Magnetism and Matter Notes for NEET, Class 12 NCERT

Q: How many questions come from Magnetism and Matter in NEET 2027?

You can expect 1 question from this chapter in NEET 2027. The chapter has medium PYQ frequency. Bar magnet field and torque, Curie's law, classification of magnetic materials and basics of Earth's magnetism are favourites.

Q: What is the magnetic dipole moment of a bar magnet?

A bar magnet with pole strength m_pole and length 2 L has dipole moment vector p_m equals m_pole times 2 L, directed from south to north pole. Unit: A m squared (or J per T). For a current loop of N turns, area A and current I, the moment is m equals N I A.

Q: What is the field of a bar magnet?

On the axis at distance r far from the magnet, B equals 2 mu_0 m over (4 pi r cubed). On the equator (perpendicular bisector), B equals mu_0 m over (4 pi r cubed). Axial is twice equatorial. Both fall as 1 over r cubed.

Q: What are the elements of Earth's magnetic field?

Three numbers describe Earth's field at any place: declination (angle between true geographic north and magnetic north on a map), dip or inclination (angle B makes with the horizontal plane), and horizontal component B_H. Total field B equals B_H over cos(dip).

Q: What is the difference between diamagnetic, paramagnetic and ferromagnetic substances?

Diamagnetic: weakly repelled by a magnet. chi is small and NEGATIVE (around -10⁻⁵). Examples: bismuth, copper, water. Paramagnetic: weakly attracted. chi is small and POSITIVE (around +10⁻⁴). Examples: aluminium, oxygen. Ferromagnetic: strongly attracted, can be permanently magnetised. chi is large positive (10² to 10⁵). Examples: iron, nickel, cobalt.

Q: What is Curie's law?

For a paramagnetic substance, the magnetisation M is proportional to the applied field H and inversely proportional to absolute temperature T: M equals C H over T, where C is the Curie constant. Equivalently, susceptibility chi equals C over T. Heating reduces magnetisation.

Q: What is a hysteresis loop?

For a ferromagnetic material, the M vs H curve traces a loop instead of a straight line. Starting from zero, M rises with H to saturation. When H is reversed, M does not retrace the same path: there is residual magnetism (retentivity) at H equals 0, and M only reaches zero at a reverse field called the coercivity. The area of the loop equals the energy lost per cycle as heat.

Introduction

A bar magnet behaves like a magnetic dipole. This chapter takes the dipole picture and uses it for permanent magnets, Earth's field, and the three classes of magnetic substances. Expect 1 NEET question; favourites are bar magnet field on axis vs equator, classification of magnetic materials and Curie's law.

Bar magnet and magnetic dipole

A bar magnet of pole strength m_pole and length 2 L can be modelled as two equal and opposite poles. Magnetic dipole moment:

p_{m} = m_{pole} \cdot 2 L (from S to N pole) .

For a current loop of N turns, area A and current I, the moment is m = N I A. Unit: A m² (or J/T).

Field of a bar magnet

At distance r far from the magnet (r much larger than 2 L):

B_{axial} = \frac{μ _{0}}{4 π} \frac{2 m}{r ^{3}}, B_{eq} = \frac{μ _{0}}{4 π} \frac{m}{r ^{3}} .

Same form as the electric dipole, just with mu_0 over 4 pi instead of 1 over 4 pi epsilon_0.

Field of a bar magnet at distance r (much larger than its length). Axial = 2x equatorial.

Moment m: 2.00 A·m²

Distance r: 10.0 cm

Field B

400.00 µT

B = \frac{2 μ _{0} m}{4 π r ^{3}}

Torque and PE in a uniform field

Same as the electric dipole result:

τ = m \times B, U = - m \cdot B .

A bar magnet free to rotate aligns itself with the external field, just like a compass needle.

Torque on a bar magnet in a uniform external field. Tries to align m with B.

m: 0.50 A·m²

B: 50.0 mT

θ: 60°

Torque τ

2.17e-2 N·m

τ = m B sin θ

Potential energy U

-1.25e-2 J

U = - m \cdot B = - m B cos θ

Earth's magnetism

Earth itself behaves like a giant bar magnet. The field at any place is described by three elements:

Declination: angle between true geographic north and magnetic north on a horizontal plane.
Dip / inclination: angle the field makes below the horizontal.
Horizontal component B_H: projection of B on the horizontal plane.

B_{H} = B cos δ, B_{V} = B sin δ, tan δ = B_{V} / B_{H} .

At the magnetic equator, dip is 0° (field horizontal). At the magnetic poles, dip is 90° (vertical).

Earth's magnetic field at a place is described by horizontal component B_H and dip angle. The total field and vertical component follow.

B_H (horizontal): 0.400 G

Dip angle δ: 60°

Total field B

0.800 G

= 80.0 µT

B = B_{H} / cos δ

Vertical component B_V

0.693 G

B_{V} = B_{H} tan δ

H, M, susceptibility and permeability

Magnetising field H: measures the field due to free currents only. Units: A/m.
Magnetisation M: dipole moment per unit volume. Units: A/m.
Magnetic field B: total field including the response of the medium.

B = μ_{0} (H + M), M = χH, μ_{r} = 1 + χ .

Susceptibility chi is a dimensionless number describing how strongly the material magnetises in response to H.

Magnetic susceptibility chi: M = chi H. Sign and size of chi tell you the type of material.

χ (susceptibility)

Try -0.0001 (diamagnetic), 0.001 (paramagnetic), 1000 (ferromagnetic).

Relative permeability μ_r

1.001e+0

μ_{r} = 1 + χ

Material type

paramagnetic

Diamagnetic, paramagnetic and ferromagnetic substances

Diamagnetic: chi small negative. Weakly repelled by a magnet. Examples: Bi, Cu, water, Au, Hg.
Paramagnetic: chi small positive. Weakly attracted. Examples: Al, O₂, Pt, Mn.
Ferromagnetic: chi large positive (10² to 10⁵). Strongly attracted. Can be permanently magnetised. Examples: Fe, Ni, Co, Gd, alloys.

Three classes of magnetic substances. Pick one to see its properties.

Diamagnetic

χ: small negative (~ -10⁻⁵)

μ_r: slightly < 1

Alignment: opposes B (weakly)

T dependence: no T dependence

Examples: Bi, Cu, Hg, water, gold, NaCl

M = χH, B = μ_{0} (H + M), μ_{r} = 1 + χ

Practice these on the timed test

Try a free 10-question NEET mock test on Magnetism and Matter, with instant results and no sign-up needed.

Curie's law and Curie temperature

For a paramagnet, susceptibility falls inversely with absolute temperature:

χ = \frac{C}{T} (Curie’s law) .

For a ferromagnet, there is a critical temperature called the Curie temperature T_C. Below T_C the material is ferromagnetic; above T_C it becomes paramagnetic and follows the Curie-Weiss law:

χ = \frac{C}{T - T _{C}} .

Iron has T_C around 1043 K; nickel around 627 K; cobalt around 1394 K.

For a paramagnet, susceptibility chi = C / T (Curie's law). Heating reduces alignment, dropping chi.

Curie constant C: 0.050 K

Applied field H: 1000 A/m

χ at 300 K = 1.67e-4

M at 300 K = 1.67e-1 A/m

χ = \frac{C}{T}, M = χH

Hysteresis loop

For a ferromagnetic material, M vs H traces a loop (not a single curve). Key features:

Saturation: at large H, all dipoles align and M reaches a maximum.
Retentivity (residual magnetism): M when H is removed. The reason permanent magnets keep their magnetism.
Coercivity: reverse H needed to bring M back to zero.
Loop area: energy lost per cycle as heat.

Wide loops (high coercivity, high retentivity) make good permanent magnets (steel, Alnico). Narrow loops (low coercivity, low retentivity) make good transformer cores (soft iron).

M vs H curve for a ferromagnetic material. The loop encloses energy lost as heat per cycle.

Coercivity (width): 40

Retentivity (height): 60

Wide loop = HARD magnet (high coercivity). Stays magnetised. Used in permanent magnets.

Narrow loop = SOFT magnet (low coercivity). Easy to magnetise/demagnetise. Used in transformer cores.

Worked NEET problems

NEET-style problem · Bar magnet

Question

A bar magnet of moment 2 A·m² is placed at distance 0.1 m from a small compass on its axial line. Find B.

Solution

B = \frac{μ _{0}}{4 π} \frac{2 m}{r ^{3}} = 1 0^{- 7} \cdot \frac{2 \times 2}{( 0.1 ) ^{3}} = 4 \times 1 0^{- 4} T .

NEET-style problem · Equatorial field

Question

Same magnet (m = 2 A·m²) at 0.1 m on the equator. Find B.

Solution

B = \frac{μ _{0}}{4 π} \frac{m}{r ^{3}} = 1 0^{- 7} \cdot \frac{2}{0.001} = 2 \times 1 0^{- 4} T .

Half the axial value, opposite direction.

NEET-style problem · Earth's field

Question

At a place, dip = 60° and B_H = 0.3 G. Find total B.

Solution

B = B_{H} / cos 60° = 0.3/ (0.5) = 0.6 G .

NEET-style problem · Susceptibility

Question

A material has chi = -0.0001. Identify the type and find mu_r.

Solution

Negative chi: diamagnetic.

μ_{r} = 1 + χ = 1 - 0.0001 = 0.9999.

NEET-style problem · Curie's law

Question

A paramagnetic substance has chi = 5 × 10⁻⁴ at 300 K. Find chi at 600 K.

Solution

By Curie's law, chi is proportional to 1/T. Doubling T halves chi:

χ (600) = χ (300) \times (300/600) = 2.5 \times 1 0^{- 4} .

Track your accuracy on every chapter

Summary cheat sheet

Bar magnet axial: $B = (μ_{0} /4 π) (2 m / r^{3})$ .
Bar magnet equatorial: $B = (μ_{0} /4 π) (m / r^{3})$ .
Torque: $τ = m B sin θ$ .
U: $U = - m B cos θ$ .
Earth's field: $B = B_{H} / cos δ$ .
M and chi: $M = χH$ , $μ_{r} = 1 + χ$ .
Diamagnetic: chi small negative.
Paramagnetic: chi small positive, follows $χ = C / T$ .
Ferromagnetic: chi very large positive, hysteresis below T_C.
Curie-Weiss: $χ = C / (T - T_{C})$ .

Next: try the interactive widgets for bar magnet field, Curie's law and hysteresis, or work through the 32 NEET PYQs with full solutions. To time yourself, take the free 10-question mock test.

Frequently asked questions

How many questions come from Magnetism and Matter in NEET 2027?

You can expect 1 question from this chapter in NEET 2027. The chapter has medium PYQ frequency. Bar magnet field and torque, Curie's law, classification of magnetic materials and basics of Earth's magnetism are favourites.

What is the magnetic dipole moment of a bar magnet?

A bar magnet with pole strength m_pole and length 2 L has dipole moment vector p_m equals m_pole times 2 L, directed from south to north pole. Unit: A m squared (or J per T). For a current loop of N turns, area A and current I, the moment is m equals N I A.

What is the field of a bar magnet?

On the axis at distance r far from the magnet, B equals 2 mu_0 m over (4 pi r cubed). On the equator (perpendicular bisector), B equals mu_0 m over (4 pi r cubed). Axial is twice equatorial. Both fall as 1 over r cubed.

What are the elements of Earth's magnetic field?

Three numbers describe Earth's field at any place: declination (angle between true geographic north and magnetic north on a map), dip or inclination (angle B makes with the horizontal plane), and horizontal component B_H. Total field B equals B_H over cos(dip).

What is the difference between diamagnetic, paramagnetic and ferromagnetic substances?

Diamagnetic: weakly repelled by a magnet. chi is small and NEGATIVE (around -10⁻⁵). Examples: bismuth, copper, water. Paramagnetic: weakly attracted. chi is small and POSITIVE (around +10⁻⁴). Examples: aluminium, oxygen. Ferromagnetic: strongly attracted, can be permanently magnetised. chi is large positive (10² to 10⁵). Examples: iron, nickel, cobalt.

What is Curie's law?

For a paramagnetic substance, the magnetisation M is proportional to the applied field H and inversely proportional to absolute temperature T: M equals C H over T, where C is the Curie constant. Equivalently, susceptibility chi equals C over T. Heating reduces magnetisation.

What is a hysteresis loop?

For a ferromagnetic material, the M vs H curve traces a loop instead of a straight line. Starting from zero, M rises with H to saturation. When H is reversed, M does not retrace the same path: there is residual magnetism (retentivity) at H equals 0, and M only reaches zero at a reverse field called the coercivity. The area of the loop equals the energy lost per cycle as heat.