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Homework Help: Basic EMF problem

  1. Oct 27, 2009 #1
    1. The problem statement, all variables and given/known data

    A battery has an emf E and an internal resistance r. IT is connected to an external resistor R. When R=4 ohms, the potential difference is 9.5 V. When R = 6 ohms, the potential difference is 10.0 V

    2. Relevant equations

    3. The attempt at a solution
    Rtotal = R + r
    I = V/Rtotal
    E = V/Rr + V_ba
    when I equate both to try to solve for r, I still end up getting an unknown of 2 variables. Any help would greatly be appreciated. Thank you.
  2. jcsd
  3. Oct 27, 2009 #2
    When you say the potential difference is 9.5 V and then 10 V you are referring to the potential difference across what circuit element? Also, what is the question? The internal resistance of the battery?
  4. Oct 27, 2009 #3
    That's just how the problem is given. I am totally lost, and what's worse is, the rest of the homework problems I have to do builds on top of this. I been at this for the past 2 hours and I keep running into 2 unknowns. Can anyone please provide a solution for this? Thanks
  5. Oct 27, 2009 #4
    The problem is not asking a question, so there's nothing to solve for. If you want to describe the system mathematically then just solve for one variable in terms of the other variable. You may not know was both variables are, but you can derive the relationship between them.
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