Electrical Fundamentals
DC basics
Conventional current flows from positive to negative; electron flow is the opposite direction. Ohm's law V = IR and power P = VI = I²R = V²/R underpin nearly every question. In series, the current is common and resistances add; in parallel, the voltage is common and 1/R_T = 1/R₁ + 1/R₂ + … Kirchhoff's voltage and current laws formalise these rules. For metals, resistance rises with temperature (positive temperature coefficient) - useful when a question asks why a reading drifts as a component warms.
Magnetism and AC
A current creates a magnetic field; relative motion between a conductor and a field induces an EMF (Faraday), and the induced EMF opposes the change producing it (Lenz). For a sine wave, RMS = peak ÷ √2 ≈ 0.707 × peak, and peak = 1.414 × RMS. Frequency f = (poles × rpm) ÷ 120. Capacitors oppose changes in voltage, inductors oppose changes in current, so in AC the current leads in a capacitive circuit and lags in an inductive one.
Machines
Transformers trade voltage for current (V₁/V₂ = N₁/N₂). DC generators use commutators; AC alternators use slip rings. Motors convert electrical energy into mechanical energy.
The traps
Mixing RMS and peak values; adding parallel resistances arithmetically; assuming equal voltage drops in a series circuit; forgetting that a capacitor blocks steady DC.
Worked example
6 Ω in parallel with 3 Ω: R_T = (6 × 3) ÷ (6 + 3) = 2 Ω. At 12 V the total current is 6 A, with 4 A through the 3 Ω branch and 2 A through the 6 Ω branch.