An alternating current (AC) is a current that reverses direction periodically and varies continuously in magnitude over time, unlike a direct current (DC), which flows in only one direction. Alternating current is the form in which electrical energy is supplied to businesses and homes, and is the form of electrical energy commonly used by consumers when they plug kitchen appliances, televisions, fans, and lights into wall outlets. A common DC source is a flashlight battery cell. The abbreviations AC and DC are often used simply to mean directly alternating when changing current or voltage. The usual AC waveform in most electrical circuits is sinusoidal, with the positive half-cycle corresponding to the positive direction of current flow and vice versa. Certain applications, such as guitar amplifiers, use different waveforms such as triangle and square waves. Audio and radio signals transmitted over wires are also examples of alternating currents. These types of ac power carry information such as sound (audio) and images (video), sometimes transmitted by modulating an ac carrier signal. These currents typically alternate at frequencies higher than those used in power transmission.