Magnetic Effects of Electric Current
The exam checks the link between current and magnetism: field patterns, the rules for direction, the force on a current-carrying conductor, and the magnetic circuit quantities.
Magnetic quantities
Flux Φ is in webers; flux density B = Φ/A is in tesla (Wb/m²). Magnetomotive force mmf = NI ampere-turns. Reluctance S = l/(μ₀μ_r A), where μ₀ = 4π × 10⁻⁷ H/m and μ_r is relative permeability. This mirrors resistance, so series and parallel magnetic paths can be treated like resistor networks.
Rules for direction
- Right-hand grip rule: thumb along the current, curled fingers show the field around a straight conductor.
- Solenoid / coil: the end where current appears anticlockwise is the north pole.
- Fleming's left-hand rule: force on a conductor - used for motors. First finger field, second finger current, thumb motion.
Force on a conductor
F = BIl sin θ newtons, maximum when the conductor is at right angles to the field.
Worked example
A conductor 0.25 m long carries 15 A at right angles to a flux density of 0.8 T. F = BIl sin 90° = 0.8 × 15 × 0.25 × 1 = 3 N. The direction is given by Fleming's left-hand rule.
Electromagnets and losses
A soft-iron core inside a solenoid becomes an electromagnet; it loses magnetism when the current stops, which is why soft iron suits relays and bells while steel suits permanent magnets. Alternating magnetisation causes hysteresis loss (area of the B–H loop) and eddy-current loss (reduced by laminations).
Traps
- Using the right-hand rule where Fleming's left-hand rule is required.
- Confusing flux (Wb) with flux density (T).
- Forgetting sin θ, or using degrees directly in a calculator set to radians.
- Writing μ₀ without the 4π × 10⁻⁷ value.