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POTENTIAL DIFFERENCE AND RESISTANCE


If we were asked: "How can electric charges be made to flow and what factors influence their flow?" The analogy to the flow of water in a pipe would certainly help to make some of the principles clear.

If one considers the conditions necessary for the water flow through a pipe, it will not be difficult to understand that there must be a pump somewhere in the pipe-line, or else a difference in level must evidently exist between the two ends of the pipe.

In case we are dealing with electricity, the requirements necessary for current flow are similar in that there must be in the circuit an electromotive force (a battery or a generator), or else a difference of potential should exist between the two ends of the conductor.

Let us picture what would happen provided there were a conducting wire between two points of unequal potential. It is clear that in such a case there must be a flow of electrons from one of the points to the other. Since the electrons in the wire constitute the current flow, they will certainly tend to flow from the point of lower potential towards that of higher potential.

Imagine that an electric current flows from point A to point B through the conductor. In electrical terms this means that there is a potential difference between A and B, the potential at A being greater than that at B. Unless there were a flow of current between A and B in any direction, A and B would doubtless be at the same potential.

Let us return to our analogy, to the flow of water in a pipe. There is a difference in level (or potential energy) between the water at the ends A and B of the given pipe AB. This difference in level is supported by a pump which raises the level (or potential energy) of the water at B to that at A.

The flow of water per second through the pipe AB depends first on the difference in level between points A and B, just as in the electrical case the current in the conductor AB depends on the potential difference between point A and point B. The other factor that is to be considered before one is able to sayhow much water does flow through the pipe is the resistance offered by the pipe to the flow of water. In the same way, an electrical conductor, say a wire, offers some resistance to the flow of charges passing through it.

In the case we have just considered, where a conductor; connected between two points of unequal potential, we have a momentary motion of charges. The charges move until they come to a static equilibrium state, then they stop; there is no steady current, therefore. When we have such a case of static equilibrium, the surface of a conductor is equal in potential everywhere. Naturally, it does not lean that there is an equal distribution of charge over the whole surface of the conductor but it does mean that all points of the conductor are alike with respect to potential.

If we have a conductor carrying a steady current, we obviously find that the situation is different. The current owing there must be caused by a difference of potential from one point to another along the wire. Otherwise, there would be no cause that might make charges move along le wire.

Dealing with potential and potential difference, mention should also be made of the volt, since it is the volt lat is the unit used for measuring potential and potential difference.


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