Electrical Fundamentals
What you must understand
Build a solid grasp of electron theory, conductors, insulators and semiconductors, and the quantities: charge, current, voltage, resistance, power and energy. Know Ohm's law, Kirchhoff's voltage and current laws, and series/parallel resistor combinations. Understand capacitance and inductance in principle, AC theory (frequency, period, peak versus RMS, phase), and the behaviour of R, L and C in AC circuits including impedance and resonance. Study magnetism and electromagnetism, transformers (turns ratio), and the construction and characteristics of batteries, including lead-acid and nickel-cadmium, plus charging hazards. Know the function of fuses, circuit breakers and bonding/earthing.
Traps in questions
- RMS versus peak: for a sine wave, RMS = peak ÷ √2 ≈ 0.707 × peak. Questions swap them.
- Parallel resistance: total is always less than the smallest resistor; the reciprocal formula is needed for more than two.
- Power formulas: P = VI = I²R = V²/R. Choose the one using the quantities you actually know.
- Series versus parallel voltage: voltage divides in series, is common in parallel; current is common in series, divides in parallel.
- Inductive/capacitive reactance signs: they oppose each other and cancel at resonance.
Worked example
Two resistors, 6 Ω and 3 Ω, in parallel: 1/R = 1/6 + 1/3 = 1/6 + 2/6 = 3/6, so R = 2 Ω. In series they would be 9 Ω. With 12 V across the parallel pair, current = 12/2 = 6 A total, and power = VI = 72 W.
Rule of thumb
Check every answer against a limiting case: parallel resistance must be smaller than the smallest branch; series resistance must exceed the largest.