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Homework Help: Circuit problem

  1. Dec 27, 2008 #1
    1. The problem statement, all variables and given/known data


    I am really stuck on another circut question, if anyone can help with question 3b (i) , I don't really know were to start, so don't have any working/solution to show for.

    thank you!

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  2. jcsd
  3. Dec 27, 2008 #2


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    Staff Emeritus
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    Gold Member

    You know the voltage drop across the 5ohm resistor therefore you can find the current. Now what resistance is needed to yield that current with a 3V source?
  4. Dec 27, 2008 #3
    but how do i show its 0.4 ohms?

    0.4 + 5 = 5.4

    3/5.4 = 0.55 amps? is this correct?
  5. Dec 27, 2008 #4


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    Staff: Mentor

    I'm not understanding where you're going with that. Instead...

    The "internal" resistance of the battery should be shown as a resistor in series with the battery, on the same leg of the circuit as the battery. So draw a resistor next to the battery on that top line, and label it Rbat.

    Now label where you are measuring the voltage with the meter. Draw the current flowing around the circuit. See how there is some voltage drop across the Rbat caused by the current? That voltage drop decreases the measured voltage as compared to the Vbat value.

    Now write the equation(s) that represent that situation, and solve them here for us.
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