Charge is a scalar quantity which is categorised into two types.
(a) Positive charge (anciently called Vitreous)
(b) Negative charge (anciently called Resinous)
A body having no charge, is said to be neutral in nature i.e. on a neutral body the sum of positive charges is equal to the sum of negative charges.
The positive charge means deficiency of electrons, whereas the negative charge on a body implies excess of electrons.
The S.I. unit of charge is coulomb (C).
I. UNITS OF CHARGE
(a) S.I. unit of charge is coulomb (C). One coulomb of charge is that charge which when placed at
rest in vacuum at a distance of one metre from an equal and similar stationary charge repels it
and is repelled by it with a force of 9 x 109 N
(b) CGS unit of charge is stat coulomb
1 coulomb = 3 x109esu of charge = (1/10) emu of charge.
i.e. 1 coulomb = 3 x 109 = stat coulomb = 1/10 abcoulomb
(c) Practical units of charge are amp × hr = 3600 coulomb and faraday (= 96500 C)
(d) Smallest unit of charge is stat coulomb and largest is faraday.
II. METHODS OF CHARGING
(a) By Friction : In friction when two bodies are rubbed together electrons are transferred from one body to other. As a result one body becomes positively charged while the other negatively charged.
Examples : When glass rod is rubbed with silk, the rod becomes positively charged while the silk negatively charged. Clouds also become charged by friction
(b) By Induction : If a charged body of same sign is brought near a neutral body charged body will
attract charge of opposite sign and repel charge of same sign present in neutral body. The nature as induced charge is always opposite to that of inducing charge. Charging of body by Induction is shown as :
(c) By Conduction : If a charged body is in direct contact with an uncharged body, charge flows from
former to latter till both are at same potential. This flow of charges is due to mutual repulsions between same kind of charges on charged body.
III. PROPERTIES OF ELECTRIC CHARGE
(a) Like point charges repel each other and unlike charges attract each other.
(b) Charge Conservation
The algebraic sum of all the charges in an isolated system is a constant. In crude language we
can say that charge can neither be created nor be destroyed, however it can simply be
transferred from one body to the other.
(c) Relativistic Invariance
Charge on a body is relativistically invariant. i.e. charge on the body at rest equals the charge
on the body at relativistic speeds. However charge density is not relativistically invariant.
Mathematically
(q)at rest = (q)in motion
Or (q)at rest = (q)at relativistic speeds
(d) Charge Quantisation
Charge on a body q must always exist as an integral multiple of some fundamental unit of charge (called electronic charge) e , where e = 1.6 x 10-19 C.
Mathematically, q = ± ne, n = 1, 2, 3, .....
From here we conclude that a neutral body can have + 1 C of charge when it falls deficient of
6.25 x 1018 electrons.
(e) A charged body can attract light uncharged body. (Due to charging by induction)
(f) Charges are always added algebraically.
For example, if a neutral body is first given a charge of + 5 C and subsequently a charge of
-7 C , then it will finally have a charge of + 5 C - 7 C = - 2 C.
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