If, for example, you rub a hard rubber rod with a woollen cloth, charged particles are exchanged between one substance and the other. Some electrons from the previously neutral wool are rubbed off onto the surface of the rubber. The rubber then has an excess number of electrons and is thus negatively charged. At the same time the wool has a deficit of electrons and is thus charged positively. A similar effect can be observed if you pull a tight nylon sweater over your head. That can really make your hair stand on end.

In current sources such as batteries or generators, positive and negative charges that exist in all materials are separated from one another by the effects of some energy. One terminal of the source thus has an excess of electrons (the negative pole) while the other displays a deficit (the positive pole). An electric field thus exists between these two charges and the system will try to even this imbalance out so that charges flow from one terminal to the other and generate a so-called electric current

If both poles are connected via conductors the charges seek to even out by passing along these conductors and giving rise to a current. This involves the current source (e.g. a battery) exerting an amount of work W on the charge Q that has been transported. The voltage of the source is now defined as the quotient of the work and the charge: 

Basic Electrical Properties

(Note: usage of as the symbol for voltage is conventional in some European countries; elsewhere, the letter V may be used instead.)

The unit of electrical voltage is called the Volt, or "V" for short. A voltage can only exist between two points (e.g. the poles of an electricity source).

The meaning of the concept of voltage can be illustrated by an analogy from the world of physics. The electricity source is equated with a water pump. A pump is able to exert an amount of lifting work on each litre of water it raises up to a certain maximum height. Thus for each litre of water it provides a certain amount of energy. Considering the two cases sketched in the following graphic, Pump P2 (right) has to exert twice the amount of work on each litre as Pump P1 (left) since the height is twice as high. The quotient of the work and the volume is thus twice as large. The analogy with electricity would then suggest that the right-hand pump had double the "voltage".

Basic Electrical Properties

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