Illustration 31.6: Voltage and Current Phasors

frequency ( f ) = Hz

Please wait for the animation to completely load.

Assume ideal components. The bottom graph shows the voltage as a function of time across the source (red), the resistor (blue), and the capacitor (green). Current is shown in black (voltage is given in volts, current is given in milliamperes, and time is given in seconds). Restart.

You cannot simply use V = I R when working with AC circuits, because you must account for the phase differences in the voltages and currents. Notice that when you look at Voltages Only, the voltages across the power supply, the resistor, and the capacitor are not in phase. One way to account for the phase differences is to describe the voltage with phasors as shown in the animation in the top right box. The voltage of each component is represented by a vector that rotates at the frequency of the source. The angle between the vectors represents the phase difference between the voltages, while the length of the vectors represents the peak voltage across each circuit element. Illustration 31.7 and Explorations 31.5 and 31.6 develop this idea further. We can also use this phasor representation to describe the current. Look at Voltage and Current and notice that the voltage from the source is out of phase with the current. So, instead of using V = I R, Ohm's Law becomes V = I Z, where Z is the impedance and includes the frequency response and phase shift associated with the various circuit components.

Illustration authored by Anne J. Cox.
Script authored by Wolfgang Christian and Anne J. Cox.