The Physics
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Voltage of a Car Battery

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Bibliographic Entry Result
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Brown, Theodore L. LeMay, Eugene, Bursten, Jr., Bruce. Chemistry: the Central Science. New Jersey: Prentice-Hall, 1997. "A 12 V lead-storage battery consists of six cells, each producing 2 V." 12 V
Andrea, J. David, Flynn, Michael S. "Automobile." World Book. Chicago: World Book, 1998: Vol. 1, 954. "A 12 volt battery"stores energy for the starter, which begins the operation of the electrical system by cranking the engine to life." 12 V
"Hints & Kinks." QST. December 1997: 69. "The most convenient power source is usually the 12 V battery in the vehicle that gets us there, and modern solid-state devices work fine at 12 V, or less." 12 V
Giancoli, C. Douglass. Physics: Principles with Applications. New Jersey: Prentice-Hall, 1980. "Thus the voltage between the ends of the 2 flashlight batteries in Figure 15-5 a is 3 V while the six 2 V cells of the storage battery in part b give 12 V." 12 V
Cutnell, John D., Johnson, Kenneth W. Physics. New York: Wiley, 1995. "The voltage rises by 12.0 V due to the battery's emf. However, the voltage drops by 0.10 V because of the potential difference across the internal resistance. Therefore, the terminal voltage is 12.0 V–0.10 V = 11.9 V" 11.9 V
(at 10 A)
"When the current through the battery is 100.0 A, the amount of voltage needed to make the current go through the internal resistance is V= (100 A)(0.01 ohm) = 1.0 V. The terminal voltage of the battery now decreases to 12.0 V–1.0 V = 11.0 V." 11 V
(at 100 A)

One of the most common and useful batteries is the lead-storage battery used in automobiles. The cell of the lead storage battery consists of alternate plates of lead (cathode) and lead coated with lead dioxide (anode) immersed in an electrolyte of sulfuric acid solution. A 12 V lead-storage battery consists of six cells, each producing approximately 2 V. The actual standard cell potential is obtained from the standard reduction potentials.

E = E (cathode)–E (anode)
E = (+1.685 V)–(−0.356 V)
E = + 2.041 V
6 cells × 2.041 V/cell = 12.246 V

Because of the positive and negative charges on the battery terminals, an electric potential difference exists between them. This potential difference is called the electromotive force (emf) of the battery. This is a poor term because emf is not a force but an energy per unit charge quantity, like potential. The SI unit of emf is the volt (V = J/C = joule/coulomb).

A car battery is an example of a source of emf. An ideal source of emf maintains a constant potential difference between its terminals, independent of the current, I, through it or the resistance, R, across it. The formula for an ideal source of emf is V= IR, which is known as Ohm's Law. The potential difference across a real source in a circuit, however, is not equal to the ideal emf. The reason is that charge moving through the material of any real source encounters an internal resistance r and experiences a drop in potential difference equal to Ir. Thus the equation for a source with internal resistance is V = emf − Ir and this potential is called the terminal voltage. Therefore the voltage in a car battery is always less than 12 V while it is producing a current.

Thi Meagan Le -- 2001