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Negative current?

  1. Sep 11, 2013 #1
    1. The problem statement, all variables and given/known data

    Calculate the current through R3.
    2. Relevant equations

    V=IR

    3. The attempt at a solution

    my attempt is attached. My question revolves around my answer...is a negative current a legitimate answer? What does it mean?
     

    Attached Files:

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  2. jcsd
  3. Sep 11, 2013 #2

    lewando

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    It is a legitimate answer provided you show a direction arrow (which you have done). It means that the current is not travelling in the direction indicated by your direction arrow. It is going in the opposite direction.
     
  4. Sep 11, 2013 #3

    lewando

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    Looking at your loop equations, they seem a bit suspect. If you are summing the changes in voltage across each element along a loop, equating the sum to zero, you should get something different.
     
  5. Sep 12, 2013 #4

    ehild

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    Your equations 1 and 2 are wrong. Note, that the potential decreases in the direction of the current.

    ehild
     
  6. Sep 12, 2013 #5
    What should my equation for loop 1 be?
     
  7. Sep 12, 2013 #6

    ehild

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    You start at the negative pole of the battery, and follow the change of potential in the direction, indicated by the green arrow. The potential increases by 5 V traversing the battery. Then it decreases by I3R3 along R3. Then again, it decreases by R1I1, when you return to your initial position. The net change of potential is zero: 5-I3R3-I1R1=0.
    Do the same for the other loop. Note that going in direction opposite to the current, the potential would increase.

    ehild

    attachment.php?attachmentid=61707&d=1378954564.jpg
     
  8. Sep 12, 2013 #7
    okay I think I am following you so for loop 2 the equation is then -10V+I3R3+I2R2=0?
     
  9. Sep 12, 2013 #8

    ehild

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    Yes.

    ehild
     
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