Electricity and Magnetism
What the exam tests
Electricity and Magnetism covers charge, electric fields, potential difference, resistance, series and parallel circuits, electrical power, capacitance, magnetic fields and electromagnetic induction. Circuit calculations and graph interpretation dominate.
Core rules
- Coulomb's law: F = kQ₁Q₂/d², attractive for opposite charges.
- Ohm's law: V = IR holds at constant temperature for a metallic conductor.
- Resistance from resistivity: R = ρL/A; doubling the area halves the resistance.
- Series: R total = R₁ + R₂ + …, current the same everywhere, voltage shared.
- Parallel: 1/R total = 1/R₁ + 1/R₂ + …, voltage the same, current shared.
- Power: P = VI = I²R = V²/R.
- Capacitance C = Q/V; energy stored = ½CV².
- Magnetic flux Φ = BA; induced emf = −N dΦ/dt (Faraday), with Lenz's law giving the direction.
Worked example
A 12 V battery drives a 6 Ω resistor in parallel with a 3 Ω resistor. Find the total resistance and the current from the battery. 1/R = 1/6 + 1/3 = 3/6, so R = 2 Ω. I = V/R = 12/2 = 6 A. Check: 4 A through the 6 Ω and 2 A through the 3 Ω, total 6 A.
Traps
- Adding parallel resistors directly.
- Using the supply voltage across every resistor in a series circuit.
- Mixing mA and A, or kW and W.
- Saying Lenz's law opposes the current rather than the change in flux.
- Forgetting that an ammeter goes in series and a voltmeter in parallel.