Magnetism is the physical phenomenon by which moving electric charges and magnetic materials exert forces on one another. It is described by a magnetic field, with magnetic flux Φ measured in webers and flux density B measured in teslas.
Learning objectives
- Define magnetic flux and flux density
- Calculate flux density from flux and area
- Calculate the force on a current-carrying conductor
- Explain how a coil produces a magnetic field
- Describe the role of magnetic cores and their losses
Progressive explanation
Start with the plain-language idea, then move into the engineering detail. Each level stands on its own, so you can stop at the depth you need.
Beginner The core idea
Magnetism matters in electrical engineering because motors, generators, transformers and relays all work by shaping and changing magnetic fields.
Intermediate How it behaves in a circuit
The magnetic field of a coil is proportional to the current and the number of turns; a ferromagnetic core concentrates the flux because its permeability is much higher than that of air. Saturation limits how much flux a given core can carry.
Advanced Limits, assumptions, engineering detail
Formulas and variables
Units of measurement
| Quantity | Symbol | Unit |
|---|---|---|
| Magnetic flux | Φ | weber (Wb) |
| Flux density | B | tesla (T) |
| Magnetic field strength | H | ampere per metre (A/m) |
| Force | F | newton (N) |
Worked examples
Flux density in a core
Problem. A magnetic flux of 2 mWb passes through a cross-section of 0.005 m². Find the flux density.
Solution. B = Φ/A = 0.002/0.005.
Force on a conductor
Problem. A 0.3 m conductor carrying 20 A sits at 90° to a 0.5 T field. What force acts on it?
Solution. F = BIL sin 90° = 0.5 × 20 × 0.3 × 1.
Air-core solenoid
Problem. A 0.5 m long air-core solenoid has 500 turns and carries 2 A. Estimate the flux density inside it.
Solution. B = µ0NI/l = (4π × 10⁻⁷ × 500 × 2)/0.5.
Practical applications
- Electric motors and generators
- Transformers and inductors
- Relays and contactors
- Magnetic sensors and encoders
- Magnetic separation and lifting
- Magnetic resonance imaging
Automotive applications
Hall and variable-reluctance sensors detect rotation through magnetic fields, and motors, alternators, ignition coils and relays all depend on magnetism. Electric-vehicle traction motors use strong permanent magnets or excited rotors to produce torque.
Industrial applications
Industrial motors, transformers and lifting magnets are designed around magnetic circuits, and non-destructive testing uses magnetic particles to reveal surface cracks in ferromagnetic parts.
Measurement procedure
- Use a gaussmeter or teslameter to measure flux density, and a fluxmeter for flux.
- Keep magnetised tools and strong magnets away from instruments and magnetic storage.
- Measure in a defined direction, because flux density is a vector and probes are directional.
- Account for the probe area and orientation when converting between flux and flux density.
Common mistakes
- Confusing magnetic flux with flux density; they differ by the area.
- Using degrees instead of radians, or forgetting sin θ, in the force formula.
- Ignoring saturation and assuming flux rises in proportion to current indefinitely.
- Assuming the field is uniform when the geometry concentrates it.
Safety considerations
- Strong magnetic fields can attract metal objects violently and interfere with pacemakers and implants.
- Keep magnets away from magnetic storage media and sensitive electronic devices.
- Magnetic components can heat up during operation; allow them to cool before handling.