Calculate resistance and power from voltage and current.
Ohm's Law states V = I × R — voltage equals current times resistance. Given any two values, the third is derived directly, and power (P = V × I) follows from the same two inputs.
This is the single most foundational relationship in electrical work, used everywhere from circuit design to diagnosing why a device is drawing more current than expected.
A common mix-up is confusing which two values you actually have. If you measured across a component with a multimeter, you likely have voltage and current directly — resistance and power both fall out from there without needing to measure them separately.
At 12V and 2A: resistance is 12 ÷ 2 = 6Ω, and power is 12 × 2 = 24W.
Voltage = I × R. Rearrange the formula for whichever value is missing — the relationship holds regardless of which two you start with.
Power dissipated as heat is P = I²R — current squared times resistance. This is why higher resistance in a wire (especially with high current) generates more heat, a key consideration in wire gauge selection.
For purely resistive AC loads, yes. For circuits with capacitors or inductors, impedance (which includes reactance) replaces simple resistance, and the math becomes more involved.