Electrical Fundamentals and Circuit Principles
Electrical Fundamentals and Circuit Principles - what the exam tests
Ohm's law and power, series and parallel networks, voltage drop, Kirchhoff's laws, magnetism and relays, cable and fuse selection, and basic component behaviour. Questions here are numeric and circuit-logic based, so show the calculation.
Core laws
- V = I × R; P = V × I = I²R = V²/R.
- Series: one path, same current everywhere, voltages add, R_total = R1 + R2 + …
- Parallel: same voltage across branches, currents add, 1/R_total = 1/R1 + 1/R2 + …
- Kirchhoff: current into a node equals current out; voltages around a closed loop sum to zero.
- Cable voltage drop V = I × R_cable. Keep total circuit drop low, commonly 0.5 V or about 3% of system voltage.
Circuit logic
- A fuse protects the cable, not the device; size it near 125% of expected load current.
- A relay lets a small switch current control a large load current. Terminals: 30 supply, 87 load out, 85 and 86 coil, 87a rest contact.
- Resistance rises with temperature, but a cold filament has low resistance and draws a large inrush - typically about ten times steady current - which is why bulbs fail at switch-on.
Worked example
A 55 W headlamp on 12 V draws I = 55/12 = 4.6 A. The feed and ground path together measure 0.15 Ω. Voltage drop = 4.6 × 0.15 = 0.69 V, about 5.8% of system voltage - too much, and the lamp is dim. Fit a heavier cable and clean the ground. Reasoning: drop is proportional to current, so a poor ground hurts high-current circuits most.
Traps that cost marks
- Applying series rules to a parallel branch.
- Measuring resistance with the circuit live.
- Ignoring ground-side resistance.
- Fitting a bigger fuse to stop it blowing - the fault is still there.
- Judging a loaded circuit from open-circuit voltage.