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Magnetism
Theory of magnetism; properties of a magnet; magnetic field around a conductor and the right-hand grip rule; magnetomotive force, flux, flux density and permeability; the hysteresis loop; eddy currents and lamination.
Magnetism — summary notes
FreeMain ideas
- Magnetic field lines run from north to south OUTSIDE the magnet and south to north inside it. They never cross.
- Like poles repel, unlike poles attract.
- Magnetomotive force MMF = N × I (ampere-turns) drives flux around a magnetic circuit — the magnetic analogue of EMF.
- Flux Φ is measured in webers. Flux density B = Φ / A is measured in teslas. Permeability μ = B / H describes how easily a material carries flux.
- The hysteresis loop shows retentivity (remanence), coercive force, and saturation. Soft iron has a narrow loop and low loss; hard steel has a wide loop and makes permanent magnets.
- Eddy currents circulating in a solid core waste energy as heat, so cores are laminated to break up the current paths.
- The right-hand grip rule gives field direction: thumb along conventional current, fingers curl in the direction of the field.
- ⚠ Exam trap: flux Φ (webers) is not the same as flux density B (teslas). B is flux per unit area.
Key formulas
- Magnetomotive force
- MMF = N × I (ampere-turns)
- Flux density
- B = Φ / A (teslas)
- Field strength
- H = NI / l (ampere-turns per metre)
- Permeability
- μ = B / H
Solved examples
A coil of 500 turns carries 2 A. What is the magnetomotive force?
MMF = N × I = 500 × 2 = 1000 ampere-turns.
A flux of 0.006 Wb passes through a pole face of 0.003 m². Find the flux density.
B = Φ / A = 0.006 / 0.003 = 2 T.
Magnetism concept map
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Magnetism quiz
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