Voltage, Current, and Power in AC Resistive Circuits

Voltage, current, and Power are three fundamental properties of any circuit. In this post you’ll learn the behavior of voltage, current and power in AC Resistive circuits.


Let’s start off with this circuit diagram of a pure AC resistive circuit. Remember that in pure resistive AC circuit the resistors voltage and current are in phase:

Voltage and Current Waveform in AC Resistive Circuits

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)

Because the resistor simply and directly resists the flow of electrons at all periods of time, the waveform for the voltage drop across the resistor is exactly in phase with the waveform for the current through it. 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 current is zero, the instantaneous voltage across the resistor is also zero. Likewise, at the moment in time where the current through the resistor is at its positive peak, the voltage across 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.

Power in AC Resistive Circuits

We can also calculate the power dissipated by this resistor, and plot those values on the same graph: (Figure below) Remember that the Instantaneous AC power in a pure resistive circuit is always positive.

Note that the power is never a negative value. When the current is positive (above the line), the voltage is also positive, resulting in a power (p=ie) of a positive value. Conversely, when the current is negative (below the line), the voltage is also negative, which results in a positive value for power (a negative number multiplied by a negative number equals a positive number). This consistent “polarity” of power tells us that the resistor is always dissipating power, taking it from the source and releasing it in the form of heat energy. Whether the current is positive or negative, a resistor still dissipates energy.


So that was all about the behavior of voltage, current, and power in ac circuits. In AC circuits the current, voltage and, power in pure AC resistive circuits are very easy to understand.

Article extracted from Lesson in Electric Circuits AC Volume Tony R Kuphaldt under Design Science License. Heading and title are modified/added. Introductory lines and summary are also added. Some text is also modified inside the article.

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