What are AC and DC?

AC and DC are alternating current and direct current, respectively. The two terms generally speak to a voltage source and the way it causes current to flow in a circuit.

AC means the applied voltage changes polarity, and, therefore, the current flow in a circuit will change directions in response. The best example is probably the AC power grid, and the grid distributes electric power across a wide area. In the AC power grid, the voltage varies in the form of a sine wave. It varies from zero to a positive peak, back to zero, to a negative peak and back to zero. It actually changes polarity, and this cycle repeats continuously at some rate. The voltage out of the wall socket in a house in the US is AC at 60 Hertz (60 Hz), which is 60 cycles per second.

DC means the voltage source has a fixed polarity. Current in a DC circuit will flow in only one direction. The most common example is a battery, and the terminals of this voltage source are positive and negative. The voltage applied to a circuit will result in unidirectional current flow.

When we compare DC and AC circuits, we generally consider that in the former, the applied voltage has a fixed polarity and unidirectional current flow. In the latter case, the polarity of the applied voltage changes, and the direction of current flow in an attached circuit will also change.