CAPACITANCE:-A capacitor essentially consists of two conducting surfaces separately,vy a layer of an insulating medium called dielectric. The conducting surfaces may be in the formof eithercircular (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).BA parallel-plate capacitor is shown in Fig. 5.1. One plate isjoined to the positive end of the supply and the other to the negative end or is
earthed. It is experimentally found that in the presence of an earthed plate B, plate A is capable of withholding more charge than when B is not there. When such a capacitor is put across a battery, ~ere is a momentary
flow of electrons from A to B. As negatively-charged electrons are withdrawn from A, it becomes positive and as these electrons collect on B, it becomes negative. Hence, a p.d. is established between plates A and
B. The transient flow of elec~ons gives rise to charging current. The strength of the charging current is maximum v,~pn the two plates are uncharged but it then decreases and finally ceases when p.d. across the
plates becomes slowly and slowly equal and opposite to the battery e.m.f.
B
+
+
+
Fig. 5.1
5.2. Capacitance
The property of a capacitor to 'store electricity' may be called its capacitance.
As we may measure the capacity of a tank, not by the total mass or volume of water it can hold, but by the mass in kg of water required to raise its level by one metre, similarly, the capacitance of a capacitor is defined as "the amount of charge required to create a unit p.d. between its plates." Suppose we give Q coulomb of charge to one of the two plate of capacitor and if a p.d. of V volts is established between the two, then its capacitance is C = Q = charge
V potential diffence Hence, capacitance is the charge required per unit potential difference. By definition, the unit of capacitance is coulomb/volt whi~h is also calledfarad (in honour of Michael Faraday)
.. 1 farad = I 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 (J.1F),nanofarad (oF) and micro-microfarad (J..I.I.lF)or picofarad (pF) are generally employed. 1 J.IF = 10-9 F; I of = 10-9 F ; J..I.I.lF or pF = 1O-12F Incidentally, capacitance is that property of a capacitor which delays and change of voltage.
Thank you. written by. Abhishek Singh E.E
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