Electric Charges and Fields
Class 12 Notes Chapter 1
1. Electric ChargeCharge is the property associated with matter
due to which it produces and experiences electric and magnetic
effect.
2. Conductors and InsulatorsThose substances which readily
allow the passage of electricity through them are called conductors,
e.g. metals, the earth and those substances which offer high
resistance to the passage of electricity are called insulators, e.g.
plastic rod and nylon.
3. Transference of electronsis the cause of frictional electricity.
4. Additivity of ChargesCharges are scalars and they add up like
real numbers. It means if a system consists of n charges q1, q2, q3,
… ,qn, then total charge of the system will be q1+q2+ … +qn.
5. Conservation of ChargeThe total charge of an isolated system
is always conserved, i.e. initial and final charge of the system will be
same.
6. Quantisation of ChargeCharge exists in discrete amount
rather than continuous value and hence, quantised.
Mathematically, charge on an object, q=±ne
where, n is an integer and e is electronic charge. When any physical
quantity exists in discrete packets rather than in continuous
amount, the quantity is said to be quantised. Hence, charge is
quantised.
7. Units of Charge
(i) SI unit coulomb (C)
(ii) CGS system
(a) electrostatic unit, esu of charge or stat-coulomb (stat-C)
(b) electromagnetic unit, emu of charge or ab-C (ab-coulomb)
1 ab-C = 10 C, 1 C = 3 x 109stat-C
8. Coulomb’s LawIt states that the electrostatic force of
interaction or repulsion acting between two stationary point charges
is given by
