Basic Electricity, Magnetism and Electrical Units
What the exam tests
Ohm's law, series and parallel circuits, power, magnetism and SI units applied to vehicle circuits.
Core rules
- V = I × R; P = V × I = I²R = V²/R.
- Series: one current path, current is common, voltages add, resistances add.
- Parallel: voltage is common, currents add, total resistance is less than the smallest branch; 1/RT = 1/R1 + 1/R2.
- A 12 V system is nominal. Resting battery is about 12.6 V; charging is 13.8–14.8 V. Use actual measured voltage in calculations.
- Magnetism: current creates a magnetic field (right-hand grip rule). A coil creates an electromagnet; collapsing field induces a back EMF, which is why relays and injectors have suppression diodes.
- Units: volt (V), ampere (A), ohm (Ω), watt (W), coulomb (C), farad (F), henry (H), hertz (Hz).
- AC signals appear at sensors and alternator ripple; RMS × 1.414 gives peak for a sine wave.
Worked example
A 60 W headlamp is supplied through 0.03 Ω of cable and earth return from a 12.6 V battery. Current = P/V = 60/12.6 = 4.76 A. Cable drop = I × R = 4.76 × 0.03 = 0.14 V. Lamp voltage = 12.6 − 0.14 = 12.46 V. If the lamp is dim and you measure 10.8 V at the bulb, the 1.8 V difference is lost in the circuit - calculate the expected drop, then use voltage-drop testing to find the bad joint rather than guessing.
Traps
Adding parallel resistances arithmetically; ignoring return-path resistance; using 12.0 V instead of measured voltage; confusing amp-hours with amps; assuming a relay coil has no resistance.