Ohm’s law states that the voltage across a resistive element equals the current through it times its resistance: V = I × R. Combined with the power law P = V × I, it closes a four-quantity system — know any two of voltage, current, resistance, and power and the other two follow from the derived forms P = V²/R, P = I²·R, R = V²/P, V = √(P·R), and I = √(P/R). A 120 V lamp rated 60 W draws I = 60/120 = 0.5 A through a hot filament resistance of R = 120²/60 = 240 Ω, and running the (I, R) pair back through the equations reproduces the 120 V and 60 W exactly.
The derived forms carry the intuition worth memorizing: doubling the voltage across a fixed resistance doubles the current but quadruples the power, since P = V²/R — the reason a 240 V element rated from 120 V service draws four times the watts. The scope caveat matters equally: these relations describe DC circuits and AC circuits with purely resistive loads such as heaters, incandescent lamps, and resistors. For AC loads with a power factor below 1 — motors, drives, transformers — real power is P = V·I·PF and the current for a given kW runs higher than the resistive formula suggests.