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Homework Help: Find Vemf

  1. Sep 25, 2010 #1
    When a battery is connected to a 110.-ohm resistor, the current is 3.98 A. When the same battery is connected to a 400.-ohm resistor, the current is 1.11 A.

    Find the emf supplied by the battery and the internal resistance of the battery.


    Vemf = i*R

    I'm not sure how to approach this since you get two voltages (438.7 and 444) from the different resistors.

    Should I treat these resistors like they are in the same circuit? In series or in parallel?

    Please help!
  2. jcsd
  3. Sep 26, 2010 #2


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    Homework Helper
    Gold Member

    Yes, you'll get two different voltages, by using different resistors, because there is another resistance involved that is always there: the internal resistance of the battery.

    (By the way, check your math regarding the voltages you calculated.)
    Not the 110 and 400 Ω resistors, no.

    But the battery's internal resistance is always there, so yes, the internal resistance is within the circuit in both cases.
    What do you think? (Hint: look in your textbook for "internal resistance" of a battery, and the answer should be clear.)
  4. Sep 26, 2010 #3
    Thanks! that explanation makes a lot of sense. I'm sure I can figure the rest out from there!
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