Electrical Principles and Theory
Core concepts
A candidate must be fluent with Ohm's law, the power relationships (P = VI, P = I²R, P = V²/R), series and parallel networks, and Kirchhoff's voltage and current laws. Understand AC fundamentals: RMS versus peak versus average values, frequency, phase angle, power factor, and the difference between real power (watts), reactive power (VAR) and apparent power (VA). Know how inductance and capacitance behave at different frequencies, and how a transformer trades voltage for current while conserving power.
Traps in questions
Watch for unit prefixes (mA versus A, kΩ versus Ω) and for questions that ask for total resistance in a mixed series-parallel network. Candidates often forget that voltage is common across parallel branches while current is common in a series path. Power factor questions frequently hide the distinction between kW and kVA. Also be careful with conductor resistance changing with temperature and with the difference between conductor resistance and insulation resistance.
Worked example
Two resistors, 6 Ω and 3 Ω, are connected in parallel across a 12 V supply. The parallel combination is 1/R = 1/6 + 1/3 = 3/6, so R = 2 Ω. Total current is I = V/R = 12/2 = 6 A. Branch currents are 2 A and 4 A, confirming Kirchhoff's current law. Total power is P = VI = 12 × 6 = 72 W, which also equals 2²×6 + 4²×3 = 24 + 48 = 72 W.
Rule of thumb
For two resistors in parallel, R = (R1 × R2)/(R1 + R2). Always redraw a mixed network into simple series and parallel blocks before calculating.