Topic-3 - TOPIC 3. Electric Currents ELECTRIC CURRENTS What...

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TOPIC 3. Electric Currents ELECTRIC CURRENTS What causes charges to flow, and what hinders the free flow of charge? The most important practical applications of electrical phenomena are in the innumerable forms of electronic devices. In these, it is the of electrical charges that takes center stage. motion All materials contain huge numbers of both positive and negative electrical charges (in the form of protons and electrons comprising every atom of matter), and in most cases these charges are pretty much fixed in position they don't move around a lot. However in certain materials for  instance in metals charged particles can move. (In metals it is the negative charges - the electrons that actually move. We will however pretend that the moving charges are positive in order to simplify the discussion.) As noted earlier, these materials are called "conductors." In order to get the charges to start moving a force must act on them, and so an electric field must be present. You can imagine, as an example, a cylindrical piece of metal inside of which there is a uniform electric field, pointing along the length of the cylinder. If you stand at one fixed position in the cylinder and count the charges as they go past that point, you can measure the rate of flow of charge as a function of time . The term that is used to describe this rate of flow of charge is " ," and the symbol for current is . (The unit of current is the "ampere," symbol current M "A.") If an amount of charge equal to flows past a particular point in a time , the current ?? ;> M is defined by this equation: 1 ampere 1 coulomb/second œ ? ? ; > We will always assume that the flow of current is "steady," that is, that the flow of charge continues without a stop and that the rate of flow does not change. Ask yourself: In that case, will the number of charges that flow past line B every second be larger than, smaller than, or the same as the number that flow past line A every second? The answer is: the same. If fewer charges flow past line B than flow past line A, that would mean that some charges are getting "trapped" between lines A and B. If that continued long enough, something disastrous would happen maybe the wire would explode! All those charges piling up in the same place would create a huge repulsive force that could not be contained. So this could not be a characteristic of "steady" flow of current. On the other hand, if more charges flowed past line B than flow past line A, that would mean that some extra charges must be sneaking in to the wire somewhere between A and B. There would have to be some external source of charge besides the charges already present in the wire. This also is not a characteristic of "steady" flow of charge, and so it too can not happen.
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We say, then, that current is "conserved," which means that the amount of charge flowing past any one point in the material is the same as that flowing past any other point, in a given amount of time. This will still hold true even if we place some obstacle in the path of the moving
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This note was uploaded on 02/11/2012 for the course PHYSICS 112 taught by Professor Fretwell during the Summer '08 term at Iowa State.

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Topic-3 - TOPIC 3. Electric Currents ELECTRIC CURRENTS What...

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