1. Electric Charge: Charge is the property associated with 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. 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 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, quantised.
Mathematically, charge on an object, q = ±ne, where n is an integer and e is the electronic charge. When any physical quantity exists in discrete packets rather than continuous amount, it is said to be quantised.
7. Units of Charge:
(i) SI unit coulomb (C)
(ii) CGS system: (a) electrostatic unit (esu) or stat-coulomb (stat-C)
(b) electromagnetic unit (emu) or ab-C (ab-coulomb)
1 ab-C = 10 C, 1 C = 3 × 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...
Note: Electrostatic forces (Coulombian forces) are conservative forces.
9. Principle of Superposition of Electrostatic Forces: The net electric force experienced by a charge particle q₀ due to a system of charged particles is equal to the vector sum of the forces exerted on it due to all other charged particles in the system.
10. Electrostatic Force due to Continuous Charge Distribution: Region where charges are closely spaced and forms continuous distribution of charge, of three types: linear, surface, and volume distributions.
11. Electric Field Intensity (EFI): The electric field intensity at any point due to a source charge is defined as the force experienced per unit positive test charge placed at that point, expressed as E = F/q.
12. EFI due to a point charge and electric field due to a system of charges are calculated with application of Coulomb’s law and principle of superposition.
13. Electric Field Lines: Pictorial representation of the electric field around configuration of charges. Lines start on positive charges and end on negative charges. The tangent at any point on the lines gives the direction of the field at that point.
14. Electric Dipole: Two point charges equal in magnitude and opposite in nature separated by a small distance form an electric dipole. Its dipole moment (p) = charge (q) × separation distance (2l).
15. Electric Field due to a Dipole: The space around an electric dipole in which its electric effect can be experienced.
16. Torque on Electric Dipole placed in Uniform Electric Field (E): τ = p × E × sin θ. The dipole is in stable equilibrium when p and E are aligned (θ=0°) and unstable when θ=180°.
17. Net force on dipole is zero in uniform field, but in non-uniform fields, net force and torque exist. Work done in rotating dipole and potential energy formulas are derived accordingly.
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