Consider pure resistor of resistance R connected to sinusoidal AC.
Let v = vmsin ωt --- (1) be the instantaneous voltage.
According to Kirchoff’s loop rule, vmsinωt = iR; here ‘i’ is the AC current.
here im is called current amplitude (or peak current) From (1) and (2), voltage-current are in phase with each other and the phasor diagram is as shown.