An electric current that reverses its direction with a constant frequency (f). If a graph of the current against time has the form of a sine wave, the current is said to be sinusoidal. Alternating current, unlike direct current, is therefore continuously varying and its magnitude is either given as its peak value (I₀) or its root-mean-square value (I₀/√2 for a sinusoidal current). This r.m.s. value is more useful as it is comparable to a d.c. value in being a measure of the ability of the current to transmit power. The instantaneous value of a sinusoidal current (I) is given by I=I₀sin2π‎ft.

If a direct current is supplied to a circuit the only opposition it encounters is the circuit’s resistance. However, an alternating current is opposed not only by the resistance of the circuit but also by its reactance. This reactance is caused by capacitance and inductance in the circuit. In a circuit consisting of a resistance (R), an inductance (L), and a capacitance (C) all in series, the reactance (X) is equal to (2π‎fL) − (1/2π‎fC). The total opposition to the current, called the impedance (Z), is then equal to the ratio of the r.m.s. applied p.d. to the r.m.s. current and is given by $\surd (R^2+X^2)$.