The ohm is the standard unit of electrical resistance in the International System of Units ( SI ). Ohms are also used, when multiplied by imaginary numbers, to denote reactance in alternating-current ( AC ) and radio-frequency ( RF ) applications. Reduced to base SI units, one ohm is the equivalent of one kilogram meter squared per second cubed per ampere squared (1 kg times m 2 · s -3 · A -2 . The ohm is also the equivalent of a volt per ampere (V/A).
In a direct-current ( DC ) circuit, a component has a resistance of one ohm when a potential difference of one volt produces a current of one ampere through the component. In AC and RF circuits, resistive ohms behave the same as they do in DC circuits, provided the root-mean-square ( rms ) AC voltage is specified. In AC and RF circuits, reactance exists only when there is a net capacitance or inductance . Capacitive reactances have negative imaginary ohmic values; inductive reactances have positive imaginary ohmic values. The reactance of a particular capacitor or inductor depends on the frequency.
Resistances and reactances are sometimes expressed in units representing power-of-10 multiples of one ohm. A kilohm is equal to one thousand (10 3 ) ohms. A megohm is equal to one million (10 6 ) ohms. Fractional prefix multiplier s are seldom used for resistance or reactances; rarely will you hear or read about a milliohm or a microhm. Extremely small resistances and reactances are usually referred to in terms of conductance.