Magnetism and Electromagnetic Induction
What this domain tests
Magnetism and Electromagnetic Induction tests magnetic force on charges and currents, fields of wires and solenoids, Faraday's law, Lenz's law, inductance and AC generators. Expect direction rules and flux-change calculations.
Core rules
- Magnetic force on a charge: F = qvB sinθ; direction by right-hand rule. On a current wire: F = BIL sinθ.
- Field of a long straight wire: B = μ0I/(2πr). Inside a solenoid: B = μ0nI.
- Faraday's law: emf = -dΦ/dt, where Φ = BA cosθ. Lenz's law: induced current opposes the change in flux.
- Motional emf for a rod moving perpendicular to B: ε = Blv.
- Self-induced emf ε = -L dI/dt; energy stored in an inductor U = 1/2 LI².
Worked example
A 0.5 m conducting rod moves at 2 m/s perpendicular to a uniform magnetic field of 0.4 T. Find the induced emf. ε = Blv = 0.4 × 0.5 × 2 = 0.4 V. The reasoning requires the rod, velocity and field to be mutually perpendicular; if the velocity makes an angle θ with B, use the perpendicular component v sinθ.
Traps
Magnetic force is zero when velocity is parallel to B. Do not use the left-hand rule for negative charges without reversing direction. Induced emf depends on rate of change of flux, not on flux itself. Lenz's law gives direction, not magnitude. In a solenoid, n is turns per metre, not total turns.