Electrical Principles and Circuit Fundamentals
Electrical Principles and Circuit Fundamentals is the foundation domain. The trade test checks whether you can calculate voltage, current, resistance and power in vehicle circuits, recognise series and parallel networks, and explain short, open and high-resistance faults. You must be able to apply Ohm's law and Kirchhoff's laws to a wiring diagram, not just quote formulas.
Core rules
- Ohm's law: V = I × R. Power: P = V × I = I²R = V²/R.
- Series: one path, same current, voltages add, resistances add.
- Parallel: same voltage across branches, branch currents add, 1/Rt = 1/R1 + 1/R2 + ...
- Kirchhoff: current into a node equals current out; voltage rises and drops around a loop sum to zero.
- Conventional current flows positive to negative; electron flow is opposite. Vehicle bodies are the return path on negative-earth systems.
Worked example
A 12 V battery supplies a 3 Ω resistor in series with a 6 Ω resistor. Total resistance = 9 Ω, so I = 12/9 = 1.33 A. The 3 Ω drop is 3 × 1.33 = 4.0 V; the 6 Ω drop is 6 × 1.33 = 8.0 V. The two drops add to 12 V, confirming Kirchhoff's voltage law. Total power = 12 × 1.33 = 16 W, or I²R = 1.33² × 9 = 16 W. If the 6 Ω item is a lamp designed for 12 V, it runs dim because it receives only 8 V.
Traps that cost marks
Adding parallel resistances directly instead of using reciprocals. Assuming a battery remains at 12.0 V while cranking. Testing resistance on a live circuit. Ignoring resistance in the return path, terminals and grounds. Treating a short circuit as an open circuit; a short has near-zero resistance and should blow protection, while an open stops current.