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CBSE Class 12 · Physics

Electromagnetic Induction

Official NCERT chapter from Physics Part I–II (book code leph1). ExamMaster notes are original teaching at CBSE Class 12 depth.

This lesson follows the official chapter “Electromagnetic Induction” in Physics Part I–II. The words below are ExamMaster’s teaching, not a paste from the book. Use the NCERT chapter for the classroom sequence; use these notes to hold the idea without copying exercises or figures.

  • CBSE Class 12
  • Medium level
  • 8 concepts

1The Experiments of Faraday and Henry

Faraday and Henry: a changing magnet-look near a loop made a current as the school experiments. The look forced a new law. Experiment is a look-plus-claim, not a biography dump.

A name-list with no changing-B is a mute history.

Figure. Faraday and Henry: the galvanometer kicks while the magnet is pushed in or pulled out. A parked magnet is silent.

How it works

  1. Name a changing magnet or a moving loopThe look.
  2. Say a current appearedThe claim.
  3. Keep the biography shortA look-story.

2Magnetic Flux

Magnetic flux: Φ = B A cosθ as taught. Flux is the through-count of B. A change of Φ is what induction watches. Weber is the unit if named.

Using B without A as flux is a miss.

Figure. Magnetic flux through a flat loop is B times the area when B is perpendicular to the face. Tilt the loop and the piercing count falls.

How it works

  1. Name B, A, and the angleThe inputs.
  2. Form B A cosθFlux.
  3. Keep a change of Φ as the next heading’s fuelThe link.

3Faraday’s Law of Induction

Faraday’s law: emf = − dΦ/dt as taught (or N times that for N turns). A faster change means a larger emf. Faraday is a rate-of-flux, not a new Coulomb.

A steady B through a still loop as a leftover emf is a miss.

Figure. Faraday’s law: emf equals the rate of change of flux. A large flux that never changes induces nothing; a falling flux does.

How it works

  1. Watch Φ change with timeThe given.
  2. Form −dΦ/dtThe emf.
  3. Keep N if a coil was givenHonest.

Flux-rate emf

Φ falls from 0.4 Wb to 0.1 Wb in 0.1 s. Find |emf| (one turn).

  • ΔΦ0.3 Wb
  • |ΔΦ/Δt|3 V
  • Read3 V as the size

Pro tip. Rate of flux, not Φ itself.

4Lenz’s Law and Conservation of Energy

Lenz: the induced current opposes the change as taught — conservation of energy in a sentence. If Φ-in is rising, the loop makes a field out. Lenz picks the sign the minus already hinted.

A current that helps the change is the leftover Lenz refuses.

Figure. Lenz: the induced current makes a field that opposes the change. An approaching N is met by an induced N, so you do work to push the magnet in.

How it works

  1. Name what Φ is doing (up or down)The change.
  2. Say the induced look fights that changeLenz.
  3. Keep energy as the whyNot a mood.

5Motional Electromotive Force

Motional emf: a rod moving on rails in B as taught, ε = B ℓ v when the taught right-angle sits. Motional is a moving-wire story, still Faraday (area changes). One sliding-rod is enough.

A still rod in a still B as motional emf is a miss.

Figure. Motional emf: the rod slides, the loop area grows, flux through the loop changes, so a current runs around the rails.

How it works

  1. Name B, ℓ, v and the right-angleThe given.
  2. Form B ℓ vMotional emf.
  3. Keep it as changing area if askedFaraday still.

6Inductance

Inductance: self L as emf = −L dI/dt as taught; mutual M if two coils. Henry is the unit. Inductance is an inertia-of-current, not a resistor. A solenoid’s L grows with n^2 if named.

Using R as L because both sit in a coil is a miss.

Figure. Inductance is how strongly a changing current in a coil makes flux — in itself (self) or in a neighbour (mutual) — and so an emf.

How it works

  1. Name L (or M) and a changing IThe given.
  2. Form −L dI/dtThe emf.
  3. Keep it an inertia-wordNot R.

7AC Generator

AC generator: a coil rotating in B as taught, ε = ε0 sinωt. Generator is Faraday on a loop that keeps changing flux. Slip rings if named. One rotate-story is enough.

A battery labelled generator because it has terminals is a miss.

Figure. An AC generator spins a coil in a field. Slip rings keep the contacts; each half-turn the induced emf reverses, so the output is alternating.

How it works

  1. Name a rotating coil in BThe generator.
  2. Write a sine-emf as taughtThe output.
  3. Keep Faraday as the whyChanging Φ.

8A definition is a test you can run

Induction-word is a test: flux, dΦ/dt, Lenz, motional, L, or a generator. If you only say “magnet”, you have a heading.

A spinning-sticker is not the test.

Figure. A definition is a test: flux is what you get when you measure how much B pierces a named area A. If you cannot name B and A, you do not have flux yet.

How it works

  1. Name Φ-change or Lenz or LThe object.
  2. Give the school sentenceThe test.
  3. Then the word has contentThe definition ran.
Faraday’s emf tracks
  1. How fast flux changes
  2. B alone
  3. q v B only

A rate.

Notes

  • Mapped to the official NCERT chapter “Electromagnetic Induction”. Original teaching only — no textbook sentences.
  • Science here is Physics, Chemistry and Biology ideas at this class, never a language or social-science chapter.

Formulas

  • Φ=B A cosθ
  • emf=−dΦ/dt
  • ε=B ℓ v
  • emf=−L dI/dt

Recap

Hold these pegs from the official chapter “Electromagnetic Induction”. The wording is ExamMaster’s teaching, not a textbook recap.

The Experiments of Faraday and Henry
Faraday and Henry: a changing magnet-look near a loop made a current as the school experiments.
Magnetic Flux
Magnetic flux: Φ = B A cosθ as taught. Flux is the through-count of B. A change of Φ is what induction watches. Weber is the unit if named.
Faraday’s Law of Induction
Faraday’s law: emf = − dΦ/dt as taught (or N times that for N turns).
Lenz’s Law and Conservation of Energy
Lenz: the induced current opposes the change as taught — conservation of energy in a sentence.
Motional Electromotive Force
Motional emf: a rod moving on rails in B as taught, ε = B ℓ v when the taught right-angle sits.
Inductance
Inductance: self L as emf = −L dI/dt as taught; mutual M if two coils.

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