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Homework Help: Internal resistance problem

  1. Sep 16, 2006 #1
    Here is the question I need to solve:

    A battery has an emf E and an internal resistance r. When the battery is connected to a 25 ohm resistor, the current through the battery is 0.75 A. When the battery is connected to a 53 ohm resistor, the current is 0.45 A. Find the emf of battery and its internal resistance.

    Okay, so I have a feeling I will be using the equation V = IR, but I don't knnow how to incorporate an internal resistance into the equation or approach. Can someone please help me outline the steps to solve this? Thanks!
     
  2. jcsd
  3. Sep 16, 2006 #2

    Gokul43201

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    1. Treat the internal resistance as a resistor (r) in series with the battery (E).
    2. Draw the circuit diagram for both cases.
    3. Solve the two circuits - write down the two equations.
    4. Solve for the 2 unknowns from the 2 equations.
     
    Last edited: Sep 16, 2006
  4. Sep 16, 2006 #3
    Would you be able to explain a bit more for step #1? How would my equation(s) be set up?
     
  5. Sep 19, 2006 #4

    Gokul43201

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    You do not set up the equation until you have a circuit diagram. Once you draw the circuit as described in #1, you set up the equation using Kirchoff's voltage law (which is a fancy extension of Ohm's law): algebraic sum of all EMFs = algebraic sum of all voltage drops across resistors (these voltage drops calculated by Ohm's Law).

    http://en.wikipedia.org/wiki/Kirchhoff's_circuit_laws
     
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