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. 139Magnetic Field Lines
8 MCQs9-step worked example
Source: NCERT Magnetic Effects of Current and MagnetismOfficial key: NTA-verifiedLast updated: 8 Oct 2026
Try this first
Which statement best describes the magnetic field lines of a bar magnet or of a current-carrying solenoid?
- A.They begin at the north pole and end at the south pole only.
- B.They form continuous closed loops.
- C.They begin on a positive pole and escape to infinity.
- D.They intersect at the centre of the magnet.
Tap to see the answer
Answer: B. B is correct: NCERT states that the magnetic field lines of a magnet or a solenoid form continuous closed loops, unlike the lines of an electric dipole (NCERT Class 12 Physics, Chapter 5, page 137).
A is wrong: A is wrong because outside the magnet the lines run from north to south, but they continue through the magnet and close; they do not end on the south pole.
C is wrong: C is wrong because that describes electric dipole lines, which begin on a positive charge and end on a negative one or escape to infinity.
D is wrong: D is wrong because field lines never intersect; at a crossing the direction of the field would not be unique.
Magnetic Field Lines, explained for NEET
The trap is the electric-field picture carried over to magnetism. Electric field lines start on a positive charge and end on a negative one; magnetic field lines do neither, and a diagram that shows them spraying out of a point is wrong.
NCERT calls magnetic field lines a visual and intuitive realisation of the magnetic field. The lines of a magnet, or of a solenoid, form continuous closed loops. That is unlike the electric dipole, whose lines begin on the positive charge and end on the negative charge or escape to infinity (NCERT Class 12 Physics, Chapter 5, page 137).
Three more properties are listed on the next page. The tangent to a field line at a point gives the direction of the net field B there. The larger the number of lines crossing per unit area, the stronger the field. And field lines do not intersect, because at a crossing the direction of B would not be unique (NCERT Class 12 Physics, Chapter 5, page 138). A small compass needle placed at various positions maps the direction of the field, point by point (NCERT Class 12 Physics, Chapter 5, page 138).
The NEET-relevant consequence is the zero net flux: magnetic field lines can never emanate from a point, and over any closed surface as many lines enter as leave (NCERT Class 12 Physics, Chapter 5, page 144).
One naming point matters. NCERT avoids the name "magnetic lines of force": unlike electrostatics, the field lines in magnetism do not indicate the direction of the force on a moving charge (NCERT Class 12 Physics, Chapter 5, page 137). Do not read a force direction off the tangent.
Watch-out: when a sketch has lines that start at one point, cross each other, or stop in empty space, it cannot be a magnetic field.
How do you solve a Magnetic Field Lines question? A worked example
- 1
Given
A bar magnet lies entirely inside a closed surface. In a sketch of its field, 36 field lines pass outward through the part of the surface near the north pole.
- 2
Required
The number of field lines that pass inward through the rest of the surface.
- 3
Concept
Magnetic field lines form continuous closed loops and can never emanate from a point, so over any closed surface the net flux of B is zero: as many lines enter the surface as leave it (NCERT Class 12 Physics, Chapter 5, pages 137 and 144).
- 4
Formula
Lines leaving = lines entering (net flux of B over a closed surface = 0).
- 5
Substitution
Lines entering = lines leaving = 36.
- 6
Calculation
Net count = 36 outward − (number inward) = 0, so the number inward is 36. No physical constants are needed.
- 7
Final answer
36 field lines pass inward, mostly near the south pole.
- 8
Common trap
Answering 0 on the grounds that the lines "start at the north pole and end at the south pole," which is the electric-dipole picture. Inside the magnet the lines continue from south to north and close, so every line that leaves also returns.
- 9
Similar NEET-style question
A closed surface is drawn around a short length of a long straight current-carrying wire, and 20 field lines pass inward through it. How many pass outward? (Answer: the net flux of B over a closed surface is zero, so the lines leaving equal the lines entering: 20 lines pass outward.)
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Can you answer these Magnetic Field Lines 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
Which statement best describes the magnetic field lines of a bar magnet or of a current-carrying solenoid?
Show answer and why every option is right or wrong
Answer: B. B is correct: NCERT states that the magnetic field lines of a magnet or a solenoid form continuous closed loops, unlike the lines of an electric dipole (NCERT Class 12 Physics, Chapter 5, page 137).
Why A is wrong: A is wrong because outside the magnet the lines run from north to south, but they continue through the magnet and close; they do not end on the south pole.
Why C is wrong: C is wrong because that describes electric dipole lines, which begin on a positive charge and end on a negative one or escape to infinity.
Why D is wrong: D is wrong because field lines never intersect; at a crossing the direction of the field would not be unique.
MCQ 2Easy RecallPractice
What does the tangent to a magnetic field line at a point represent?
Show answer and why every option is right or wrong
Answer: D. D is correct: the tangent to the field line at a given point represents the direction of the net magnetic field B at that point (NCERT Class 12 Physics, Chapter 5, page 138).
Why A is wrong: A is wrong because NCERT notes that magnetic field lines do not indicate the direction of the force on a moving charge.
Why B is wrong: B is wrong because the tangent gives the direction of B in space, not the direction of a current.
Why C is wrong: C is wrong because magnetic field lines describe B, not an electric field.
MCQ 3Easy RecallPractice
Why can two magnetic field lines never cross each other?
Show answer and why every option is right or wrong
Answer: A. A is correct: if two field lines intersected, the direction of the magnetic field would not be unique at the point of intersection (NCERT Class 12 Physics, Chapter 5, page 138).
Why B is wrong: B is wrong because forming closed loops is itself an accepted property of magnetic field lines, so it cannot be a reason to forbid crossing.
Why C is wrong: C is wrong because the field is not zero along a line; the tangent gives its direction and the line density its strength.
Why D is wrong: D is wrong because the reason is the ambiguity of direction, not a loss of flux.
MCQ 4Direct ApplicationPractice
In a sketch of magnetic field lines, 12 lines per cm² cross a small surface perpendicular to the lines at point P, and 3 lines per cm² cross a similar surface at point Q. Which statement is correct?
Show answer and why every option is right or wrong
Answer: C. C is correct: the larger the number of field lines crossing per unit area, the stronger the magnetic field, so B is larger at P (NCERT Class 12 Physics, Chapter 5, page 138).
Why A is wrong: A is wrong because it reverses the rule; the region with more lines per unit area has the stronger field.
Why B is wrong: B is wrong because the line densities differ, 12 against 3 per cm², so the magnitudes differ.
Why D is wrong: D is wrong because the magnitude is read from the line density alone; the direction is read separately from the tangent.
MCQ 5Direct ApplicationPractice
In a diagram, 24 magnetic field lines pass outward through a closed surface drawn in a magnetic field. How many lines must pass inward through the same surface?
Show answer and why every option is right or wrong
Answer: B. B is correct: over any closed surface the net flux of B is zero, so as many lines enter the surface as leave it (NCERT Class 12 Physics, Chapter 5, page 144).
Why A is wrong: A is wrong because zero inward lines would mean the surface encloses a source of lines, and NCERT says magnetic field lines can never emanate from a point.
Why C is wrong: C is wrong because half the outward number would leave a net outward flux; the numbers must match exactly.
Why D is wrong: D is wrong because it doubles the outward number, which would leave a net inward flux.
MCQ 6Direct ApplicationPractice
A student reasons: "A magnetic field line at point P runs towards the east, so a charge moving at P feels a force towards the east, because field lines are lines of force." What does NCERT say about this reasoning?
Show answer and why every option is right or wrong
Answer: A. A is correct: NCERT avoids the name "magnetic lines of force" because, unlike electrostatics, the field lines in magnetism do not indicate the direction of the force on a (moving) charge (NCERT Class 12 Physics, Chapter 5, page 137).
Why B is wrong: B is wrong because it states the electrostatic habit that NCERT warns against; the tangent gives the direction of B, not of the force.
Why C is wrong: C is wrong because the sign of the charge does not turn the tangent into a force direction.
Why D is wrong: D is wrong because NCERT does not say there is no force; it says the lines do not show the force direction.
MCQ 7CalculationPractice
Three students sketch magnetic field lines. P draws straight lines spreading outward from a single point like spokes. Q draws continuous closed loops that pass through a bar magnet and close outside it. R draws two lines that cross at a point. Which sketches can represent a magnetic field?
Show answer and why every option is right or wrong
Answer: C. C is correct: only Q obeys the rules. Field lines form closed loops (page 137), can never emanate from a point (page 144), and never intersect (page 138) (NCERT Class 12 Physics, Chapter 5, pages 137, 138 and 144).
Why A is wrong: A is wrong because lines spreading out of a single point imply a source of flux, and magnetic field lines can never emanate from a point.
Why B is wrong: B is wrong because P is not a valid magnetic pattern, even though Q is.
Why D is wrong: D is wrong because R has two lines crossing, which would make the field direction ambiguous at the crossing.
MCQ 8CalculationPractice
At point P a magnetic field line runs northward and 4 lines per cm² cross a perpendicular surface. At point Q the line runs eastward and 9 lines per cm² cross a perpendicular surface. Which statement about the field is correct?
Show answer and why every option is right or wrong
Answer: D. D is correct: more lines per unit area means a stronger field, so B is larger at Q (9 against 4), and the tangent to the line gives its direction, east at Q (NCERT Class 12 Physics, Chapter 5, page 138).
Why A is wrong: A is wrong on both counts: P has the fewer lines per unit area, so the weaker field, and the direction at Q is east, not north.
Why B is wrong: B is wrong because the direction at Q is given by the line at Q, which runs east; the northward line belongs to P.
Why C is wrong: C is wrong because it takes the stronger field at P, but P has 4 lines per cm² against 9 at Q.
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What to remember before solving Magnetic Field Lines questions
7 NCERT lines
For a needle of magnetic moment m and moment of inertia I in a uniform field B, the restoring torque mB sinθ gives simple harmonic motion for small θ. The square of the angular frequency is ω^2 = mB/I and the time period is T = 2π√(I/mB) (5.4), or B = 4π^2 I/(m T^2) (5.5).
-- NCERT Class 12 Physics (pre-2023 edition), Chapter 5, p. 178Some textbooks assign a magnetic charge (also called pole strength) +qm to the north pole and -qm to the south pole of a bar magnet of length 2l, and magnetic moment qm (2l
-- NCERT Class 12 Physics (pre-2023 edition), Chapter 5, p. 177(ii) The tangent to the field line at a given point represents the direction of the net magnetic field B at that point. (iii) The larger the number of field lines crossing per unit area, the stronger is the magnitude of the magnetic field B.
-- NCERT Class 12 Physics, Ch. 5, p. 138(iv) The magnetic field lines do not intersect, for if they did, the direction of the magnetic field would not be unique at the point of intersection.
-- NCERT Class 12 Physics, Ch. 5, p. 138One way is to place a small magnetic compass needle at various positions and note its orientation. This gives us an idea of the magnetic field direction at various points in space.
-- NCERT Class 12 Physics, Ch. 5, p. 138Key Fact
Example 5.3: field lines never emanate from a point; net flux of B over a closed surface is zero
Magnetic field lines can never emanate from a point, as shown in figure. Over any closed surface, the net flux of B must always be zero, i.e., pictorially as many field lines should seem to enter the surface as the number of lines leaving it.
-- NCERT Class 12 Physics, Ch. 5, p. 144Magnetic Field Lines: NEET previous year questions (PYQs) with answers
18 questions in Magnetic Effects of Current and Magnetism
No question in our NEET 2020–2025 set targets this topic directly.
All 18 past-paper questions from Magnetic Effects of Current and Magnetism →
More in Magnetic Effects of Current and Magnetism: 3 exam traps and mistakes · 11 formulas · 5 question patterns from its other lessons.
Sources
NCERT refs: Class 12 Physics Chapter 5, p.137 | Class 12 Physics Chapter 5, p.138 | Class 12 Physics Chapter 5, p.144
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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