In an A.C. circuit containing inductance L and resistance R, the applied voltage V is the phasor sum of VR and VL, and thus the current I lags the applied voltage V by an angle lying between 0o and 90o (depending on the values of VR and VL). 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 = √ (VR2 + VL2)
The phase angle Φ can be determined from:
tan Φ = VL / VR
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