Eddy Currents

8 MCQs9-step worked example

Source: NCERT Electromagnetic Induction and Alternating CurrentsOfficial key: NTA-verifiedLast updated: 26 Sep 2026

Eddy Currents, explained for NEET

The confusion that costs marks here is treating eddy currents as a different law. They are not. An eddy current is ordinary induced current — Faraday's law, Lenz's law, no new physics — that happens to circulate inside the bulk of a conductor instead of along a wire loop.

NCERT Class 12 Physics (pre-2023 edition), Chapter 6, page 218 (NCERT removed eddy currents from the current book in 2023; the NEET (UG) 2026 syllabus still lists them) describes what happens when a solid metal plate moves through a non-uniform magnetic field, or sits in a changing one. The flux through any closed path you can trace inside the metal changes. The metal is a conductor, so that path is a circuit. Current flows. Because the conducting body is continuous rather than a single defined loop, the currents close on themselves in swirling patterns — hence "eddy."

Two consequences follow, and NEET tests the pair.

Direction. Lenz's law fixes it: the eddy current opposes the change that produced it. A metal plate swinging into a field region is retarded. A magnet dropped down a copper pipe falls slowly. The plate is not magnetic; the opposition is entirely induced.

Dissipation. The eddy current flows through the metal's own resistance, so energy appears as heat. In a transformer core this is a loss — which is why cores are laminated: thin sheets separated by insulating varnish break the large circulating paths, cutting the current and the I²R heating. In an induction furnace or an induction cooktop the same heating is the point.

The design lesson is worth holding: lamination does not oppose the flux, it only cuts the conducting paths transverse to the eddy loops. Sheets are stacked parallel to the flux direction.

Watch-out: eddy-current braking gives a force that vanishes when motion stops, because zero relative motion means zero flux change. It cannot hold a stationary object — a classic wrong option.

Can you answer these Eddy Currents MCQs?

Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.

MCQ 1Easy RecallPractice

Eddy currents are best described as

Show answer and why every option is right or wrong

Answer: A. A is correct. NCERT Class 12 Physics (pre-2023 edition), Chapter 6, page 218 defines eddy currents as currents induced in the body of a conductor when the magnetic flux linked with it changes; they circulate in closed loops within the bulk of the metal.

Why B is wrong: B is wrong because eddy currents are not restricted to the surface, and they arise in ordinary conductors such as copper and aluminium that are not magnetised at all.

Why C is wrong: C is wrong because no contact is required — the changing flux does all the work, which is why a magnet falling through a copper pipe without touching it is still retarded.

Why D is wrong: D is wrong because it inverts cause and effect: the conductor need not be magnetised, and it is the external changing flux, not any intrinsic magnetisation, that drives the current.

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MCQ 2Concept TrapPractice

A copper plate is made to swing as a pendulum so that it passes between the poles of a strong magnet. Its oscillations die out much faster than those of the same plate swinging with the magnet removed. The correct reason is that

Show answer and why every option is right or wrong

Answer: B. B is correct. As the plate enters and leaves the pole region the flux through it changes, inducing eddy currents; by Lenz's law their effect opposes the motion, and the currents dissipate the plate's kinetic energy as resistive heat — the damping described in NCERT Class 12 Physics (pre-2023 edition), Chapter 6, page 218.

Why A is wrong: A is wrong because copper is not ferromagnetic and acquires no appreciable magnetisation; the retarding effect appears in any good conductor and disappears if the plate is replaced by an insulator of the same shape.

Why C is wrong: C is wrong because a magnetic field does not alter the air's viscous drag; the damping persists and is in fact stronger in vacuum-free laboratory demonstrations where drag is unchanged.

Why D is wrong: D is wrong because radiative losses at pendulum speeds are utterly negligible; the energy is accounted for by I²R heating in the plate.

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MCQ 3Concept TrapPractice

Two identical metal plates are swung between the poles of a magnet. One is solid; the other has several deep slots cut into it. The slotted plate swings noticeably longer before stopping. The reason is that the slots

Show answer and why every option is right or wrong

Answer: C. C is correct. NCERT Class 12 Physics (pre-2023 edition), Chapter 6, page 218 notes that cutting slots in the plate breaks up the large eddy-current loops; the induced currents are then confined to small regions, are much weaker, and dissipate far less energy, so the damping falls.

Why A is wrong: A is wrong because less mass would mean less kinetic energy to remove as well, and the observed effect is a change in the damping rate, not in the starting energy; a lighter solid plate is still damped strongly.

Why B is wrong: B is wrong because the slots are narrow and the flux through the plate's outline is essentially unchanged — what changes is the availability of conducting paths for the induced current to follow.

Why D is wrong: D is wrong because cutting slots does not raise permeability, and in any case a higher permeability would strengthen the flux rather than prevent induction.

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MCQ 4Easy RecallPractice

The core of a transformer is built from thin sheets of iron insulated from one another rather than from a single solid block. The purpose of this lamination is to

Show answer and why every option is right or wrong

Answer: A. A is correct. NCERT Class 12 Physics (pre-2023 edition), Chapter 6, page 218 gives laminated cores as the standard remedy for eddy currents: the insulating layers confine the induced currents to individual thin sheets, greatly reducing the resistive heating in the core.

Why B is wrong: B is wrong because the number of turns depends on the winding, not on whether the core is laminated; a solid core of the same dimensions would accept the same windings.

Why C is wrong: C is wrong because lamination addresses induced currents, not retentivity; permanent magnetisation of the core is managed by choosing a soft magnetic material, not by slicing it.

Why D is wrong: D is wrong because thermal expansion is not the design concern here — the sheets are insulated from each other precisely to block current, and a solid core would tolerate the temperature rise equally well mechanically.

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MCQ 5Concept TrapPractice

In a laminated transformer core, the thin sheets should be stacked so that their planes are

Show answer and why every option is right or wrong

Answer: B. B is correct. The eddy-current loops lie in planes perpendicular to the flux, so the insulating boundaries must slice across those loops while leaving the flux path continuous — achieved by stacking the laminations parallel to the flux direction, as required by the eddy-current remedy described in NCERT Class 12 Physics (pre-2023 edition), Chapter 6, page 218.

Why A is wrong: A is wrong because insulating gaps across the flux path would introduce air gaps in the magnetic circuit, sharply reducing the flux the core carries and defeating the core's purpose.

Why C is wrong: C is wrong because no such compromise is needed or used: one orientation simultaneously preserves the flux path and cuts the current loops, so there is nothing to trade off.

Why D is wrong: D is wrong because the insulation is only effective against current crossing it — current can still circulate freely within the plane of each sheet, so orientation decides whether the large loops survive.

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MCQ 6Direct ApplicationPractice

A strong bar magnet is released from rest at the top of a long vertical copper pipe whose inner diameter is slightly larger than the magnet. The magnet is observed to descend far more slowly than it would in free fall. Copper is non-magnetic. The retarding force arises because

Show answer and why every option is right or wrong

Answer: D. D is correct. As the magnet moves, the flux through each horizontal ring of copper changes; eddy currents are induced in those rings and, by Lenz's law, act to oppose the relative motion — the induced-current opposition set out in NCERT Class 12 Physics Chapter 6 (page 160).

Why A is wrong: A is wrong because copper is not ferromagnetic and takes on no significant magnetisation; if the effect were magnetisation it would persist when the magnet is held stationary, which it does not.

Why B is wrong: B is wrong because the clearance around the magnet lets air pass freely, and the same magnet falls quickly through a plastic pipe of identical bore — isolating the conductor, not the air, as the cause.

Why C is wrong: C is wrong because the magnet's field does not appreciably change copper's resistance, and a static field produces no force on a stationary non-magnetic conductor at all.

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MCQ 7Concept TrapPractice

A moving metal disc is brought to rest by an electromagnetic (eddy-current) brake. Once the disc has completely stopped and the electromagnet is left switched on, the braking force on the disc is

Show answer and why every option is right or wrong

Answer: A. A is correct. Eddy currents require a changing flux; a stationary disc in a steady field has constant flux, so no emf, no current and no force. This follows directly from the induction condition underlying eddy currents in NCERT Class 12 Physics (pre-2023 edition), Chapter 6, page 218.

Why B is wrong: B is wrong because position inside the field does not matter — a constant flux induces nothing however strong the field, which is why an eddy-current brake cannot serve as a parking brake.

Why C is wrong: C is wrong because the braking force depends on the rate of flux change, not on the field alone; as the disc slows the force falls with it and reaches zero at rest.

Why D is wrong: D is wrong because Lenz's law opposes the change that produces the current, and with no motion there is no change to oppose — an induced force can retard motion but cannot spontaneously start it.

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MCQ 8CalculationPractice

An induction cooktop heats a steel vessel placed on it, while the glass surface beneath stays comparatively cool. The best explanation is that

Show answer and why every option is right or wrong

Answer: B. B is correct. The coil's alternating field induces eddy currents in any conductor placed on it; the power appears as resistive heating in the vessel itself, which is the Joule heating of the induced current. Glass is an insulator, carries no eddy current, and so stays cool — the heating application of eddy currents noted in NCERT Class 12 Physics (pre-2023 edition), Chapter 6, page 219.

Why A is wrong: A is wrong because glass is an insulator in which no eddy current is induced; it warms only by conduction from the hot vessel above it, not the other way round.

Why C is wrong: C is wrong because the heating comes from induced currents flowing in the conducting metal; a changing flux through a conductor always drives such currents, and their Joule heating is what warms the vessel.

Why D is wrong: D is wrong because a steady field gives no changing flux and so induces no emf and no current; induction heating needs an alternating field.

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How do you solve a Eddy Currents question? A worked example

1
Given
• Side of square plate, a = 2.0 × 10⁻¹ m• Initial field, B₁ = 8.0 × 10⁻¹ T• Final field, B₂ = 2.0 × 10⁻¹ T• Time interval, Δt = 3.0 × 10⁻¹ s• Field perpendicular to the plate's face throughout.
2
Required
Magnitude of the induced emf around the plate's outer boundary, and the sense of the eddy current.
3
Concept
The plate is a conductor sitting in a changing magnetic field. The flux through any closed path in the metal is changing, so an emf is induced around that path and current circulates in the bulk — an eddy current. The emf is governed by Faraday's law; the direction is fixed by Lenz's law, which requires the induced current to oppose the decrease in flux, and therefore to maintain flux in the original direction.
4
Formula
ε = −dΦ_B/dt, with Φ_B = B·A for a uniform field perpendicular to the plane of area A (NCERT Class 12 Physics Chapter 6, page 157).

For a uniform rate of change, the magnitude is |ε| = A·|ΔB| / Δt.
5
Substitution
A = a² = (2.0 × 10⁻¹ m)² = 4.0 × 10⁻² m²

|ΔB| = B₁ − B₂ = 8.0 × 10⁻¹ T − 2.0 × 10⁻¹ T = 6.0 × 10⁻¹ T

|ε| = (4.0 × 10⁻² m²) × (6.0 × 10⁻¹ T) / (3.0 × 10⁻¹ s)
6
Calculation
Numerator: (4.0 × 10⁻²) × (6.0 × 10⁻¹) = 2.4 × 10⁻² Wb

|ε| = (2.4 × 10⁻² Wb) / (3.0 × 10⁻¹ s) = 8.0 × 10⁻² V

The exponent 2 in a² is a counting integer from the geometry of a square, not a measured quantity, so it does not contribute to the significant-figure count. All four given quantities carry two significant figures, so the answer is quoted to two.
7
Final answer
|ε| = 8.0 × 10⁻² V.

The flux out of the plate is decreasing, so by Lenz's law the eddy current flows in the sense that would maintain it — that is, anticlockwise as seen by an observer looking back along the field direction (looking from the side the field points towards). This current closes on itself inside the metal; the emf drives it against the plate's own resistance, and the energy appears as heat.
8
Common trap
The trap is answering "no current, because aluminium is not magnetic." The plate's response has nothing to do with magnetisation — it is ordinary induction in a conductor. The second trap is applying Lenz's law backwards: because the field is falling, the induced current supports the existing flux rather than opposing it. Read whether the flux is rising or falling before assigning the sense.
9
Similar NEET-style question
The same square plate is now held stationary in a field that is held constant at 8.0 × 10⁻¹ T. Then the plate is pulled sideways at constant speed entirely within that uniform-field region, remaining perpendicular to the field. What eddy current flows in the plate, and why? *(Expected reasoning: none of consequence — within a uniform field, translating the plate does not change the flux through any interior loop, so no emf is induced. Eddy currents in a moving plate appear at the boundary of the field region, where the flux through the plate is actually changing.)*

What to remember before solving Eddy Currents questions

Key Fact

Eddy currents

In the NEET syllabus; removed from current NCERT.

When bulk pieces of conductors are subjected to changing magnetic flux, induced currents are produced in them whose flow patterns resemble swirling eddies in water (discovered by Foucault): eddy currents. They oppose the motion (a copper plate swinging between magnet poles is damped; slots in the plate reduce the damping). Eddy currents heat the metallic cores of transformers and motors and dissipate energy, so cores are laminated, the laminations insulated and parallel to the field. Uses: magnetic braking in trains, electromagnetic damping (galvanometers), induction furnaces, and electric power meters.

-- NCERT Class 12 Physics (pre-2023 edition), Chapter 6, p. 218

More in Electromagnetic Induction and Alternating Currents: 4 exam traps and mistakes · 11 formulas · 6 question patterns from its other lessons.

Eddy Currents questions from past NEET papers

No question in our NEET 2020–2025 set targets this topic directly.

All 20 past-paper questions from Electromagnetic Induction and Alternating Currents →

Sources

Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.

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