Electrical Theory and Fundamentals
Core concepts
A candidate must be fluent in Ohm's law (E = I × R), power relationships (P = I × E, and the single-resistor forms P = I²R and P = E²/R), and series/parallel circuit behavior. Know that series circuits share one current and divide voltage, while parallel circuits share one voltage and divide current. Understand AC fundamentals: RMS versus peak (V_peak = V_RMS × 1.414), frequency, phase, inductance and capacitance reactance (X_L = 2πfL, X_C = 1/(2πfC)), and impedance as the vector combination of resistance and reactance. Power factor, apparent power (VA) versus true power (W), and three-phase relationships (P = √3 × E × I × PF) are frequent topics. Magnetism, induction, and transformer turns ratios (E₁/E₂ = N₁/N₂, with current in inverse ratio) round out the fundamentals.
Traps
Watch for unit confusion (milliamps, kilowatts), mixing RMS and peak values, and forgetting that power factor reduces true power below the VA product. Questions often ask for total resistance of mixed networks-simplify parallel groups first. Another trap is assuming neutral carries no current in unbalanced multiwire circuits.
Worked example
A 240 V single-phase load draws 20 A at 0.8 power factor. True power = 240 × 20 × 0.8 = 3,840 W (apparent = 4,800 VA). If asked for resistance of a purely resistive equivalent, use R = E/I = 12 Ω only when PF = 1; with PF < 1 the impedance is 12 Ω but resistance is 9.6 Ω.
Rule of thumb
For quick three-phase estimates: kW ≈ 1.732 × volts × amps × PF ÷ 1,000.