Pure resistive AC circuit: voltage and current are in phase.
If we were to plot the current and voltage for a very simple AC circuit consisting of a source and a resistor, (figure above) it would look something like this: (figure below)
Voltage and current “in phase” for resistive circuit.
Because the resistor allows an amount of current directly proportional to the voltage across it at all periods of time, the waveform for the current is exactly in phase with the waveform for the voltage. We can look at any point in time along the horizontal axis of the plot and compare those values of current and voltage with each other (any “snapshot” look at the values of a wave are referred to as instantaneous values, meaning the values at that instant in time). When the instantaneous value for voltage is zero, the instantaneous current through the resistor is also zero. Likewise, at the moment in time where the voltage across the resistor is at its positive peak, the current through the resistor is also at its positive peak, and so on. At any given point in time along the waves, Ohm’s Law holds true for the instantaneous values of voltage and current.
We can also calculate the power dissipated by this resistor, and plot those values on the same graph: (figure below)
Instantaneous AC power in a resistive circuit is always positive.
by Gary Elinoff