Magnetism
Magnetic quantities
A current produces a magnetic field; a coil concentrates it. Flux Φ (weber) is the total field through an area; flux density B (tesla) is flux per unit area, B = Φ/A. Magnetomotive force mmf = NI (ampere-turns) drives flux through a magnetic circuit, opposed by reluctance S = l/(µ₀µ_r A). Flux Φ = mmf/reluctance - the magnetic analogue of Ohm's law. Field strength H = NI/l (A/m) and B = µ₀µ_r H.
Materials
Ferromagnetic materials (iron, nickel, cobalt) have high permeability and concentrate flux. Soft iron is used for cores and pole pieces because it magnetises and demagnetises easily (small hysteresis loop), while steel retains magnetism and makes permanent magnets. Saturation limits flux density, and above the Curie temperature a magnet permanently loses its magnetism.
The B-H curve shows initial magnetisation, saturation and hysteresis. The loop area represents energy lost as heat per cycle, which is why AC machines use laminated soft-iron cores.
Traps
- Flux (Wb) and flux density (T) are frequently swapped in questions.
- The field is strongest where flux lines are closest - at the poles, not midway along a magnet.
- Cutting a magnet produces two magnets, each with a north and a south pole.
- The relative permeability of air is effectively 1; inserting iron raises flux for the same mmf.
Worked example
A coil of 500 turns on a core of reluctance 2 x 10⁵ A/Wb carries 0.4 A: mmf = 200 A-turns, Φ = 200 / (2 x 10⁵) = 1 mWb.