Electrical and Electronic Fundamentals
Ohm's law and network rules
Everything in vehicle electrics rests on V = I × R and P = V × I = I²R = V²/R. In series, the current is common, voltages add and resistances add. In parallel, the voltage is common, currents add and 1/RT = 1/R1 + 1/R2 + … Two equal resistors in parallel give half the value of one. Kirchhoff's laws simply state that voltage drops around a loop sum to the supply, and currents into a node equal currents out.
Magnetism explains relays, solenoids, motors and sensors: current creates a field (right-hand rule), a changing field induces a voltage (induction - the basis of ignition coils, alternators and inductive crank sensors), and the Hall effect gives a non-contacting switch or position signal. Semiconductors: a silicon diode conducts at about 0.6–0.7 V forward and blocks in reverse; LEDs, zener diodes, transistors (as switches and drivers) and MOSFETs appear throughout control circuits. Thermistors are NTC (resistance falls as temperature rises) or PTC; photocells and piezo elements produce their own signal.
Traps
Treating voltage as something that 'flows'; forgetting the reciprocal in parallel; dividing voltage across components when the circuit is open (no current, no drop); assuming resistance stays constant with temperature; and confusing a component's rated power with its resistance.
Worked example. A 12 V supply feeds 3 Ω in series with a parallel pair of 6 Ω and 6 Ω. The pair is 3 Ω, total is 6 Ω, so I = 2 A. Each block drops 6 V, and total power is 12 V × 2 A = 24 W.