaces may be in the form of either circular (or rectangular) plates or be of spherical or cylindrical shape. The purpose of a capacitor is to store electrical energy by electrostatic stress in the dielectric (the word ‘condenser is a misnomer since a capacitor does not ‘condense’ electricity as such, it merely stores it).

Capacitance

The property of a capacitor to ‘store electricity’ may be called its capacitance.

similarly, the capacitance of a capacitor is defined as “the amount of charge required to create a unit potential difference between its plates.”

Suppose we give Q coulomb of charge to one of the two plates of the capacitor and if a potential difference. of V volts is established between the two, then its capacitance is

C =Q/V = Charge/Potential difference

Hence, capacitance is the charge required per unit potential difference.

By definition, the unit of capacitance is coulomb/volt which is also called farad (in honour of Michael Faraday)

∴ 1 farad = 1 coulomb/volt

One farad is defined as the capacitance of a capacitor which requires a charge of one coulomb to establish a p.d. of one volt between its plates.

One farad is actually too large for practical purposes. Hence, much smaller units like microfarad (μF), nano-farad (nF), and micro-microfarad (μμF) or picofarad (pF) are generally employed.

Incidentally, capacitance is that property of a capacitor which delays and change of voltage across it.

Different types of capacitors

There are two major types of capacitors:

Fixed Capacitors
Variable Capacitors

These two contain different types of capacitors including

Non-polarized
Polarized

Fixed capacitors of different types:

Ceramic capacitors
Electrolytic capacitors
Film and paper capacitors
Supercapacitors
Glass, air-gap, vacuum, silicon, silver mica capacitors

Capacitors in Series

With reference to the Fig Series circuit,

let

C1, C2, C3 = Capacitances of three capacitors

V1, V2, V3 = p.ds. across three capacitors.

V = applied voltage across the combination

C = combined or equivalent or joining capacitance.

In series combination, the charge on all capacitors is the same but the potential difference across each is different.

∴ V = V1 + V2 + V3

or Q/C = Q/C1+Q/C2+Q/C3

or 1/C = 1/C1+1/C2+1/C3

For a changing applied voltage,

dV/dt = dV1/dt/dV2/dt dV3/dt

We can also find values of V1, V2, and V3 in terms of V.

Now, Q = C1V1 = C2V2 = C3V3 = CV

where C =(C1C2C3)/(C1C2+C2C3+C3C1) = (C1C2C3)/ (ΣC1C2)

∴ C1V1 = C V

or V1 = V(C1/C ) =V(C2C3)/ (ΣC1C2)

Similarly, V2 = =V(C1C3)/ (ΣC1C2)

V3 = =V(C1C2)/ (ΣC1C2)

Capacitors in Parallel

In this case, the potential difference across each is the same but the charge on each is different (Fig.Parallel Circuit).

∴ Q = Q1+Q2+Q3

or CV = C1V = C2V = C3V

or C = C1 + C2 + C3

For such a combination, dV/dt is the same for all capacitors.