Electricity and Magnetism
Electricity and Magnetism
This domain tests current, potential difference, resistance, series and parallel circuits, electrical power and heating, plus magnetic fields, force on a current-carrying conductor, electromagnetic induction and domestic safety.
Core rules
- Ohm’s law: V = IR at constant temperature.
- Series: same current; R_total = R1 + R2 + ... .
- Parallel: same voltage; 1/R_total = 1/R1 + 1/R2 + ... .
- Electrical power P = VI = I²R = V²/R; energy in kWh = power in kW × time in hours.
- A fuse is in series with the live wire and melts when current exceeds its rating; earth wire gives a low-resistance path for leakage.
- Magnetic field lines run from north to south outside a magnet and never cross; field is strongest where lines are closest.
- Fleming’s left-hand rule gives force on a current-carrying conductor in a magnetic field; Fleming’s right-hand rule gives induced current direction.
- Electromagnetic induction needs relative motion between conductor and magnetic field.
Worked example
A 6 Ω and a 3 Ω resistor are connected in parallel across 12 V. Parallel resistance: 1/R = 1/6 + 1/3 = 1/6 + 2/6 = 3/6, so R = 2 Ω. Total current I = V/R = 12/2 = 6 A. Power drawn P = VI = 12×6 = 72 W. The 6 Ω branch carries 2 A and the 3 Ω branch 4 A, confirming the larger current in the smaller resistance.
Traps
Do not add parallel resistances directly. Keep current in ampere, time in second for joule heating, but use hours for kWh. A fuse is not placed in the neutral wire. Induced current opposes the change producing it, not necessarily the motion you expect.