Definition and Explain of Capacitor

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:
  1. Fixed Capacitors
  2. Variable Capacitors
These two contain different types of capacitors including
  1. Non-polarized
  2. 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 + V
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 = C3
or         C = C1 + C2 + C3 
For such a combination, dV/dt is the same for all capacitors.












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