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Magnetism & Matter revision notes

Every idea in the Magnetism & Matter chapter for JEE Main, with the usual mistakes and the fast methods. Topic 1 is open to everyone; sign in free to read all 12 and download the PDF.

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How we count: 7× asked means 7 past JEE Main questions in the PrepWiser bank needed that concept, and "last" is the latest year one appeared. ★ marks ideas asked 8 or more times.

Bar magnet and field lines

Bar magnet and Gauss's law for magnetism
2× asked, last 2025
m=qm (2l),∮B⃗⋅dA⃗=0m = q_m\,(2l),\qquad \oint \vec B\cdot d\vec A = 0
qmq_m = pole strength, 2l2l = magnetic length; m⃗\vec m is S to N. Far away, a magnet's field equals a solenoid's of equal moment.
NS
Field of a bar magnet (traced): N to S outside, S to N inside (arrow); every line closes, so net flux through any closed surface is zero
  • Not 'monopoles exist'. Breaking a magnet always gives smaller dipoles. Field lines are closed loops (N to S outside, S to N inside), so net flux through any closed surface is zero.
Bending a magnet
1× asked, last 2024
m′=2mπ  (semicircle),m′=m2  (L, bent at the middle)m'=\frac{2m}{\pi}\;(\text{semicircle}),\qquad m'=\frac{m}{\sqrt2}\;(\text{L, bent at the middle})
Pole strength unchanged; m′=qm×m'=q_m\times straight S–N distance. Arc of angle θ\theta (rad): m′=2msin⁡(θ/2)/θm'=2m\sin(\theta/2)/\theta.
SN
Straight: mm
SN
L at the middle: m/2m/\sqrt2
SN
Semicircle: 2m/π2m/\pi
  • Not the same moment after bending. Length along the wire does not count, only the straight distance between the poles. A wire of length LL bent into a semicircle has its ends 2L/π2L/\pi apart.

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10 more concepts in 4 topics

Field of a magnetic dipole, Magnet in a uniform field, Magnetisation and intensity and Magnetic materials and hysteresis. Read the first part free. Sign in free with Google to read the rest and download the PDF.

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Field of a magnetic dipole

  • Short magnet: axial and equatorial3×
  • Magnetic potential of a dipole1×

Magnet in a uniform field

  • Torque and potential energy6×
  • Work to rotate a magnet7×
  • Small oscillations of a magnet5×
  • Also remember2

Magnetisation and intensity

  • Magnetisation and magnetic intensity7×
  • Susceptibility and permeability11×

Magnetic materials and hysteresis

  • Dia, para and ferromagnetism7×
  • Curie's law and domains6×
  • Hysteresis6×

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