Explain AC circuit for circuit with only resistor.

Explanation

AC current through a resistor is in phase with voltage. But this is not so in the case of an inductor, a capicator or a combination of these circuit elements.
In order to show phase relationship between voltage and current by using notion of phasors.
A phasor is a vector which rotates about the origin with angular speed `omega ` as shown in figure .

The vertical components of phasors V an dI are `V_(m) sin omega t ` and `I_(m) sin omega t ` where `V_(m)` and `I_(m)` are oscillating quantities or indicates peak values.
According to figure a resistor is connected with ac source.

Figure (a) shows the voltage and current phasors and their relationship at time `t_(1)` corresponding to the circuit.
The projection of V and I phasors on vertical axis are `V_(m) sin omega t_(1)` and `I_(m) sin omega t ` respectively represent the value of voltage and current at that instant.
The rotating vectors with frequency `omega` is shown in figure (b) are generated.
Phasors V and I for the case of resistor are in the same direction. This means that the phase angle between voltage and the current is zero.