Electrical Fundamentals
Core content
You must be confident with Ohm's law, series and parallel circuits, Kirchhoff's voltage and current laws, electrical power, and the behaviour of resistors, capacitors and inductors. In DC circuits, know how voltage divides in series and current divides in parallel. In AC circuits, understand root-mean-square values, frequency, period, reactance, impedance, phase angle, resonance and power factor. Know how transformers change voltage and current, how batteries store energy, and how magnetism links current to force and induction. Capacitors oppose changes in voltage; inductors oppose changes in current.
Traps
The most common trap is confusing peak and RMS values: for a sine wave, peak equals RMS multiplied by about 1.414. Another is assuming parallel resistance adds directly; it does not, and the total is always less than the smallest branch. In AC, current and voltage may be out of phase, so power is not simply volts times amps unless the load is resistive. Watch for leading and lagging: capacitor current leads voltage, inductor current lags voltage. Also check units: reactance and impedance are in ohms.
Worked example
Two resistors of 6 ohms and 3 ohms are connected in parallel. Use reciprocals: 1/R = 1/6 + 1/3 = 1/6 + 2/6 = 3/6, so R = 2 ohms. The result is less than either resistor, which is a useful check. Rule of thumb: for two parallel resistors, R = (R1 x R2)/(R1 + R2).
Key points
- Ohm's law: V = I x R, with consistent units.
- Parallel resistance uses reciprocals and is less than the smallest branch.
- Sine-wave peak voltage is about 1.414 times the RMS value.
- Capacitor current leads voltage; inductor current lags voltage.
- Transformer power is approximately constant, so voltage and current change inversely.