Ch0209 - Chapter 9 Electric Current, EMF, Ohm's Law 63 9...

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Unformatted text preview: Chapter 9 Electric Current, EMF, Ohm's Law 63 9 Electric Current, EMF, Ohm's Law We now begin our study of electric circuits. A circuit is a closed conducting path through which charge flows. In circuits, charge goes around in loops. The charge flow rate is called electric current. A circuit consists of circuit elements connected together by wires. A capacitor is an example of a circuit element with which you are already familiar. We introduce some more circuit elements in this chapter. In analyzing circuits, we treat the wires as perfect conductors and the circuit elements as ideal circuit elements. There is a great deal of variety in the complexity of circuits. A computer is a complicated circuit. A flashlight is a simple circuit. The kind of circuit elements that you will be dealing with in this course are two-terminal circuit elements. There are several different kinds of two-terminal circuit elements but they all have some things in common. A two-terminal circuit element is a device with two ends, each of which is a conductor. The two conductors are called terminals. The terminals can have many different forms. Some are wires, some are metal plates, some are metal buttons, and some are metal posts. One connects wires to the terminals to make a circuit element part of a circuit. An important two-terminal circuit element is a seat of EMF 1 . You can think of a seat of EMF as an ideal battery or as an ideal power supply. What it does is to maintain a constant potential difference (a.k.a. a constant voltage) between its terminals. One uses either the constant name E (script E) or the constant name V to represent that potential difference. To achieve a potential difference E between its terminals, a seat of EMF, when it first comes into existence, has to move some charge (we treat the movement of charge as the movement of positive charge) from one terminal to the other. The one terminal is left with a net negative charge and the other acquires a net positive charge. The seat of EMF moves charge until the positive terminal is at a potential E higher than the negative terminal. Note that the seat of EMF does not produce charge; it just pushes existing charge around. If you connect an isolated wire to the positive terminal, then it is going to be at the same potential as the positive terminal, and, because the charge on the positive terminal will spread out over the wire, the seat of EMF is going to have to move some more charge from the lower-potential terminal to maintain the potential difference. One rarely talks about the charge on either terminal of a seat of EMF or on a wire connected to either terminal. A typical seat of EMF maintains a potential difference between its terminals on the order of 10 volts and the amount of charge that has to be moved, from one wire whose dimensions are similar to that of a paper clip, to another of the same sort, is on the order of a pC ( C 10 1 12 ). Also, the charge pileup is almost instantaneous, so, by the time...
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Ch0209 - Chapter 9 Electric Current, EMF, Ohm's Law 63 9...

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