Electricity and Electromagnetism
This domain examines electric fields and circuits, then magnetic fields and induction. You should handle Coulomb forces, field strength and potential, capacitors, Kirchhoff’s laws, resistivity, EMF and internal resistance, the magnetic force F = BIL, and Faraday’s and Lenz’s laws.
Core rules
- Coulomb’s law: F = kQ1Q2/r². Electric field strength E = F/q; between parallel plates E = V/d.
- For capacitors, Q = CV, energy = ½CV², and series/parallel combinations follow the opposite rules to resistors.
- Kirchhoff: current into a junction equals current out; sum of EMFs equals sum of p.d.s around a loop. Terminal p.d. V = E − Ir.
- Resistance R = ρL/A; resistivity depends on material and temperature, not on the sample dimensions alone.
- A current-carrying conductor in a magnetic field experiences F = BIL sin θ; induced EMF magnitude is NΔΦ/Δt, with Lenz’s law giving the direction.
Worked example
A coil of 200 turns and area 0.020 m² sits perpendicular to a field that rises from 0.10 T to 0.30 T in 0.40 s. The flux change per turn is (0.30 − 0.10)(0.020) = 4.0 × 10⁻³ Wb. Using Faraday’s law, EMF = NΔΦ/Δt = 200(4.0 × 10⁻³)/0.40 = 2.0 V. The induced current opposes the increase, so its field points opposite the applied field.
Traps
- Treating EMF as terminal p.d. when current flows.
- Forgetting that flux Φ = BA cos θ depends on orientation.
- Adding capacitor series values like resistors.
- Ignoring internal resistance in power calculations.
- Using electron flow direction for conventional current.