Basic Electricity I - Ohm's Law, Power, and Circuits
Volts, amps, ohms, and how they combine
Basic electricity questions reward fluency with Ohm's law (E = I × R), power relationships (P = E × I and P = I²R), and the behavior of series and parallel circuits. In series, current is common and voltages add, and resistances add. In parallel, voltage is common and branch currents add, and total resistance falls below the smallest branch. Know the difference between AC and DC, single-phase and three-phase, transformers that step control voltage down, and motor overloads that protect windings from sustained overcurrent.
Connect meters correctly: voltmeter in parallel with the component, ammeter in series with the circuit, ohmmeter only on a de-energized circuit. A clamp-on ammeter is the practical exception for measuring load current without breaking the circuit.
Traps: mixing peak and RMS values; assuming a device with voltage present is functioning (it may be open internally); measuring resistance with power applied; confusing overload heaters with fuses; forgetting that a three-phase motor reverses by swapping any two of its three leads.
Worked example: a 240 V single-phase circuit with a 12 Ω resistive load draws I = 240 ÷ 12 = 20 A and dissipates P = 240 × 20 = 4,800 W. Two such 12 Ω resistors in parallel give 6 Ω and 40 A - parallel paths add current, not resistance. A 20 A breaker on the 20 A load is marginal; conductor and device sizing must be verified rather than assumed.