In an A.C. circuit containing inductance L and resistance R,
the applied voltage V is the phasor sum of V_{R} and V_{L},
and thus the current I lags the applied voltage V by an angle lying between
0^{o} and 90^{o} (depending on the values of V_{R} and V_{L}).
In any A.C. series circuit the current is common to each component and is thus taken as the reference phasor.

The resultant voltage is:

V = √ (V_{R}^{2} + V_{L}^{2})

The phase angle Φ can be determined from:

tan Φ = V_{L} / V_{R}

In an a.c. circuit, the ratio applied voltage V / current I is called the impedance Z

Z = V / I (Ω)

If each side of the voltage triangle divided by current I then the ‘impedance triangle’ is derived