If we were to plot the current and voltage
for a very simple AC circuit consisting of a source and a
resistor, it would look something like this:
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.
