Electrical Theory and Principles
Core relationships
Candidates must be fluent with Ohm's law, the power relationships, and how they combine in series and parallel circuits. In series, current is common and voltage divides; in parallel, voltage is common and current divides. Know Kirchhoff's voltage and current laws as bookkeeping tools and be able to compute equivalent resistance for simple networks.
AC and magnetism
Understand RMS versus peak versus average values, frequency, phase relationships, and power factor. Real power is measured in watts, reactive power in VARs, apparent power in volt-amperes, and the power triangle ties them together. Know how inductance and capacitance produce reactance, how impedance combines resistance and reactance vectorially, and why transformers trade voltage for current. Magnetism underlies motors, transformers, and induction, so know flux, reluctance, and the right-hand rules.
Traps in questions
Frequent errors include adding resistances vectorially when they should be added directly, treating RMS as peak, ignoring power factor when computing current from kVA, and assuming a parallel branch changes voltage across other branches. Another trap is a unit mismatch: mixing milliamps with amps or watts with volt-amperes.
Worked example
A 240 V single-phase load draws 15 A at 0.8 power factor. Real power = 240 x 15 x 0.8 = 2880 W. Apparent power = 240 x 15 = 3600 VA. Reactive power = sqrt(3600 squared minus 2880 squared) = 2160 VAR. Rule of thumb: convert everything to base units first, then apply the triangle.
Practice approach
Redraw every circuit, label knowns and unknowns, and check whether the answer is physically sensible before selecting it.