Electromagnetic Induction — Class 12 MCQs with Answers

Electromagnetic Induction — important questions & MCQs with answers (Class 12 Physics)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   Induced EMF =

   AdΦ/dtB−dΦ/dtCΦ·tDΦ/t
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   Answer: −dΦ/dt

   Explanation: Faraday's law gives EMF = −dΦ/dt; the minus sign (Lenz's law) shows the induced EMF opposes the flux change that caused it. Dropping the sign leaves only magnitude, not direction.

2. Q2 of 6Easy

   Lenz's law follows from:

   ACharge quantisationBNewton's first lawCCoulomb's lawDConservation of energy
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   Answer: Conservation of energy

   Explanation: Lenz's law follows from conservation of energy — if induced current aided the change instead of opposing it, the system would gain energy for free, which is forbidden.

3. Q3 of 6Medium

   Motional EMF: ε =

   AB/vBBLvCL/BDBLv²
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   Answer: BLv

   Explanation: Motional EMF for a rod of length L sweeping speed v through field B is ε = BLv — linear in each quantity, so doubling v exactly doubles ε.

4. Q4 of 6Medium

   Assertion (A): When the North pole of a bar magnet is moved away from a closed coil, the face of the coil nearer the magnet acquires North polarity. Reason (R): The induced current in a closed coil flows in a direction that opposes the change in magnetic flux that produces it.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: A is false but R is true

   Explanation: A is false: the flux through the coil decreases, so the induced current tries to maintain it, making the near face a South pole that attracts the receding magnet. R is true; it is Lenz's law.

5. Q5 of 6Hard

   Mutual inductance unit:

   ATeslaBHenryCWeberDVolt
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   Answer: Henry

   Explanation: Mutual inductance, like self-inductance, is defined through ε = −M dI/dt, so it carries the same SI unit, the henry.

6. Q6 of 6Hard

   Assertion (A): The mutual inductance of a small circular coil placed coaxially at the centre of a much larger circular coil (r₁ ≪ r₂) can be found by taking the larger coil's field to be uniform over the small coil's area. Reason (R): The field of the larger coil changes very little over the small region near its centre that the smaller coil occupies.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: Both A and R are true and R is the correct explanation of A

   Explanation: A is true and R is true. Because r₁ ≪ r₂, the larger coil's field near its centre is nearly the same over the small area, so the flux is simply πr₁² times the centre field; R is exactly why the approximation works.

Electromagnetic Induction notes: key concepts (Class 12 Physics)

How a changing magnetic field creates an EMF: magnetic flux, Faraday's law and Lenz's law with its basis in energy conservation, motional EMF, eddy currents and their uses, self and mutual inductance, the energy stored in an inductor, and the AC generator.

Magnetic flux

The product of field, area and the cosine of the angle between the field and the area's normal, measured in webers. It is a scalar.

Faraday's law of induction

The induced EMF equals the negative rate of change of magnetic flux. What matters is the RATE of change, not the size of the flux itself.

Ways of changing flux

Flux can be changed by varying the field, the area, or the orientation of the coil — each of these induces an EMF.

Lenz's law

The induced current always opposes the change producing it. The minus sign in Faraday's law is this law in mathematical form.

Lenz's law and energy

If the induced current aided the change, energy would be created from nothing. Lenz's law is therefore conservation of energy applied to induction.

Motional EMF

A conductor of length L moving with speed v across a field B develops an EMF BLv, which can be derived directly from the magnetic force on its free electrons.

Eddy currents

Circulating currents induced in bulk conductors by changing flux. They dissipate energy as heat, which is a loss in transformers but useful elsewhere.

Uses and reduction of eddy currents

Exploited in induction furnaces, electromagnetic braking and induction cooktops; suppressed in transformer cores by laminating them.

Self-inductance

A coil's opposition to a change in its own current, arising from the flux it links with itself. It is measured in henry.

Inductance of a solenoid

Proportional to the square of turns per unit length, the area and the length, so winding more turns raises inductance sharply.

Mutual inductance

The EMF induced in one coil by a changing current in a neighbouring coil — the principle on which every transformer works.

Energy stored in an inductor

Equal to ½LI², held in the magnetic field. This is why breaking an inductive circuit produces a spark as the stored energy discharges.

AC generator

A coil rotating in a magnetic field produces a sinusoidal EMF, converting mechanical energy into electrical energy. Its peak EMF is NBAω.

Induced electric field

A changing magnetic field creates an electric field even where no conductor exists. Unlike the electrostatic field, it is non-conservative.