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
- 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.
- 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
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
Magnetism
Magnetism quiz
Magnetism, quiz
1. A coil of 200 turns carries 3 A. Its magnetomotive force is:
600 ampere-turns66.7 ampere-turns203 ampere-turns2. The left-hand rule (Fleming's) applied to a current-carrying conductor in a magnetic field gives the direction of:
The force on the conductorThe induced EMFThe magnetic flux3. Outside a bar magnet, magnetic field lines run:
From north to southFrom south to northIn both directions equally4. A coil of 500 turns carries 2 A. The magnetomotive force is:
1000 ampere-turns250 ampere-turns502 ampere-turns5. Flux density B is measured in:
TeslasWebersHenries6. A flux of 0.006 Wb passes through 0.003 m². The flux density is:
2 T0.5 T18 T7. Transformer and machine cores are laminated in order to:
Reduce eddy-current lossesIncrease the flux densityReduce copper losses8. The ability of a material to retain magnetism after the field is removed is called:
RetentivityCoercivityPermeability9. The right-hand grip rule relates:
Current direction to magnetic field directionForce to motion in a generatorVoltage to current in a resistor10. Permeability μ is defined as:
B / HH / BN × I