Electrical polarity
Electrical polarity is present in every electrical circuit. Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

s flow from the negative pole to the positive pole. In a direct current
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...

 (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
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....

 (AC) circuit the two poles alternate between negative and positive and the direction of the electron flow continually reverses.

Positive and negative

In DC circuits (e.g. for a car battery
Car battery
An automotive battery is a type of rechargeable battery that supplies electric energy to an automobile. Usually this refers to an SLI battery to power the starter motor, the lights, and the ignition system of a vehicle’s engine...

) positive is usually marked red and negative is usually marked black, but other color schemes are very common in automotive and telecommunications systems.

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.

Ground (earth)

Modern cars
An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...

 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.

See also

  • Anode
    An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....

  • Cathode
    A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...

  • Electric charge
    Electric charge
    Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...

  • Polarity symbols
    Polarity symbols
    Many electrical devices use direct current power, often provided by a AC to DC adapter, also known as a wall wart or power brick...

  • Polarity (physics)
  • Dot convention
    Dot convention
    In circuit analysis, the dot convention is a convention used to denote the voltage polarity of two mutually inductive components, such as winding on a transformer....

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