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: In a domestic mains system this is not quite true. There is one wire that is designated "hot" and colored black while the other wire is designated neutral, colored white, and is bonded to earth at the source. The power to the wires alternates in polarity and varies in potential from 0-120 volts depending at which point in the generation cycle you analyze. At one point in time, the white wire will be at negative with reference to the black wire while 180° later in the cycle the opposite is true. There is actually a point in the cycle where the wires have no potential difference to each other as the alternation passes through the zero potential point while moving to reversal, which explains why the peak AC voltage is greater than its effective voltage as compared to DC voltage. The frequency of this cycle is 50 or 60 times per second, depending on which country you are in. On such a grounded system, 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). This is because your body capacitance to earth is creating a high impedance circuit and current will flow from the ungrounded wire to earth through your body capacitance. Since the neutral (white) wire is grounded at the source, it has no potential difference to earth ground and touching the neon test lamp to it will produce no current flow and the lamp will not light. In the US, national standards call for the hot side of the line to be a black wire, or in the case of 120-240 VAC, the second hot wire (2nd phase) is to be red. The neutral side of the line (transformer secondary center tap)is to be white. The neutral wire is required to be bonded to earth at the service entrance point of a building. This is primarily for the prevention of lightning injury to occupants and damage to the building from fire caused by a lightning strike on the incoming lines. There is also a ground conductor that must be ran along with any circuit wires to provide for ground bonding of all metal housings on equipment and also for bonding outlet boxes to ground potential. That ground wire is never connected to the neutral line except at the main service entrance panel. It can be bare (or insulated if colored green) and can be either copper or aluminum. Along with augmenting the lightning strike protection, another important safety factor is provided by bonding this ground wire to any metal appliance housing. This makes it impossible for the metal housing to ever become live with reference to ground since it would constitute a short circuit and would trip the circuit breaker. This ground conductor will never carry any current under normal operation. Its primary purpose is to carry fault currents.
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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