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Electrical polarity (positive and negative) is the direction of current flow in an electrical circuit. Current flows from the positive pole (terminal) to the negative pole. Electrons flow from negative to positive. In a direct current (DC) circuit, current flows in one direction only, and one pole is always negative and the other pole is always positive. In an alternating current (AC) circuit the two poles alternate between negative and positive and the direction of the current (electron flow) reverses periodically.
Conventions for identification
In DC circuits, the positive pole is usually marked red (or "+") and the negative pole is usually marked black (or "−"), but other color schemes are sometimes used in automotive and telecommunications systems. Polarity symbols are often used where DC is supplied via a coaxial power connector.
On a car battery, the positive pole usually has a larger diameter than the negative pole.
Modern cars often have a "negative earth" electrical system. In this case the negative terminal of the battery is connected to the vehicle's chassis (the metallic body work) and the positive terminal provides the "live" wire to the various systems. However, some older cars were built with a "positive earth" electrical system, in this case the positive terminal of the battery is bonded to the chassis and the negative terminal for the live.
In AC systems the two wires of a circuit alternate polarity many times per second. In electrical power systems, all wires carrying the same instantaneous polarity at any moment will have a common identifying marking scheme, such as wire color. Depending on the conventions used for wiring the power system, the color coding or other marking may also indicate additional properties of the conductor, such as its role as neutral in a power circuit. In a polyphase AC system, identifying the wires belonging to a common phase is important to ensure proper operation of the circuit.
Where alternating current circuits are used to carry signals such as audio, polarity is also required to ensure proper function of the system. For example, a set of loudspeakers used for stereo music reproduction will have all device terminals and wiring marked to ensure the same instantaneous polarity, so that the resulting sound produced by each speaker element is in the same phase and add correctly at the listener's ear.
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