Electric Charges and Fields Class 12 Notes Chapter 1

1. Electric Charge Charge is the property associated with the matter due to which it produces and experiences electric and magnetic effect.
2. Conductors and Insulators Those 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 electrons is the cause of frictional electricity.
4. Additivity of Charges Charges are scalars and they add up like real numbers. It means if a system consists of n charges q1, q2, q3 , … ,qn, then the total charge of the system will be q1 +q2 + … +qn.
5. Conservation of Charge The total charge of an isolated system is always conserved, i.e. initial and final charge of the system will be the same.
6. Quantisation of Charge Charge exists in discrete amount rather than continuous value and hence, quantized.
Mathematically, charge on an object, q=±ne
where, n is an integer and e is an electronic charge. When any physical quantity exists in discrete packets rather than in continuous amount, the quantity is said to be quantized. Hence, the charge is quantized.
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 109 stat-C
8. Coulomb’s Law It states that the electrostatic force of interaction or repulsion acting between two stationary point charges is given by