If you want to make charge carriers flow through a resistor, you must establish a potential difference between the ends of the device. One way to do this is to connect each end of the resistor to one plate of a charged capacitor. The trouble with this scheme is that the flow of charge acts to discharge the capacitor, quickly bringing the plates to the same potential. When that happens, there is no longer an electric field in the resistor and thus the flow of charge stops. ,”produce a steady flow of charge, you need a “charge pump,” a device that by doil1g’~work on the charge carriers-maintains a potential difference between a pair of terminals. We call such a device an emr device, and the device is said to provide an emr’l!:, which means that it does work on charge carriers. An emf devicei sometimes called a seat of emf The term emf comes from the outdated phrase electromotive force, which was adopted before scientists clearly understood the function of an emf device. In Chapter 27, we discussed the motion of charge carriers through a circuit in terms of the electric field set up in the circuit-the field produces forces that move the charge carriers. In this chapter we take a different approach: We discuss the motion of the charge carriers in terms of the required energy-an emf device supplies the energy for the motion via the work it does. A common emf device is the battery, used to power a wide variety of machines from wristwatches to submarines. The emf device that most influences our daily ives, however, is the electric generator, which, by means of electrical connections(wires) from a generating plant, creates a potential difference in our homes and workplaces. The emf devices known as solar cells, long familiar as the winglike panels on spacecraft, also dot the countryside for domestic applications. Less familiar
emf devices are the fuel cells that power the space shuttles and the thermopiles that provide onboard electrical power for some spacecraft and for remote stations in Antarctica and elsewhere. An emf device does not have to be an instrument-e-Iiving systems, ranging from electric eels and human beings to plants, have physiological emf devices. .
Although the devices we have listed differ widely in their modes of operation, they all perform the same basic function-they do work on charge carriers and thus maintain a potential difference between their terminals.