Electrical Fundamentals
What you must know
Candidates must master electron flow versus conventional current, voltage, current and resistance, Ohm's law (V = IR), power (P = VI = I²R = V²/R), series and parallel circuits, Kirchhoff's laws, and the magnetic effects of current. You also need capacitance (series and parallel rules are the reverse of resistors), inductance, AC fundamentals (frequency, period, RMS versus peak, phase), and the basics of transformers and batteries.
Traps to watch
- Series versus parallel rules: resistance adds in series; in parallel the reciprocal adds, so total resistance is always less than the smallest branch.
- Capacitors invert the resistor rules: they add in parallel and combine reciprocally in series.
- RMS versus peak: for a sine wave, RMS = peak / √2 ≈ 0.707 x peak. Aircraft 115 V AC is RMS; the peak is about 163 V.
- Power in AC: P = VI only for purely resistive loads; power factor matters otherwise.
- Kirchhoff's current law: current entering a node equals current leaving; do not forget branch currents.
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. Across a 12 V supply, total current I = V/R = 12/2 = 6 A. Branch currents: 12/6 = 2 A and 12/3 = 4 A, which sum to 6 A, confirming Kirchhoff's current law.
Rule of thumb
For two resistors in parallel, R = (R1 x R2)/(R1 + R2). For any parallel network, the total is smaller than the smallest branch.