Electrostatics and Magnetism (NEET Physics)
Why This Chapter Matters
Electrostatics and Magnetism together carry 8-12 marks in NEET. Coulomb's law, electric field, capacitors, magnetic force, and electromagnetic induction are all tested with numerical problems.
Core Concepts
1. Electrostatics
Coulomb's Law: F = kq₁q₂/r² | k = 9×10⁹ N·m²/C²
Electric field: E = F/q₀ = kQ/r² (due to point charge)
Electric potential: V = kQ/r | E = -dV/dr
Gauss's Law: ∮E·dA = q_enc/ε₀
Capacitor:
C = Q/V = ε₀A/d (parallel plate)
With dielectric κ: C = κε₀A/d (κ increases capacitance)
Energy stored: U = ½CV² = Q²/2C
Series: 1/C = 1/C₁ + 1/C₂ | Parallel: C = C₁ + C₂
2. Current Electricity Review
Ohm's Law: V = IR | P = VI = I²R = V²/R
Kirchhoff's Laws (KVL + KCL)
Wheatstone bridge: P/Q = R/S (balanced condition)
3. Magnetic Effects of Current
Biot-Savart Law: dB = (μ₀/4π)(Idl × r̂)/r²
Magnetic field due to infinite straight wire: B = μ₀I/2πr
Circular loop (centre): B = μ₀I/2R
Solenoid (inside): B = μ₀nI
Lorentz Force: F = q(v × B) = qvBsinθ
No work done by magnetic force.
Circular motion in B: r = mv/qB, T = 2πm/qB (period independent of speed)
Force on current-carrying conductor: F = BILsinθ
Force between parallel wires: F/L = μ₀I₁I₂/2πd
Same direction: attract | Opposite: repel
4. Electromagnetic Induction
Faraday's Laws:
ε = -dΦ/dt (Lenz's law gives negative sign)
Lenz's Law: Induced current opposes the change causing it.
Self-inductance: ε = -L(dI/dt) | Energy stored in inductor: U = ½LI²
AC Generator: e = e₀sinωt | e₀ = NBAω (N=turns, B=field, A=area, ω=angular velocity)
Transformer: V₁/V₂ = N₁/N₂ | Step-up: N₂ > N₁ | Step-down: N₂ < N₁
Power: V₁I₁ = V₂I₂ (ideal transformer, no losses)
PYQs (NEET)
NEET 2023: Two charges +4μC and -4μC separated by 0.2 m. Electric potential at midpoint?
At midpoint (0.1m from each): V = kq₁/r₁ + kq₂/r₂ = 9×10⁹×4×10⁻⁶/0.1 + 9×10⁹×(-4×10⁻⁶)/0.1 = 0 V
(Equal and opposite potentials cancel)
NEET 2022: A proton is moving in a magnetic field. If speed is doubled, radius of circular path?
r = mv/qB → r doubles (r ∝ v, since m, q, B unchanged)
NEET 2021: Transformer output voltage is 220 V from 11000 V input. Turns ratio N₁/N₂?
V₁/V₂ = N₁/N₂ → 11000/220 = 50. Step-down transformer, N₁:N₂ = 50:1