Law
Lenz's law
The direction of induced current opposes the change in flux that produced it (consequence of energy conservation; provides the negative sign in Faraday's law).
-- NCERT Class 12 Physics, Ch. 6, p. 160Lenz's Law
8 MCQs1 revision card9-step worked example
Source: NCERT Electromagnetic Induction and Alternating CurrentsPYQ coverage: NEET 2024Official key: NTA-verifiedLast updated: 23 Sep 2026
Lenz's Law, explained for NEET
NEET's induction questions rarely fail students on magnitude. The documented error mode in this question pattern is the sign — picking the induced current in the wrong direction.
Lenz's law fixes that direction. The induced current always flows so that its own magnetic field opposes the change in flux — not the flux itself, and not the external field. NCERT Class 12 Physics Chapter 6 (Electromagnetic Induction) states it on page 159, and it lives inside Faraday's law as the minus sign in ε = −dΦ_B/dt.
The confusion worth naming: students read "opposes" and oppose the field. With B out of the page they answer clockwise every time. That is right only while the flux is increasing. If the same field is weakening, the loop's current runs anticlockwise to prop the flux up. Direction is decided by whether the flux is growing or shrinking, never by the field direction alone.
Four steps, in order: (1) fix the direction of B through the loop; (2) decide whether that flux is increasing or decreasing; (3) the induced current's own field must point against that change; (4) use the right-hand grip rule to turn that field direction into a current sense.
Why the minus sign has to be there: energy conservation. If the induced current aided the change, a nudge would amplify itself — the magnet would accelerate into the coil while the coil delivered current, with nothing supplying the energy. The opposition is exactly what forces you to do work, and that work is the electrical energy you get out.
The mechanical form is faster in the exam hall: induced effects oppose the motion that produces them. An approaching magnet is repelled; a receding magnet is attracted.
Watch out for the null case. No change, no current. A loop drifting inside a large region of uniform field encloses constant flux, so nothing is induced — however strong B is.
Can you answer these Lenz's Law 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
According to Lenz's law, the induced current in a closed loop flows in the direction that
Show answer and why every option is right or wrong
Answer: C. Lenz's law is stated in terms of the change in flux, not the flux or the field: the induced current's own field acts against whatever change is occurring. NCERT Class 12 Physics Chapter 6, page 159.
Why A is wrong: A is wrong because it reverses the law; an induced current that increased the flux would amplify its own cause and create energy from nothing.
Why B is wrong: B is wrong because it is the single most common misreading of the word 'opposes'. If the flux is decreasing, the induced current's field points along the external field to support it, not against it.
Why D is wrong: D is wrong because the induced current only slows the change; it never cancels the flux. A steady non-zero flux produces no induced current at all.
MCQ 2Easy RecallPractice
Lenz's law is a direct consequence of which conservation principle?
Show answer and why every option is right or wrong
Answer: A. The opposition is what makes external work necessary to keep the flux changing; that work becomes the electrical energy delivered. NCERT Class 12 Physics Chapter 6 develops this argument alongside the law on page 159.
Why B is wrong: B is wrong because magnetic flux through a loop is not a conserved quantity — the whole phenomenon exists because flux changes.
Why C is wrong: C is wrong because charge conservation holds in every circuit, induced or not, and says nothing about which direction the induced current takes.
Why D is wrong: D is wrong because no angular-momentum conservation argument is used anywhere in the statement or justification of Lenz's law.
MCQ 3Easy RecallPractice
In the expression ε = −dΦ_B/dt, the minus sign indicates that
Show answer and why every option is right or wrong
Answer: D. The minus sign is where Lenz's law is embedded in Faraday's law — it carries the direction information, while the magnitude comes from |dΦ_B/dt|. NCERT Class 12 Physics Chapter 6, page 159.
Why A is wrong: A is wrong because the sign of ε depends on the sign of dΦ_B/dt; when flux decreases, ε comes out positive in the same convention.
Why B is wrong: B is wrong because flux carries whatever sign follows from the chosen normal direction; there is no convention making it inherently negative.
Why C is wrong: C is wrong because it confuses a directional minus sign with a decay in time. A uniformly changing flux gives a constant induced EMF that does not diminish.
MCQ 4Direct ApplicationPractice
The north pole of a bar magnet is moved towards the flat face of a circular coil. As seen by an observer standing on the magnet's side and looking at that face, the induced current in the coil is
Show answer and why every option is right or wrong
Answer: B. The approaching north pole increases the flux, so the near face must become a north pole to repel it; by the right-hand grip rule a north pole facing the observer means anticlockwise current as seen by that observer. NCERT Class 12 Physics Chapter 6, page 159.
Why A is wrong: A is wrong because a south pole on the near face would attract the magnet and speed it up, which would generate energy with no work done.
Why C is wrong: C is wrong on the geometry, not the physics: a near face acting as a north pole requires anticlockwise, not clockwise, current when viewed from that side.
Why D is wrong: D is wrong because induction needs only relative motion and a changing flux; no contact is involved, and the current flows throughout the approach.
MCQ 5Direct ApplicationPractice
The same bar magnet, north pole still facing the coil, is now withdrawn from the coil. As seen by the observer on the magnet's side, the induced current is
Show answer and why every option is right or wrong
Answer: D. Withdrawal decreases the flux, so the near face becomes a south pole to attract the departing north pole and oppose the separation; a south pole towards the observer corresponds to clockwise current from that viewpoint. NCERT Class 12 Physics Chapter 6, page 159.
Why A is wrong: A is wrong on both counts. It carries over the approach answer unchanged, which is the direction error this pattern is built to catch — the current reverses when the motion reverses.
Why B is wrong: B is wrong because repulsion would help the magnet leave, requiring no work from the person pulling it while still producing current.
Why C is wrong: C is wrong because attraction is correct but the sense is not: a south pole facing the observer means the current runs clockwise as that observer sees it.
MCQ 6Concept TrapPractice
Suppose the induced current in a loop flowed so as to assist the change in flux rather than oppose it. The most direct consequence would be that
Show answer and why every option is right or wrong
Answer: A. A magnet nudged towards such a coil would be pulled in faster, increasing the flux change, which strengthens the attraction — a runaway that creates energy from nothing. This impossibility is the argument NCERT Class 12 Physics Chapter 6 gives for the law's direction, page 159.
Why B is wrong: B is wrong because the magnitude |dΦ_B/dt| is unaffected by direction. Reversing the sign changes where the current goes, not whether an EMF exists.
Why C is wrong: C is wrong because the current would still be limited by the loop's resistance at every instant; the problem is that it would grow without bound over time, not that it starts infinite.
Why D is wrong: D is wrong because the flaw is a violated conservation law, not a change in dissipation — and the loop would in fact heat more, since the runaway drives ever larger currents.
MCQ 7Direct ApplicationPractice
Two coaxial coils P and S lie close together. A steady current has been flowing in P for some time. The switch in P's circuit is then suddenly opened. During the brief interval while P's current dies away, the induced current in S
Show answer and why every option is right or wrong
Answer: C. The flux through S is collapsing, so by Lenz's law S's induced current circulates so as to maintain that flux — which means in the same sense as the dying current in P. NCERT Class 12 Physics Chapter 6, page 159.
Why A is wrong: A is wrong on duration. Once P's current has fallen to zero the flux is constant again and the induced current stops, however long the switch stays open.
Why B is wrong: B is wrong because it judges the moment before the switch opened. The question concerns the decay interval, during which dΦ_B/dt is large and non-zero.
Why D is wrong: D is wrong because it applies the opposition to the current's direction instead of to the change. Opposing a decrease means supporting the existing flux, not reversing it.
MCQ 8CalculationPractice
A square conducting loop is pushed at constant speed from a field-free region into a region of uniform magnetic field directed into the page, and keeps moving until it lies entirely inside the field region. Which statement describes the loop while it is entering, and later when it is fully inside?
Show answer and why every option is right or wrong
Answer: B. While entering, into-the-page flux is increasing, so the induced current is anticlockwise (its own field points out of the page inside the loop) and the resulting force opposes the motion; once fully inside, the enclosed flux is constant and no current is induced. NCERT Class 12 Physics Chapter 6, page 159.
Why A is wrong: A is wrong on the sense: clockwise current would reinforce the into-page flux that is already increasing, which is the direction Lenz's law forbids.
Why C is wrong: C is wrong because a force aiding the motion would let the loop accelerate through the boundary while delivering current, and because a constant enclosed flux inside the region induces nothing.
Why D is wrong: D is wrong twice over: the entering sense is anticlockwise, and no current at all flows once the loop is fully inside, so there is nothing left to reverse.
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Lenz's Law: quick recall before you leave
How do you solve a Lenz's Law question? A worked example
- 1
Given
A circular coil of 200 turns (exact) and radius r = 5.0 × 10⁻² m lies with its plane perpendicular to a uniform magnetic field directed along its axis and pointing towards the observer. The field increases steadily from 0.10 T to 0.50 T in 0.20 s. The coil's total resistance is 4.0 Ω.
- 2
Required
The magnitude of the induced current, and its direction as seen by the observer.
- 3
Concept
The flux through the coil is changing because B is changing while area and orientation stay fixed. Faraday's law gives the magnitude; Lenz's law gives the direction. The flux towards the observer is increasing, so the induced current must produce a field away from the observer inside the loop.
- 4
Formula
ε = −N dΦ_B/dt, with Φ_B = B A and A = πr². Then I = ε/R.
- 5
Substitution
A = π(5.0 × 10⁻² m)² = π × 2.5 × 10⁻³ m² = 7.85 × 10⁻³ m²
dB/dt = (0.50 T − 0.10 T) / 0.20 s = 2.0 T s⁻¹
|ε| = N A (dB/dt) = 200 × 7.85 × 10⁻³ m² × 2.0 T s⁻¹ - 6
Calculation
|ε| = 200 × 1.571 × 10⁻² V = 3.14 V
I = |ε| / R = 3.14 V / 4.0 Ω = 0.785 A
The turn count 200 and the constant π are exact and do not limit the significant figures. The measured inputs (5.0 × 10⁻² m, 0.10 T, 0.50 T, 0.20 s, 4.0 Ω) all carry two significant figures, so the answer is quoted to two. - 7
Final answer
I = 0.79 A, flowing clockwise as seen by the observer. A clockwise current viewed from a given side produces a field pointing away from that viewer inside the loop, which is what opposes the increasing flux coming towards them.
- 8
Common trap
Answering "anticlockwise" by reasoning that the induced current opposes the field. It opposes the change. Had the same field been decreasing from 0.50 T to 0.10 T over the same interval, the magnitude would be identical — 0.79 A — but the current would run anticlockwise. Magnitude questions and direction questions are decided by different halves of ε = −dΦ_B/dt, and the direction half depends entirely on the sign of dΦ_B/dt.
- 9
Similar NEET-style question
A circular coil of 150 turns (exact) and radius 4.0 × 10⁻² m has its axis along a uniform field that falls steadily from 0.60 T to zero in 0.30 s. The coil's resistance is 3.0 Ω. Find the induced current, and state whether, viewed from the side the field originally pointed towards, it runs clockwise or anticlockwise.
What to remember before solving Lenz's Law questions
Which Lenz's Law formulas do you need for NEET?
1 formula — click to collapse
Faraday's law of induction
Induced EMF in circuit equals negative rate of change of magnetic flux. Lenz's law in the sign.
| Symbol | Quantity | SI Unit |
|---|---|---|
| epsilon | induced EMF | V |
| Phi_B | magnetic flux | Wb |
Valid when
- Closed loop
- Flux change real (motion or B change)
More in Electromagnetic Induction and Alternating Currents: 4 exam traps and mistakes · 10 formulas · 5 question patterns from its other lessons.
Lenz's Law questions from past NEET papers
1 question from NEET 2024. Answers verified against NTA official keys. — click to collapse
NEET 2024Revised key
1AB and DC
2BA and CD
3AB and CD
4BA and DC
NTA Answer: Option 1(revised_final)
All 20 past-paper questions from Electromagnetic Induction and Alternating Currents →
How does NEET ask about Lenz's Law?
1 recurring pattern from past papers — click to collapse
EMF induced in coil by changing flux. Lenz's law sign: opposes change.
InterpretationMedium
Common distractors
wrong sign direction
Picks induced current in wrong direction
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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