Magnetism and Matter: Bar Magnet, Earth's Field & Magnetic Properties
Study bar magnets, magnetic dipoles, Earth's magnetism and magnetic properties of materials like diamagnetic, paramagnetic and ferromagnetic for JEE Main Physics.
The central idea of this chapter
Matter responds differently to magnetic fields based on its atomic structure. This chapter connects bar magnets, Earth's field and magnetic properties of materials through dipole concepts and susceptibility.
What should you understand first?
Magnetism in matter arises from atomic currents and electron spin. Once you understand dipole behaviour and classification of materials, most problems become direct applications.
Core concepts
- Bar magnet & magnetic dipole m = pole strength × 2l
- Magnetic field lines Closed loops
- Earth's magnetism Declination, dip
- Magnetic intensity & flux density H, B, μ
- Magnetic properties of matter Diamagnetic, paramagnetic, ferromagnetic
Useful building blocks
- Field on axis B = (μ₀/4π) · 2m/r³
- Field on equator B = (μ₀/4π) · m/r³
- Torque on dipole τ = mB sinθ
- Potential energy U = −mB cosθ
- Susceptibility χ = M/H
Magnetism and Matter formula sheet
Use dipole analogy consistently for bar magnets. Memorise field expressions and properties of different magnetic materials.
| Topic | Formula / Relation | Meaning or use |
|---|---|---|
| Magnetic dipole moment | m = pole strength × 2l | Strength of magnetic dipole |
| Field on axial line | B = (μ₀/4π) · 2m/r³ | Short dipole at distance r on axis |
| Field on equatorial line | B = (μ₀/4π) · m/r³ | Short dipole at distance r on equator |
| Torque on dipole | τ = mB sinθ | Dipole in uniform magnetic field at angle θ |
| Potential energy | U = −mB cosθ | Energy of dipole in magnetic field |
| Work done in rotation | W = mB(cosθ₁ − cosθ₂) | Rotating dipole from θ₁ to θ₂ |
| Earth's magnetic elements | BH = B cosδ, BV = B sinδ | Horizontal and vertical components, δ is angle of dip |
| Magnetic intensity | H = B/μ | Magnetising field intensity |
| Intensity of magnetisation | M = m/V | Magnetic moment per unit volume |
| Magnetic susceptibility | χ = M/H | Measure of how easily a material is magnetised |
| Relative permeability | μr = 1 + χ | Ratio of material permeability to μ₀ |
| Curie's law | χ ∝ 1/T | For paramagnetic materials, χ inversely proportional to T |
How to approach magnetism and matter problems
Identify whether the problem is about field, torque, energy or material properties. Use dipole formulas for bar magnets and susceptibility relations for materials.
Bar magnet & dipole
- Field at a point? Axial or equatorial
- Torque asked? τ = mB sinθ
- Energy or work? U = −mB cosθ
- Earth's field? BH, BV, δ
- Oscillation of magnet? T = 2π√(I/mB)
Magnetic materials
- Diamagnetic? χ < 0, small
- Paramagnetic? χ > 0, small
- Ferromagnetic? χ >> 0, large
- Temperature effect? Curie's law
- Permeability? μr = 1 + χ
How to prepare Magnetism and Matter
Start with bar magnet as dipole, then learn Earth's magnetism and magnetic elements. Finish with properties of diamagnetic, paramagnetic and ferromagnetic materials.
What to do
- Learn dipole field expressions on axis and equator
- Practise torque, energy and work done problems for bar magnet
- Understand Earth's magnetic elements and their relations
- Memorise properties and examples of diamagnetic, paramagnetic and ferromagnetic materials
- Revise Curie's law and temperature dependence of susceptibility
Common mistakes
- Confusing axial and equatorial field formulas for dipole
- Forgetting that potential energy is minimum when dipole aligns with field
- Mixing up signs of susceptibility for different materials
- Not using correct relation between B, H, M and χ
- Ignoring temperature effect on paramagnetic susceptibility
Ready to test Magnetism and Matter?
Revise the formula sheet, then solve mixed JEE Main problems on bar magnets, Earth's field and magnetic properties.
Magnetism and Matter FAQ
Short answers to frequently tested ideas in this chapter.
A magnetic dipole consists of two equal and opposite magnetic poles separated by a small distance. It is characterised by magnetic dipole moment m = pole strength × separation.
Earth behaves like a giant bar magnet with its magnetic south near geographic north and magnetic north near geographic south. It has horizontal and vertical components of magnetic field.
Diamagnetic materials are weakly repelled by magnetic fields. They have negative susceptibility and move from stronger to weaker parts of a non-uniform magnetic field.