Electrical Fundamentals
Core concepts
You must understand electron flow versus conventional current, voltage, current and resistance, and apply Ohm's law (V = IR) and power relationships (P = VI = I²R = V²/R). Cover series and parallel circuits, Kirchhoff's laws, resistors, capacitors (charge Q = CV, energy ½CV²), inductors, magnetism and electromagnetism, AC theory (frequency, period, peak, RMS, average values, phase), transformers, and the basics of DC machines and generators.
Traps to watch
- Series vs parallel rules: in series, current is common and voltages add; in parallel, voltage is common and currents add. Resistance in parallel is always less than the smallest branch.
- RMS vs peak: for a sine wave, RMS = peak ÷ √2 ≈ 0.707 × peak. Heating and meter readings use RMS.
- Capacitor behaviour: blocks DC once charged, passes AC; voltage across a capacitor cannot change instantly.
- Inductor behaviour: opposes change of current; passes DC, opposes AC increasingly with frequency.
- Power factor: in AC circuits with reactance, true power = VI cos φ, not simply VI.
Worked example
Two resistors, 6 Ω and 3 Ω, in parallel across 12 V. Parallel resistance = (6 × 3)/(6 + 3) = 2 Ω. Total current = 12 ÷ 2 = 6 A. Branch currents: 12 ÷ 6 = 2 A and 12 ÷ 3 = 4 A; they sum to 6 A, confirming the result.
Rule of thumb
For parallel resistors, the product-over-sum shortcut works for two branches only; for three or more, use reciprocals. Sanity-check that the parallel result is smaller than the smallest resistor.