Physics - electricity and magnetism
What the exam tests
Circuits, current, voltage, resistance, power and magnetic fields. Expect series and parallel calculations, plus questions on electromagnets, motors and electromagnetic induction.
Core rules
- Current is the rate of charge flow, measured with an ammeter in series.
- Voltage is energy per unit charge, measured with a voltmeter in parallel.
- Ohm's law: V = IR for a fixed resistance at constant temperature.
- Series: same current; resistances add; voltages add.
- Parallel: same voltage; 1/R = 1/R₁ + 1/R₂; total resistance is less than the smallest branch.
- Power: P = VI = I²R = V²/R.
- A current-carrying wire has a magnetic field around it; a coil with a core becomes an electromagnet. A changing magnetic field can induce a voltage.
Worked example
A 6.0 V battery is connected to 2.0 Ω and 3.0 Ω resistors in series. Total R = 5.0 Ω, so I = V/R = 6.0/5.0 = 1.2 A. The voltage across the 3.0 Ω resistor is V = IR = 1.2 × 3.0 = 3.6 V. Power in that resistor is I²R = 1.44 × 3.0 ≈ 4.3 W.
Traps
- Current is not “used up” around a series circuit; it is the same at every point.
- In parallel, total resistance is smaller than any branch, so total current rises.
- A voltmeter connected in series breaks the circuit; an ammeter in parallel can short it.