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Magnetism and Matter is a short chapter, and JEE Main usually takes one question from it. Almost all of those questions come from two ideas — the bar magnet treated as a magnetic dipole, and which class a substance belongs to near a magnet. You can finish this chapter in one sitting, so do not carry it to the last week. Ten questions below. Give them ten honest minutes.
What this chapter carries in JEE Main
- Usually 1 question every year, which is 4 marks.
- The bar magnet as a dipole — its moment, the torque on it, and how its field dies away — is the regular one.
- Sorting a substance into diamagnetic, paramagnetic or ferromagnetic fills the rest.
What the syllabus says
In the syllabus (Unit 13, Magnetic Effects of Current and Magnetism): Current loop as a magnetic dipole and its magnetic dipole moment, the bar magnet as an equivalent solenoid, magnetic field lines, the field of a bar magnet along its axis and perpendicular to its axis, torque on a magnetic dipole in a uniform magnetic field, para-, dia- and ferromagnetic substances with examples, and the effect of temperature on magnetic properties.
Taken out in 2024: earth’s magnetic field and magnetic elements, magnetic susceptibility and permeability, hysteresis, permanent magnets and electromagnets and the magnetic dipole moment of a revolving electron.
Taken from the NTA syllabus for JEE Main 2027. See the full syllabus, unit by unit.
Formulas you must know by heart
| Magnetic moment | M = m × 2l, and NIA for a loop |
| Axial and equatorial field | 2kM/d3 and kM/d3, with k = μ0/4π |
| Torque and potential energy | τ = MB sinθ, U = −MB cosθ |
| Work done in rotating a magnet | W = MB(cosθ1 − cosθ2) |
| Oscillating magnet | T = 2π√(I/MB) |
| Curie law | χ ∝ 1/T for a paramagnetic material |
| Three classes | diamagnetic χ small negative, paramagnetic small positive, ferromagnetic large positive |
A bar magnet behaves exactly like a small current loop. That one idea carries most of this chapter — the moment M = m × l, the torque MB sinθ, and a field that dies away as 1/r³ in every direction.
Ten questions. Give yourself ten minutes.
Write your answers down before you scroll. A question you looked up is a question you have not answered.
Q1. A bar magnet of moment M is cut into two equal pieces across its length. The magnetic moment of each piece is
- (a) M
- (b) M/2
- (c) 2M
- (d) zero
Q2. A bar magnet of magnetic moment M is placed in a uniform magnetic field B with its axis at 30° to the field. The torque on it is
- (a) zero
- (b) MB/2
- (c) MB√3/2
- (d) MB
Q3. On the axis of a short bar magnet, at a distance r from its centre, the magnetic field falls off as
- (a) 1/r
- (b) 1/r²
- (c) 1/r³
- (d) 1/r⁴
Q4. A substance is weakly repelled by a magnet and has no magnetic moment of its own. It is
- (a) paramagnetic
- (b) diamagnetic
- (c) ferromagnetic
- (d) antiferromagnetic
Q5. A bar magnet is placed in a uniform magnetic field. The net force on it is
- (a) zero
- (b) MB
- (c) MB sinθ
- (d) 2MB
Q6. The work done in turning a magnet of moment M through 180° in a uniform field B, starting from alignment, is
- (a) MB
- (b) 2MB
- (c) MB/2
- (d) zero
Q7. A circular loop of area A carries a current I. It behaves like a magnetic dipole of moment
- (a) IA
- (b) I/A
- (c) A/I
- (d) I²A
Q8. A magnetic dipole of moment M is turned from alignment with a uniform field B until it is at 90° to the field. The work done is
- (a) zero
- (b) MB/2
- (c) MB
- (d) 2MB
Q9. A diamagnetic substance placed in a non-uniform magnetic field moves towards the region where the field is
- (a) stronger
- (b) weaker
- (c) unchanged
- (d) reversed
Q10. The SI unit of magnetic moment is
- (a) A·m
- (b) A·m2
- (c) A/m
- (d) weber
Answer key, with a hint for each
| Q1 | (b) | The pole strength stays the same but the length halves, so M = m × l is halved. |
| Q2 | (b) | τ = MB sinθ. At θ = 30°, sinθ = 1/2, so τ = MB/2. The torque is zero only when the magnet is already lined up with the field, and largest at 90°. |
| Q3 | (c) | A bar magnet is a magnetic dipole, and a dipole field falls as 1/r³ both on the axis and on the equator. Only the constant differs — the axial field is twice the equatorial one at the same distance. |
| Q4 | (b) | A diamagnetic substance has no permanent magnetic moment; the field induces one that opposes it, so the substance is pushed away weakly. Paramagnetic and ferromagnetic substances are both pulled in. |
| Q5 | (a) | A uniform field pulls the north and south poles equally hard in opposite directions, so the two forces cancel and only a torque is left. A net force on a magnet needs a field that is not uniform — which is why a magnet is pulled towards the sharp end of another magnet. |
| Q6 | (b) | W = MB(cos 0° − cos 180°) = MB(1 + 1). |
| Q7 | (a) | M = IA, with the direction given by the right hand rule. This one equivalence is the reason a bar magnet and a current carrying solenoid behave alike — the magnet is simply a bundle of atomic current loops. |
| Q8 | (c) | U = −MB cosθ. At θ = 0 the energy is −MB, at θ = 90° it is zero, so the work done is MB. Turning it all the way round to 180° would take 2MB. |
| Q9 | (b) | Diamagnetic substances are repelled by a magnetic field, so they drift away from the strong region. |
| Q10 | (b) | From M = NIA, which is ampere times square metre. |
Now mark yourself honestly
JEE Main marks the way it marks. Use the same rule here:
- +4 for a right answer
- −1 for a wrong one
- 0 for a blank
So ten questions here are out of 40. In the real paper, 25 Physics questions are out of 100.
If you scored 32 or more, the basics of this chapter are secure. Move to harder questions, not more of these.
If you scored below 24, do not feel bad about it. You have just found out something useful before the exam did. Go back to the formula table above, learn it properly, and sit these ten again tomorrow.
And if you guessed a few and they landed, count those as wrong. A guess that works once will not work in January.
The full chapter, on a clock
Ten questions tell you a little. Twenty-five tell you the truth.
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