Electricity and Magnetism
Domain 5: Electricity and Magnetism
This domain spans electrostatics, DC circuits, magnetism, and electromagnetic induction. Expect Coulomb's law, field and potential, series and parallel networks, Kirchhoff's rules, forces on moving charges and wires, and Faraday and Lenz reasoning.
Core rules
- Coulomb: F = kq1q2/r²; field E = F/q; potential V = kq/r. Field is a vector; potential is a scalar.
- Ohm: V = IR; P = IV = I²R = V²/R. Series: same current, resistances add. Parallel: same voltage, reciprocal resistances add.
- Kirchhoff: junction currents sum to zero; loop voltage changes sum to zero.
- Magnetic force: F = qvB sinθ on a charge, F = BIL sinθ on a wire; direction by right-hand rule. A charge moving parallel to B feels no force.
- Faraday: emf = −N ΔΦ/Δt; Lenz's law says the induced current opposes the change in flux, not the flux itself.
Worked example
A 12 V battery connects to a 4 Ω resistor in series with a parallel pair of 6 Ω and 3 Ω. The pair is 1/(1/6 + 1/3) = 2 Ω, so total R = 6 Ω and I = 2 A. The parallel voltage is 2 A × 2 Ω = 4 V, so the 6 Ω branch carries 4/6 = 0.67 A and the 3 Ω branch 1.33 A. Branch currents sum to 2 A.
Traps
- Saying current is 'used up' around a circuit; charge is conserved.
- Adding parallel resistances directly or assuming voltage splits equally in parallel.
- Reversing Lenz's law: induced flux opposes the change, and may aid the original flux when it decreases.
- Using the right-hand rule for a negative charge without flipping the direction.
- Ignoring internal resistance, which lowers terminal voltage under load.