Electricity and Magnetism
Electricity and magnetism questions cover Coulomb's law, field and potential, DC circuits, Kirchhoff's rules, magnetic forces and electromagnetic induction.
Core rules
- Coulomb: F = kq₁q₂/r², k = 9×10⁹ N m² C⁻². Electric field E = F/q; field lines point away from positive charge.
- Ohm: V = IR. Series: R = R₁ + R₂ + …, same current. Parallel: 1/R = 1/R₁ + 1/R₂ + …, same voltage.
- Power: P = VI = I²R = V²/R.
- Kirchhoff: currents into a junction equal currents out; around a loop, sum of emfs equals sum of IR drops.
- Magnetic force on a current: F = BIL sinθ; on a moving charge: F = qvB sinθ, giving circular radius r = mv/(qB).
- Faraday: ε = −dΦ/dt with Φ = BA cosθ. Lenz's law: induced current opposes the change in flux.
Worked example
A 6 Ω and a 3 Ω resistor are connected in parallel across a 12 V supply. The equivalent resistance is (6×3)/(6+3) = 2 Ω, so the total current is 12/2 = 6 A. The 6 Ω branch carries 12/6 = 2 A and the 3 Ω branch carries 4 A. Total power is VI = 12×6 = 72 W.
Traps
Believing a parallel combination is larger than the smallest resistor; ignoring internal resistance when it is stated; reversing Lenz's-law direction; using peak voltage where rms is required; mixing mA and A in power calculations.