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Homework Help: Internal Resistance

  1. Sep 17, 2008 #1
    How can I go about calculating internal resistance? I have variables I, R, and V but no idea on how to find the 'internal' resistance. Is this some sort of special case?
  2. jcsd
  3. Sep 17, 2008 #2
    Your question needs to be a bit more specific.

    Internal resistance is a concept used to account for the fact that the terminal voltage of a a real battery (as opposed to an ideal voltage source, whose voltage is constant, regardless of the current drawn from the source) drops as the current drawn from the battery increases. A simple electrical model of a battery is an ideal voltage source in series with a resistor representing the internal reistance. As current increases,, more voltage is dropped across the internal R and the battery terminal voltage decreases.
  4. Sep 17, 2008 #3
    Well, the question says "A 20 V battery delivers 107 mA of current when connected to a 74 ohm resistor. Determine the internal resistance of the battery."
  5. Sep 18, 2008 #4


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    Homework Helper

    Just draw a circuit consisting of the 74 ohm resistor, the unknown resistance r in series and the 20V batteries. Then solve for r given the info in the question.
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