21.2 Electromotive Force: Terminal Voltage When you forget to turn off your car lights, they slowly dim as the battery runs down. Why don't they simply blink off when the battery's energy is gone? Their gradual dimming implies that battery output voltage decreases as the battery is depleted. Furthermore, if you connect an excessive number of 12-V lights in parallel to a car battery, they will be dim even when the battery is fresh and even if the wires to the lights have very low resistance. This implies that the battery's output voltage is reduced by the overload. The reason for the decrease in output voltage for depleted or overloaded batteries is that all voltage sources have two fundamental parts-a source of electrical energy and an internal resistance. Let us examine both. Electromotive Force You can think of many different types of voltage sources. Batteries themselves come in many varieties. There are many types of mechanicalelectrical generators, driven by many different energy sources, ranging from nuclear to wind. Solar cells create voltages directly from light, while thermoelectric devices create voltage from temperature differences. This OpenStax book is available for free at http://cnx.org/contenticol11406/1.9 Chapter 21 | Circuits and DC Instruments 817 A few voltage sources are shown in Figure 21.8. All such devices create a potential difference and can supply current if connected to a resistance. On the small scale, the potential difference creates an electric field that exerts force on charges. causing current. We thus use the name electromotive force, abbreviated emf. Emfis not a force at all; it is a special type of potential difference. To be precise, the electromotive force (emf) is the potential difference of a source when no current is flowing. Units of emf are volts. Figure 21.8 A veriety of voltage sources (clockwise from top left): the Brazos Wind Farm in Fluvanna. Texas (credit: Lesfiet, Wikimedia Commons); the Kraznoyarsk Dam in Russia (credit: Alex Polezhaev); a solar farm (credit U.S. Department of Energy); and a group of nickel metal hydride batteries (credit Tiaa Monto). The voltage output of each depends on its construction and load, and equals emf only if there is no load. 2700 PHILIPS