Class 12 Physics Electromagnetic Induction Notes

Class 12 Physics Electromagnetic Induction Notes

6.1 Introduction

Electromagnetic induction explains how a changing magnetic field produces an electric current. This principle is fundamental to generators, transformers, and electrical machines.

6.2 The Experiments of Faraday and Henry

  • Faraday’s Experiment (1831): Moving a magnet inside a coil induces current.
  • Henry’s Experiment: Changing current in one coil induces current in a nearby coil.

Key Observation: A changing magnetic field is necessary to produce an induced current.

6.3 Magnetic Flux

Magnetic flux through a surface is the total magnetic field passing through it:ΦB=B⃗⋅A⃗=BAcos⁡θ\Phi_B = \vec{B} \cdot \vec{A} = BA \cos\thetaΦB​=B⋅A=BAcosθ

  • B = magnetic field strength
  • A = area of surface
  • θ = angle between B and normal to surface
  • Unit: Weber (Wb)

6.4 Faraday’s Law of Induction

Faraday’s Law:
The induced EMF in a circuit is equal to the negative rate of change of magnetic flux through the circuit:E=−dΦBdt\mathcal{E} = – \frac{d\Phi_B}{dt}E=−dtdΦB​​

  • The negative sign represents Lenz’s Law (opposition to change).

6.5 Lenz’s Law and Conservation of Energy

  • Lenz’s Law: Induced current always flows in a direction to oppose the change in magnetic flux.
  • Ensures conservation of energy; work is required to change the magnetic flux.

Example: Pulling a magnet out of a coil requires effort because the induced current opposes the motion.

6.6 Motional Electromotive Force

When a conductor moves in a magnetic field, a motional EMF is induced:E=Bℓv\mathcal{E} = B \ell vE=Bℓv

  • ℓ = length of conductor
  • v = velocity perpendicular to magnetic field
  • B = magnetic field strength
  • This principle is used in rail generators and electric meters.

6.7 Inductance

  • Self-Inductance (L): Property of a coil to oppose change in current through it.

EL=−LdIdt\mathcal{E}_L = – L \frac{dI}{dt}EL​=−LdtdI​

  • Unit: Henry (H)
  • Energy stored in inductor:

U=12LI2U = \frac{1}{2} L I^2U=21​LI2

  • Mutual inductance: Change of current in one coil induces EMF in a nearby coil.

6.8 AC Generator

  • An AC generator converts mechanical energy into alternating electrical energy.
  • Works on Faraday’s Law of Induction.
  • EMF produced in a rotating coil:

E=NABωsin⁡(ωt)\mathcal{E} = NAB\omega \sin(\omega t)E=NABωsin(ωt)

  • N = number of turns, A = area of coil, B = magnetic field, ω = angular velocity

Applications: Power generation in homes, industries, and power stations.