⚡
Class 12 PhysicsChapter Notes
2 chapters · Definitions, key points, formulas & exam tips
Ch 1
Electric Charges and Fields
Key Definitions
Electric Charge: A fundamental property of matter. Two types: positive and negative. Unit: Coulomb (C).
Coulomb's Law: Force between two point charges: F = kq₁q₂/r². Directly proportional to product of charges, inversely to square of distance.
Electric Field: Force per unit positive charge at a point. E = F/q. Unit: N/C or V/m.
Key Points to Remember
- →Charge is quantised: q = ne where e = 1.6 × 10⁻¹⁹ C.
- →Charge is conserved — total charge in an isolated system remains constant.
- →Electric field lines: start from positive, end at negative charge. Never cross.
- →Gauss's Law: total electric flux through a closed surface = q/ε₀.
- →Electric field inside a conductor = 0.
- →Field due to infinite plane sheet of charge: E = σ/2ε₀.
Formulas & Equations
F = kq₁q₂/r² = q₁q₂/(4πε₀r²)
E = F/q = kq/r²
Electric flux: Φ = E·A cosθ
Gauss's Law: Φ = q_enclosed/ε₀
Exam Tips
💡
Principle of superposition: net force = vector sum of individual forces.
💡
Electric dipole: two equal and opposite charges separated by distance 2l. p = q × 2l.
💡
Torque on dipole in field: τ = pE sinθ.
Ch 2
Electrostatic Potential and Capacitance
Key Definitions
Electric Potential: Work done per unit positive charge in bringing it from infinity to a point. Unit: Volt (V).
Capacitance: Ability to store charge. C = Q/V. Unit: Farad (F).
Dielectric: Insulating material between capacitor plates that increases capacitance.
Key Points to Remember
- →Equipotential surface: potential is same at all points. Field is perpendicular to it.
- →Work done in moving charge on equipotential surface = 0.
- →Capacitors in series: 1/C = 1/C₁ + 1/C₂. In parallel: C = C₁ + C₂.
- →Energy stored: U = ½CV² = Q²/2C = QV/2.
- →Dielectric increases capacitance by factor K (dielectric constant).
- →Van de Graaff generator: accumulates high voltage using electrostatic principles.
Formulas & Equations
V = kq/r
C = ε₀A/d (parallel plate)
Energy: U = ½CV²
With dielectric: C = Kε₀A/d
Exam Tips
💡
Potential due to a dipole: V = kp cosθ/r² — direction matters.
💡
Common capacitor questions: find equivalent capacitance of networks.
💡
Effect of inserting dielectric: if battery connected — charge increases, V unchanged; if disconnected — V decreases, charge unchanged.