Key Fact
Bar magnet as solenoid
A bar magnet behaves like a current-carrying solenoid. Its magnetic moment m = N I A. Field along axis (axial) and perpendicular (equatorial) follow dipole formulas.
-- NCERT Class 12 Physics, Ch. 5, p. 139Bar Magnet As Solenoid
8 MCQs4 revision cards9-step worked example
Source: NCERT Magnetic Effects of Current and MagnetismPYQ coverage: NEET 2024Official key: NTA-verifiedLast updated: 23 Sep 2026
Bar Magnet As Solenoid, explained for NEET
The equivalence claim is narrow, and aspirants over-read it. A bar magnet behaves like a solenoid only in the far field — at points far compared with the magnet's own length. Inside the bar, and close to it, the analogy collapses: a real solenoid has a near-uniform interior field running from S-end to N-end, while inside a bar magnet the field is set by aligned atomic moments, not by a current you can point to.
What the equivalence actually says: a solenoid of N turns, current I and cross-sectional area A carries a magnetic moment m = NIA, and far from it the field falls as an inverse cube along the axis and along the equator, with the axial value twice the equatorial value at equal distance. A bar magnet of moment m produces the same far-field pattern. NCERT Class 12 Physics Chapter 5 opens (page 137) by using this correspondence to justify treating the bar magnet as a magnetic dipole in the first place — the solenoid is the bridge from Chapter 4's currents to Chapter 5's magnets.
Two consequences worth holding. First, the equivalence explains why magnetic poles cannot be separated: cut the solenoid in half and each half is still a solenoid with its own two ends, so each fragment of a bar magnet is a complete magnet. Second, the "poles" of a bar magnet are not point charges sitting at the tips; the effective pole separation is shorter than the magnet's geometric length, which is why a bar magnet's measured moment is not simply pole-strength × full length.
Watch out for: the axial-vs-equatorial factor of 2 belongs to the dipole-field lesson, and the μ₀nI interior formula belongs to the solenoid lesson. Here the only claim under test is when and why the two objects are interchangeable.
Can you answer these Bar Magnet As Solenoid 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
The equivalence between a bar magnet and a current-carrying solenoid holds for the field at points that are
Show answer and why every option is right or wrong
Answer: B. The bar-magnet/solenoid correspondence is a far-field statement — it reproduces the dipole field pattern at distances large compared with the object's length. NCERT Class 12 Physics Chapter 5, page 137, introduces the bar magnet as a dipole on exactly this basis.
Why A is wrong: A is wrong because the interior of a bar magnet is where the analogy fails hardest — a solenoid's interior field comes from a conduction current, the magnet's from aligned atomic moments.
Why C is wrong: C is wrong because near the tips the field is strongly geometry-dependent and the simple dipole form does not apply.
Why D is wrong: D is wrong because distances comparable to the object's own dimensions are precisely the near-field regime the equivalence excludes.
MCQ 2Direct ApplicationPractice
A solenoid of N turns, each of cross-sectional area A, carries a steady current I. The magnetic moment of the equivalent bar magnet is
Show answer and why every option is right or wrong
Answer: A. Each turn contributes a moment IA, and N turns in series carry the same current, so the moments add to give m = NIA. This is the quantity matched to the bar magnet's moment when the two are called equivalent.
Why B is wrong: B is wrong because dividing by N reverses the effect of adding turns — more turns increase the moment, they do not dilute it.
Why C is wrong: C is wrong because the moment scales with area, not inversely with it; a wider solenoid of the same turns and current has a larger moment.
Why D is wrong: D is wrong because the turns contribute linearly. Squaring N would require the current itself to scale with N, which it does not.
MCQ 3Concept TrapPractice
A bar magnet is cut into two equal halves perpendicular to its axis. Using the solenoid picture, what best explains why each half is a complete magnet with two poles?
Show answer and why every option is right or wrong
Answer: B. Cutting the equivalent solenoid across its length gives two shorter solenoids, each still a closed winding with a start end and a finish end — so each piece has both poles. This is the standard argument for the non-existence of isolated magnetic poles.
Why A is wrong: A is wrong because no external induction is needed; the second pole is intrinsic to the fragment, as the shortened-solenoid picture makes immediate.
Why C is wrong: C is wrong because magnetic monopoles have never been observed; the whole point of the solenoid analogy here is that no such object is required.
Why D is wrong: D is wrong because repulsion is a consequence of the poles existing, not a mechanism that creates them.
MCQ 4Easy RecallPractice
In the solenoid picture of a bar magnet, the magnetic moment arises physically from
Show answer and why every option is right or wrong
Answer: B. The equivalence rests on both objects' moments having the same origin in circulating current: a conduction current in the coil, and aligned atomic-scale current loops in the magnetised material. NCERT Class 12 Physics Chapter 5, page 137.
Why A is wrong: A is wrong because magnetic charge (a monopole) is not a physical object; the moment is a current-loop property, not a charge accumulation.
Why C is wrong: C is wrong because static electric charge produces an electric field, and moving it would be required for any magnetic effect — a magnetised bar has no such surface charge.
Why D is wrong: D is wrong because the Earth's field is not stored in the magnet; a bar magnet retains its moment when shielded from the Earth's field.
MCQ 5Concept TrapPractice
Two students describe a bar magnet's interior field. Student P says it points from the N-end to the S-end inside the material, matching the exterior lines. Student Q says it points from the S-end to the N-end inside, as in a solenoid. Who is right, and why?
Show answer and why every option is right or wrong
Answer: B. Magnetic field lines are continuous closed loops, so the interior direction must be opposite to the exterior one — S-end to N-end inside, exactly as in the equivalent solenoid. This closed-loop property is what the solenoid analogy makes visible.
Why A is wrong: A is wrong because N-to-S describes the field only outside the magnet; treating it as universal breaks the continuity of the field lines.
Why C is wrong: C is wrong because the interior field has a perfectly well-defined direction; it is simply the reverse of the exterior one.
Why D is wrong: D is wrong because the interior field is not zero — it is the strongest part of the line pattern, which is why the lines close through the material.
MCQ 6CalculationPractice
A solenoid is unwound and rewound on a former of the same length but twice the cross-sectional area, using the same total length of wire and carrying the same current. Assuming the wire length fixes the product (turns × circumference), by what factor does the magnetic moment of the equivalent bar magnet change?
Show answer and why every option is right or wrong
Answer: B. Doubling the area doubles A but increases the radius by √2, so the circumference rises by √2 and the fixed wire length allows only N/√2 turns. Then m = NIA scales as (1/√2) × 2 = √2. NCERT Class 12 Physics Chapter 5, page 137, for the moment of the equivalent solenoid.
Why A is wrong: A is wrong because it assumes the area gain and the turns loss cancel exactly; they do not, since area scales as radius squared while circumference scales as radius.
Why C is wrong: C is wrong because it uses the doubled area while holding N fixed, ignoring that a larger circumference consumes more wire per turn.
Why D is wrong: D is wrong because it applies only the turns reduction and drops the area increase entirely, reversing the net direction of the change.
MCQ 7Direct ApplicationPractice
A short bar magnet and a short solenoid are said to be equivalent. Which single quantity must be matched between them for the claim to hold?
Show answer and why every option is right or wrong
Answer: C. The far field of a dipole is fixed entirely by its magnetic moment, so matching moments is necessary and sufficient for the two objects to produce the same distant field.
Why A is wrong: A is wrong because two objects of different lengths can carry equal moments, and the far field cannot distinguish them.
Why B is wrong: B is wrong because mass plays no part in the field expression at all.
Why D is wrong: D is wrong because current alone is not the matched quantity — it appears only through the product NIA, and pole strength alone is likewise incomplete without an effective separation.
MCQ 8CalculationPractice
A bar magnet of geometric length L has measured magnetic moment m. A student computes a pole strength as q = m/L and then predicts the field by treating two point poles ±q sitting exactly at the tips, separated by L. Compared with the true moment, this model
Show answer and why every option is right or wrong
Answer: B. The effective magnetic length of a bar magnet is shorter than its geometric length, so reproducing the same moment m over that shorter separation demands a larger pole strength than m/L. The solenoid picture makes this natural — the moment is a distributed current-loop property, not two charges pinned at the tips.
Why A is wrong: A is wrong because the tips are not the effective pole positions; m/L therefore does not reproduce the correct field near the magnet.
Why C is wrong: C is wrong because it inverts the geometry — the effective separation is shorter than the geometric length, not longer.
Why D is wrong: D is wrong because the conclusion follows from the effective-length relation alone; the cross-sectional area is not needed to see the direction of the error.
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Bar Magnet As Solenoid: quick recall before you leave
How do you solve a Bar Magnet As Solenoid question? A worked example
- 1
Given
A solenoid is wound with N = 5.00 × 10² turns on a cylindrical former of cross-sectional area A = 4.00 × 10⁻⁴ m², and carries a steady current I = 1.50 A. The solenoid's length is 20.0 cm.
- 2
Required
The magnetic moment of the equivalent bar magnet, and the maximum geometric length of a bar magnet whose pole strength would be 1.00 A·m if modelled with poles at its tips.
- 3
Concept
Far from either object, the field is that of a magnetic dipole and depends only on the magnetic moment. Equating the two objects means equating their moments. The solenoid's moment is the sum of the moments of its individual turns, all carrying the same current.
- 4
Formula
m = NIA. For the crude tip-pole model, m = q × ℓ, where q is pole strength and ℓ the separation.
- 5
Substitution
m = (5.00 × 10²)(1.50 A)(4.00 × 10⁻⁴ m²)
- 6
Calculation
5.00 × 10² × 1.50 = 7.50 × 10². Then 7.50 × 10² × 4.00 × 10⁻⁴ = 3.00 × 10⁻¹ A·m². The turn count N = 5.00 × 10² is a counting integer and does not limit the significant-figure count; the sig-fig count here is set by I and A, each given to three significant figures. Note also that the solenoid's 20.0 cm length does not enter this calculation at all — it would matter for the interior field, not for the moment.
- 7
Final answer
m = 3.00 × 10⁻¹ A·m². For the tip-pole model, ℓ = m/q = (3.00 × 10⁻¹ A·m²)/(1.00 A·m) = 3.00 × 10⁻¹ m. Because the effective magnetic length is shorter than the geometric length, a real bar magnet carrying this moment with pole strength 1.00 A·m would be physically longer than 30.0 cm.
- 8
Common trap
Reaching for the solenoid length because it was supplied. The moment m = NIA has no length in it; the length belongs to the interior-field expression covered in the Ampere's-law-for-solenoid lesson. A stated quantity is not automatically a used quantity.
- 9
Similar NEET-style question
Two solenoids carry the same current. Solenoid X has 2.00 × 10² turns of area 6.00 × 10⁻⁴ m²; solenoid Y has 3.00 × 10² turns of area 4.00 × 10⁻⁴ m². Which produces the stronger field at a distant axial point at the same distance, and by what factor?
What to remember before solving Bar Magnet As Solenoid questions
More in Magnetic Effects of Current and Magnetism: 3 exam traps and mistakes · 11 formulas · 5 question patterns from its other lessons.
Bar Magnet As Solenoid questions from past NEET papers
1 question from NEET 2024. Answers verified against NTA official keys. — click to collapse
NEET 2024Revised key
1M
2M/2
32M
4M/√3
NTA Answer: Option 2(revised_final)
All 17 past-paper questions from Magnetic Effects of Current and Magnetism →
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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