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Homework Help: Connecting a known and unknown battery

  1. Mar 24, 2008 #1
    [SOLVED] Connecting a known and unknown battery

    1. The problem statement, all variables and given/known data
    An unmarked battery has an unknown internal resistance. If the battery is connected to a fresh 5.60 V battery (negligible internal resistance) positive to positive and negative to negative, the current through the circuit is 10.0 mA. If the polarity of the unknown battery is reversed, the current increases to 25.0 mA. Determine the emf and internal resistance of the unknown battery. Assume that in each case the direction of the current is negative to positive within the 5.60 V battery.

    2. Relevant equations

    3. The attempt at a solution
    I tried making two equations using the loop rule for each setup.

    5.6 - .01*r - e = 0
    5.6 - .025*r + e = 0

    e = 2.4 V
    r = 320 ohms

    Would that be right? The resistance looks a little too high to me.

    When it says the batteries are connected positive to positive, does that mean the batteries are connected in series in the circuit?
  2. jcsd
  3. Mar 24, 2008 #2
    The answer is right. Positive to positive just means that the positive sides are connected to each other.

    This has nothing to do with being in series, because the batteries will be in series no matter how you connect them.
  4. Mar 24, 2008 #3
    All right, thanks kamerling!
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