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Find Internal Resistance From 2 Circuits w/ The Same Battery

  1. Jun 10, 2016 #1
    1. The problem statement, all variables and given/known data
    A battery with an emf of 60 V and an internal resistance (r) is connected to a 10 ohm external resistance. The power lost inside the battery is 50w. The same battery is then connected to a 4.0 ohm resistance. The power loss is 200w. What is the internal resistance of the battery?
    FullSizeRender (3).jpg
    2. Relevant equations

    V = emf - Irint

    P= VI

    V= IR


    P= V^2/R

    3. The attempt at a solution
    P= V^2/R
    V= √ PR
    V= √50 x 10
    V= 22.36 V

    I= V/R
    = 22.36/ 10
    = 2.23 A
    Rint= emf - V/ I
    Rint= 60 - 22.3/ 2.23
    = 16.9 ohms

    This is obviously wrong and I didn't even incorporate the second circuit yet. I'm pretty sure I could finish this question if I knew how to start. Any help is appreciated :)
    Last edited: Jun 10, 2016
  2. jcsd
  3. Jun 10, 2016 #2


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    Please show your work. Do not just describe it with words.
  4. Jun 10, 2016 #3


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    Your calculation of V is incorrect. 50w is the power dissipated in the battery yet you used the value of the external resistor.

    Note there is a difference between "power delivered" by the battery and "power dissipated" by the battery.
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