Alternating Current

Alternating current behaves differently, depending on what components are in a circuit.

Alternating Current (AC) is an electric current that reverses direction cyclically – unlike direct current (DC), which always travels the same way, as is the case with anything powered by a battery, for example. AC is the current running through the electric wires and appliances in your home. The magnitude of AC current varies, growing from zero to a positive maximum and then reducing back to zero before the reversal of the current causes the current to gradually reach a negative maximum and then return to zero once again. The number of times an alternating current repeats a full cycle per second is the frequency and the maximum the current reaches in either direction is its amplitude. The waveform of an alternating current power circuit is a sine wave.

Placing various components in a circuit powered by an AC source can affect the sine waves for the current and the voltage across the circuit that triggers the current flow, as demonstrated in this tutorial. Illustrated below is a simple circuit with an alternating current power supply. An ideal Resistor, Capacitor or Inductor can be placed in the circuit by making the appropriate selection from the Choose a Component pull-down menu. The voltage (measured in volts) and current (measured in amps) in the circuit fluctuate due to the alternating current, as seen in the readings on the meters in the circuit.

The relationship between the voltage and current is further emphasized by the phasor diagram in the bottom left corner, which shows their oscillations as rotating vectors. When a vector points upward along the y-axis, the voltage or current has reached its positive maximum value, and when it points downward along the same axis, the negative maximum has been reached. The horizontal x-axis indicates a value of zero. In the bottom right corner of the tutorial window a graph indicates the amplitude (y-axis) of both the voltage and the current over time (x-axis). Observe the changes in the diagram and graph when different components are placed in the circuit.

When the circuit contains only a pure resistor, the current and voltage are continuously in phase with one another. When a pure capacitor is in the circuit, however, the current is at its maximum peak when the voltage is at zero; in this case, the current is said to lead the voltage by 90 degrees. If the pure inductor is selected, the reverse occurs: the voltage leads the current by 90 degrees. Of course, pure resistance, capacitance and inductance generally do not occur in real-world applications. As a result, the differences in the phase relationship between current and voltage can vary significantly from the idealized correlation presented in the tutorial.

You can adjust how fast this interactive animation runs with the Applet Speed slider. Slow it down to take a good look at the relationships among the different components of the tutorial. Speed it up for a slightly more realistic sense as to how fast these changes actually occur. The tutorial cannot illustrate the true speed of fluctuations in AC voltage, which goes through a full cycle – changing from positive to negative and back again – 60 times a second in the U.S., 50 times a second in Europe.

Last modified on 6 April 2020
National Maglab Logo

Magnet Academy is a free resource on magnetism & electricity brought to you by the Center for Integrating Research + Learning at the National High Magnetic Field Laboratory.