Electricity and Magnetism
Electricity and Magnetism
This domain covers electrostatics, electric fields and potential, capacitance, direct-current circuits, magnetism, and electromagnetic induction. The exam tests both conceptual reasoning and algebra with Ohm's law, Kirchhoff's rules, and Faraday's law.
Core rules and concepts
- Coulomb's law: F = kq1q2/r². Electric field E = F/q; potential V = kq/r. Field lines point from positive to negative charge.
- Capacitance C = Q/V; for parallel plates C = ε0A/d. Capacitors in parallel add; in series add reciprocally.
- Ohm's law V = IR. In series, current is the same and voltages add; in parallel, voltage is the same and currents add.
- Kirchhoff's junction rule conserves charge; loop rule conserves energy.
- Magnetic force on a moving charge: F = qvB sinθ, perpendicular to v and B. Faraday's law: emf = -ΔΦB/Δt; Lenz's law gives the direction of induced current opposing the flux change.
Worked example
Two resistors, 6 Ω and 3 Ω, are connected in parallel across a 12 V battery. Equivalent resistance: 1/R = 1/6 + 1/3 = 1/2, so R = 2 Ω. Total current: I = V/R = 6 A. The 6 Ω resistor carries 2 A; the 3 Ω resistor carries 4 A, summing to 6 A.
Traps that cost marks
Adding series resistances as reciprocals; assuming current is used up in a series circuit; forgetting that magnetic force does no work because it is perpendicular to velocity; and misapplying Lenz's law so induced current aids rather than opposes change.