Basic Units

Voltage

Voltage2 , as we will see later in this unit, quantifies the energy associated with charge moving relative to other charge. Opposite charges are attracted together and similar charges repel. When a charge moves towards other similar charges then work has to be done (energy must be added) to force this to happen. When opposite charges move towards each other then energy is given out. It is a fundamental assumption in physics that any system will always try to move to the state with the lowest energy - water will always flow down hill, for example, unless it is stopped from doing so. So, unlike charges will always try to move towards each other and like charges will move as far apart as possible, if they can. We represent this 'desire' to move apart or together by saying that there is a potential difference between the 2 points in much the same way as we talk about the potential energy of a bucket of water at the top of a hill. A voltage or potential difference exists because of the way that charge is distributed in space and a potential difference can be imposed between two points in space.

So, when a conductor has no voltage applied across it, there will be no net movement of electrons. However, if a voltage is applied across the conductor, then electrons will flow.

Voltages are defined as differences - that is, in the case of the diagram here, one end of the conductor is held at a voltage that is more positive than the other end. In this case, the electrons are attracted to the positive end.

The units of voltage are volts (V).

 

The circle in the diagram represents a voltage generator. This generator can be thought of as a reservoir of electrons that are, effectively, held separate from a corresponding positive set of charges. In a battery, this is accomplished by chemical means, for example. The electrons wish to flow towards and be re-united with the positive charges but can only do this by flowing though the external conductor to which the voltage generator is connected. So as the electrons in the conductor move out of the material towards the positive voltage, the electrons from the voltage generator flow into the conductor.

Electrons must always move in a loop to be re-united with the positive charges, effectively, the current must have somewhere to go and this gives us the concept of a circuit - a loop through which the current is flowing.


2Named after Alessandro Volta, 1745-1827 (see:http://www.ideafinder.com/history/inventors/volta.htm)