Electromagnetic Induction Class 12 Physics Notes

Q: What are energy losses in transformer Class 12?

Five losses: copper loss (I²R heating in windings use thicker wire), eddy current loss (heating in core — use laminated core), hysteresis loss (magnetising/demagnetising core each cycle use soft iron), flux leakage (not all flux links secondary use toroidal design), magnetostriction (core vibration causing humming sound).

Q: Which electromagnetic induction topics are most important for Exam?

Most important: Faraday's law with Lenz's law (4 marks), AC generator working and EMF derivation (4 marks), transformer turns ratio and losses (4 marks), self and mutual inductance (2 marks), eddy currents applications (2 marks). AC generator and transformer appear in almost every NEB Class 12 board paper.

NEB Class 12 Physics Electromagnetic Induction Notes in PDF Complete Handwritten. Physics Notes 2082: All Chapters | New Curriculum | Class 12 Physics Notes download.

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NEB Class 12 Physics Electromagnetic Induction Note Handwritten in PDF

Faraday’s Laws of Electromagnetic Induction

Faraday’s First Law: Whenever there is a change in magnetic flux through a circuit, an EMF is induced in the circuit.

Faraday’s Second Law: The magnitude of the induced EMF is directly proportional to the rate of change of magnetic flux:

ε = −dΦ/dt (for single turn)

ε = −N dΦ/dt (for N turns)

The negative sign is Lenz’s Law (explained below).

Magnetic flux: Φ = BA cosθ Flux changes when B changes, or A changes, or θ changes (coil rotates).

Lenz’s Law

Statement: The direction of induced current is always such as to oppose the change in magnetic flux that causes it.

Why the negative sign in Faraday’s law: The induced EMF always creates a current that opposes the change in flux. If flux is increasing, induced current creates field to oppose increase. If flux is decreasing, induced current creates field to oppose decrease.

Energy conservation: Lenz’s law is a consequence of energy conservation. If induced current aided the flux change instead of opposing it, we would get increasing current → increasing force → increasing EMF → more current — a self-sustaining process that creates energy from nothing. Lenz’s law prevents this.

Motional EMF

When a conductor of length L moves with velocity v perpendicular to magnetic field B:

ε = BLv (motional EMF)

Derivation: Charges in the conductor experience Lorentz force → charge separation → potential difference = work done per unit charge = BLv.

If the conductor is part of a circuit, this EMF drives a current.

AC Generator Construction and Working

Construction:

Rectangular coil (armature) with N turns
Uniform magnetic field (permanent magnets or electromagnet)
Slip rings connected to ends of coil
Brushes in contact with slip rings
External circuit connected through brushes

Working: As coil rotates with angular velocity ω:

Flux: Φ = NBA cosωt
Induced EMF: ε = −dΦ/dt = NBAω sinωt = ε₀ sinωt
Where ε₀ = NBAω is the peak EMF
The output is sinusoidal AC

Slip rings (vs commutator in DC generator):

Slip rings maintain continuous connection without reversing → sinusoidal AC output
Commutator reverses connections every half cycle → pulsating DC output

Eddy Currents

When a solid conducting material moves through a magnetic field (or when B changes through it), induced EMFs drive currents in closed loops within the material these are eddy currents.

Where eddy currents are harmful:

Transformer and motor cores: Eddy currents cause heating (energy loss)
Solution: Use laminated cores (thin sheets insulated from each other) → longer path → higher resistance → smaller eddy currents

Where eddy currents are useful:

Induction heating (induction stoves, induction furnaces)
Electromagnetic braking (in trains, roller coasters)
Metal detectors (eddy currents disturbed by metal → detected)
Speedometers in old cars (rotating magnets induce eddy currents in aluminium disc)

Self-Inductance and Mutual Inductance

Self-inductance (L): When current through a coil changes, the changing magnetic flux induces a back-EMF in the same coil.

ε_back = −L(dI/dt)

L = self-inductance (Henry), always opposes change in current (like electrical inertia).

Energy stored in inductor: U = ½LI²

Mutual inductance (M): When current in coil 1 changes, it induces EMF in nearby coil 2:

ε₂ = −M(dI₁/dt)

M depends on geometry, number of turns, and coupling between coils. Used in transformers.

Transformer Working and Efficiency

Turns ratio: V₂/V₁ = N₂/N₁

For ideal transformer (100% efficient): V₂/V₁ = N₂/N₁ = I₁/I₂

Step-up transformer: N₂ > N₁ → V₂ > V₁, I₂ < I₁ Step-down transformer: N₂ < N₁ → V₂ < V₁, I₂ > I₁

Power losses in real transformers:

Copper loss: I²R heating in windings reduced by thicker wire
Iron loss (eddy current loss): Heating in core reduced by laminated core
Hysteresis loss: Energy to repeatedly magnetize/demagnetize core reduced by soft iron core
Flux leakage: Not all flux from primary links secondary reduced by toroidal design
Magnetostriction: Core vibration (the humming sound)

Typical large power transformers achieve 98–99% efficiency.

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Frequently Asked Questions

What is Faraday’s law of electromagnetic induction Class 12?

Faraday’s law: induced EMF ε = −N dΦ/dt. Induced EMF equals negative rate of change of magnetic flux times number of turns. Negative sign represents Lenz’s law induced EMF opposes the change causing it. Flux changes when B changes, area changes, or the angle between them changes.

What is Lenz’s law in Class 12 Physics?

Lenz’s law: induced current always flows in a direction to oppose the change in magnetic flux that produces it. This is why the negative sign appears in Faraday’s law. Lenz’s law is a consequence of conservation of energy if induced current aided the flux change, energy would be created from nothing.

What is the working principle of AC generator Class 12?

Rotating coil (N turns, area A) in uniform magnetic field B generates EMF: ε = NBAω sinωt = ε₀ sinωt, where ε₀ = NBAω is peak EMF. As coil rotates, flux changes sinusoidally producing sinusoidal AC. Slip rings maintain continuous connection without current reversal giving alternating current output to the external circuit.

What is the difference between self-inductance and mutual inductance?

Self-inductance (L): coil opposes change in its own current ε = −L(dI/dt). It is electrical inertia of the coil. Mutual inductance (M): changing current in one coil induces EMF in a nearby coil ε₂ = M(dI₁/dt). Self-inductance is a coil’s own inertia; mutual inductance is coupling between two coils used in transformers.

What are eddy currents and their applications Class 12?

Eddy currents are induced currents in solid conductors when magnetic flux changes through them. Harmful in transformers (cause heating losses) reduced using laminated cores. Useful applications: induction heating (induction stoves, furnaces), electromagnetic braking (trains, roller coasters), metal detectors, and speedometers in older vehicles.

What is the transformer equation and turns ratio?

Transformer turns ratio: V₂/V₁ = N₂/N₁ = I₁/I₂ for ideal transformer. Step-up: N₂ > N₁ increases voltage, decreases current. Step-down: N₂ < N₁ decreases voltage, increases current. Power input = power output for ideal transformer. Real transformers are 95-99% efficient due to copper loss, iron loss, and flux leakage.

What are energy losses in transformer Class 12?