Electromagnetic Induction Revision Notes for NEET Physics PDF Download
⚡ ELECTROMAGNETIC INDUCTION · EMI
Faraday's law • Lenz's law • Motional EMF • Self & Mutual inductance • AC generator • Eddy currents • NEET problems
🧲 MAGNETIC FLUX & FARADAY'S LAW
Magnetic flux: ΦB = ∫ B·dA = B A cosθ (for uniform B). Unit: Weber (Wb).
Faraday's law: Induced EMF = – dΦ/dt
For N turns: ε = – N dΦ/dt
If Φ = NBA cosθ and θ varies (rotation): ε = NBA ω sin(ωt) (AC generator).
📐 Magnetic flux through a loop
🔄 LENZ'S LAW (Direction of Induced EMF)
The induced current opposes the change in magnetic flux that produced it.
Lenz's law = conservation of energy.
🖍️ Lenz's law demonstration (magnet falling through coil)
🏃 MOTIONAL EMF
When a conductor moves in a magnetic field, an EMF is induced.
For a rod moving perpendicular to B: ε = Bℓv
Direction: Fleming's right‑hand rule (thumb = motion, index = field, middle = current).
Rotating rod in B: ε = ½ B ω ℓ²
Sliding bar on rails: ε = Bℓv, current = Bℓv/R, power = (Bℓv)²/R.
📏 Motional EMF in a moving rod
🔋 SELF INDUCTION & INDUCTANCE (L)
EMF induced in a coil due to change in its own current: ε = – L (dI/dt).
Self inductance: L = NΦ/I = μ₀ n² A ℓ (solenoid). Unit: Henry (H).
Energy stored in inductor: U = ½ L I²
Inductors oppose sudden changes in current (back EMF).
🔧 Symbol for inductor & growth/decay of current in LR circuit
🔄 MUTUAL INDUCTION
EMF induced in a coil due to changing current in a nearby coil: ε₂ = – M dI₁/dt.
Mutual inductance M depends on geometry, number of turns, and medium.
For two coaxial solenoids: M = μ₀ n₁ n₂ A ℓ.
⚡ AC GENERATOR
Converts mechanical energy to electrical energy. Rotating coil in uniform B induces sinusoidal EMF.
Instantaneous EMF: ε = NBA ω sin(ωt).
Peak EMF: ε₀ = NBA ω. Frequency: f = ω/(2π).
🔄 AC generator principle
🌀 EDDY CURRENTS
Induced circulating currents in bulk conductors when subjected to changing magnetic flux.
Applications: Induction furnaces (heating), electromagnetic braking, damping in galvanometers.
Reduction: Laminating cores (thin insulated sheets) increases resistance, reduces eddy currents.
💡 NEET TIPS & SHORTCUTS
- Induced EMF depends on rate of change of flux, not flux itself.
- For a coil rotating in B, ε = NBAω sin(ωt); average EMF over half cycle = 2NBAω/π.
- In LR circuit, current grows as I = (V/R)(1 – e–Rt/L) and decays as I = I₀ e–Rt/L.
- Mutual inductance M = √(L₁L₂) for perfect coupling.
⚠️ COMMON MISTAKES
- Forgetting negative sign in Faraday's law (Lenz's law).
- Using wrong sign for induced EMF in loops.
- Confusing self‑inductance with mutual inductance.
- Assuming eddy currents are always undesirable – they have practical uses.
📌 QUICK REVISION CARD
Magnetic flux: Φ = B·A = BA cosθ
Faraday's law: ε = – dΦ/dt
Motional EMF: ε = Bℓv
Self inductance (solenoid): L = μ₀ n² A ℓ
Energy in inductor: U = ½ L I²
AC generator: ε = NBA ω sin(ωt)
LR time constant: τ = L/R
Mutual inductance: M = k √(L₁L₂)