Series RL or RC Impedance

A resistor's voltage is in phase with its current, a reactance's is a quarter cycle away, so their oppositions add as the legs of a right triangle: 40 Ω of resistance with 30 Ω of reactance gives 50 Ω, not 70 Ω. This is the everyday impedance calculation — the winding resistance and inductance of a relay coil, a contactor, a transformer primary, a loudspeaker crossover leg.

The same triangle hands you the phase angle, φ = arctan(X/R), and hence the power factor cos φ = R/Z. That is worth remembering when you meet an inductive load with no nameplate: measure the DC resistance, measure the AC current at a known voltage to get Z, and the power factor falls out of the ratio. Rough numbers are enough to tell a mostly resistive load from a mostly reactive one.