Electricity and Magnetism
What the exam tests
This domain covers electrostatics, capacitance, DC circuits, magnetic fields and electromagnetic induction. Expect calculations with Coulomb's law, capacitors, Kirchhoff's rules, internal resistance, magnetic force and Faraday's law.
Core rules
- Coulomb's law: F = kQ₁Q₂/r². Electric field strength E = F/q; potential V = kQ/r.
- Capacitance C = Q/V; energy stored = ½CV². Capacitors in series add reciprocally; in parallel add directly.
- Kirchhoff's rules: current into a junction equals current out; sum of e.m.f.s around a loop equals sum of p.d.s.
- Terminal p.d. = e.m.f. − Ir, where r is internal resistance.
- Magnetic force on a current-carrying conductor F = BIl sin θ.
- Faraday's law: induced e.m.f. = −N dΦ/dt. Lenz's law gives the direction opposing the change producing it.
Worked example
A coil of 200 turns and area 0.020 m² is in a magnetic field that increases uniformly from 0.10 T to 0.30 T in 0.50 s. Find the induced e.m.f. Change in flux density ΔB = 0.20 T. Flux change per turn ΔΦ = ΔBA = 0.20 × 0.020 = 4.0 × 10⁻³ Wb. Induced e.m.f. = N ΔΦ/Δt = 200 × (4.0 × 10⁻³)/0.50 = 1.6 V. The induced current direction opposes the increase in flux, as required by Lenz's law.
Traps
Using terminal p.d. as the e.m.f. when internal resistance is present; using the parallel formula for capacitors in series incorrectly; confusing magnetic flux with flux density; forgetting the negative sign as a direction indicator in Faraday's law; ignoring the angle between B and the conductor or coil area.