Magnetism and Matter
Magnetisation and magnetic materials
Relate H, M and B and classify magnetic materials.
Put a material inside a solenoid, and the field changes. Three quantities describe what happens:
- H, the magnetising field, set by the current alone: H = nI inside a solenoid. It does not depend on the core. Unit A/m.
- M, the magnetisation: the net magnetic moment per unit volume of the material, from its lined-up atoms. M = χH, where χ (chi) is the magnetic susceptibility.
- B, the total field: B = µ₀(H + M) = µ₀µ_r H, with relative permeability µ_r = 1 + χ.
Example: n = 500 turns/m, I = 1 A, so H = 500 A/m. With an iron core of µ_r = 1000: B = 1000 × 4π × 10⁻⁷ × 500 ≈ 0.63 T, a thousand times the empty-coil field.
Materials fall into three groups:
| Type | χ | Behaviour | Examples |
|---|---|---|---|
| Diamagnetic | small, negative | weakly pushed out of a field | bismuth, copper, water |
| Paramagnetic | small, positive | weakly pulled into a field | aluminium, sodium, oxygen |
| Ferromagnetic | very large, positive | strongly pulled in; can stay magnetised | iron, nickel, cobalt |
In a ferromagnet, the atomic magnets line up in small regions called domains. A field makes the domains grow and line up, giving a huge M.
Temperature fights this alignment. Above the Curie temperature, a ferromagnet loses its strong magnetism and becomes paramagnetic.
A superconductor pushes a field out completely (χ = –1): it is a perfect diamagnet. This is the Meissner effect, used in magnetic levitation.
Soft iron is used for electromagnets (it magnetises and demagnetises easily); hard steel and special alloys are used for permanent magnets.
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