Electrical polarity (positive and negative) is present in every electrical circuit. Electrons flow from the negative pole to the positive pole. In a direct current (DC) circuit, one pole is always negative, the other pole is always positive and the electrons flow in one direction only. In an alternating current (AC) circuit the two poles alternate between negative and positive and the direction of the electron flow reverses.
Positive and negative
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. On a car battery, the positive pole usually has a larger diameter than the negative pole.
In AC systems which are at no point connected to earth (unlike domestic mains wiring where the neutral is commonly earthed at substations and generators) polarity is not important as the two wires alternate polarity many times per second. Added Note: This is not quite true. There is one wire that is "hot" and the other one not. The power to the hot lead is switched 110-120 volts above and 110-120 volts below (positive and negative to) the other wire, either 50 or 60 times per second, depending on which country you are in. You can prove this by touching first one lead and then the other with a neon test lamp. It will light up on one wire (black), but not on the other (white). In the US, national standards call for the hot side of the line to be a black wire, or in the case of 220-240 VAC, the second hot wire (2nd phase) is to be red. The neutral side of the line is to be white. If there is also a separate neutral, it is a bare wire, usually copper.
Modern cars have a "negative earth" electrical system. In this case the negative terminal of the battery is bonded 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.
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