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Section summary

  • All voltage sources have two fundamental parts—a source of electrical energy that has a characteristic electromotive force (emf), and an internal resistance r size 12{r} {} .
  • The emf is the potential difference of a source when no current is flowing.
  • The numerical value of the emf depends on the source of potential difference.
  • The internal resistance r size 12{r} {} of a voltage source affects the output voltage when a current flows.
  • The voltage output of a device is called its terminal voltage V size 12{V} {} and is given by V = emf Ir size 12{V="emf" - ital "Ir"} {} , where I size 12{I} {} is the electric current and is positive when flowing away from the positive terminal of the voltage source.
  • When multiple voltage sources are in series, their internal resistances add and their emfs add algebraically.
  • Solar cells can be wired in series or parallel to provide increased voltage or current, respectively.

Conceptual questions

Is every emf a potential difference? Is every potential difference an emf? Explain.

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Explain which battery is doing the charging and which is being charged in [link] .

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The diagram shows two cells of e m f script E sub one equals twelve volts and internal resistance r sub one equals one ohm, and e m f script E sub two equals eighteen volts and internal resistance r sub two equals zero point five ohms, connected. The cells are connected with their positive terminals facing each other in a closed circuit.

Given a battery, an assortment of resistors, and a variety of voltage and current measuring devices, describe how you would determine the internal resistance of the battery.

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Two different 12-V automobile batteries on a store shelf are rated at 600 and 850 “cold cranking amps.” Which has the smallest internal resistance?

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What are the advantages and disadvantages of connecting batteries in series? In parallel?

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Semitractor trucks use four large 12-V batteries. The starter system requires 24 V, while normal operation of the truck’s other electrical components utilizes 12 V. How could the four batteries be connected to produce 24 V? To produce 12 V? Why is 24 V better than 12 V for starting the truck’s engine (a very heavy load)?

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Problem exercises

Standard automobile batteries have six lead-acid cells in series, creating a total emf of 12.0 V. What is the emf of an individual lead-acid cell?

2.00 V

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Carbon-zinc dry cells (sometimes referred to as non-alkaline cells) have an emf of 1.54 V, and they are produced as single cells or in various combinations to form other voltages. (a) How many 1.54-V cells are needed to make the common 9-V battery used in many small electronic devices? (b) What is the actual emf of the approximately 9-V battery? (c) Discuss how internal resistance in the series connection of cells will affect the terminal voltage of this approximately 9-V battery.

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What is the output voltage of a 3.0000-V lithium cell in a digital wristwatch that draws 0.300 mA, if the cell’s internal resistance is 2 . 00 Ω size 12{2 "." "00" %OMEGA } {} ?

2.9994 V

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(a) What is the terminal voltage of a large 1.54-V carbon-zinc dry cell used in a physics lab to supply 2.00 A to a circuit, if the cell’s internal resistance is 0 . 100 Ω size 12{0 "." "100" %OMEGA } {} ? (b) How much electrical power does the cell produce? (c) What power goes to its load?

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What is the internal resistance of an automobile battery that has an emf of 12.0 V and a terminal voltage of 15.0 V while a current of 8.00 A is charging it?

0 . 375 Ω size 12{0 "." "375" %OMEGA } {}

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(a) Find the terminal voltage of a 12.0-V motorcycle battery having a 0 . 600-Ω size 12{0 "." "600"- %OMEGA } {} internal resistance, if it is being charged by a current of 10.0 A. (b) What is the output voltage of the battery charger?

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Practice Key Terms 4

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Source:  OpenStax, College physics for ap® courses. OpenStax CNX. Nov 04, 2016 Download for free at https://legacy.cnx.org/content/col11844/1.14
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