Ohm's Law

Discovered experimentally by Georg Ohm in 1827, Ohm's law (V = I × R) is the foundation of linear circuit analysis. It links three fundamental quantities — voltage in volts, current in amperes and resistance in ohms — and lets engineers calculate any one when the other two are known. From it follow the power equations P = V × I = I² × R = V² / R, which are essential for selecting resistors by power rating.

The law holds strictly only for ohmic conductors at constant temperature. Many real elements — diodes, transistors, plasmas, filament bulbs and even resistors operating outside their derating curve — exhibit non-linear behaviour where R depends on V, I or temperature. Wirewound and metal-film resistors stay close to ideal across a wide range, while carbon-composition parts and the channel of an active device do not.

In practice engineers use Ohm's law constantly: sizing pre-charge and discharge resistors, computing voltage-divider outputs, finding the value of a sense resistor that drops 50 mV at 10 A, or back-calculating insulation resistance from a microamp leakage measurement. It remains the single most-used formula in resistor selection.