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Currents in a circuit

  1. Apr 2, 2015 #1
    1. The problem statement, all variables and given/known data
    ?temp_hash=485d4917e6369ebdd2aa0d59a4e2e720.png

    2. Relevant equations

    V=Ir
    3. The attempt at a solution
    I solved part a ) already my question is about part b). Lets say I know that the 6 ohm resistor has current I3 going across is pointing to the left, and the 3 ohm resistor has current I1 also pointing to the left across it. I have to find it as if I am starting from point B and ending at point A (I think) . we are going against the current for the 6 ohm resistor so it would be + 6I3 from that and then we are following the direction of the current for the 3 ohm resistor so that would be - 3I1 across that. So the total voltage difference would be 6I3 - 3I1 . Am I doing this correctly?
     

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  3. Apr 2, 2015 #2

    mfb

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    Let's call the intermediate point C.

    Is V_B larger or smaller than V_C? What is the sign of V_C-V_B then? Does that fit to your description? If yes, then the same works for B and A.
     
  4. Apr 2, 2015 #3
    so it doesn't matter which way the current is facing? V_C is smaller than V_B so it would be -6I3-3I1 = -(6I3+3I1)
     
  5. Apr 3, 2015 #4

    NascentOxygen

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    You need to adopt a procedure which you are confident you can work with and get it right. For the directions you indicated,
    I would start by writing ##V_B + 6I_3 - 3I_1 = V_A##
     
  6. Apr 3, 2015 #5
    then subtracting Vb from both sides you have Va-Vb = 6I3-3I1 ... so I was right originally? So you start at point be. You are going against the current for the 6 ohm resistor so there is a voltage increase across that. You are going with the current for the 3 ohm resistor so there is a voltage decrease across that. you start at voltage b. on your way to point A it increase and drops so voltage b after that increase and drop is equal to the voltage at point a. Am I correct in saying that if you go against the current the voltage will increase?
     
  7. Apr 3, 2015 #6

    NascentOxygen

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    Yes
     
  8. Apr 3, 2015 #7
    so I think I ended up with the right answer?
     
  9. Apr 3, 2015 #8

    NascentOxygen

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    In algebraic terms, yes. But you shouldn't need to ask; you have already determined the currents so can substitute their values. Only the correct answer will make sense, wrong answers will show glaring inconsistencies. That is how you can check your own work.
     
  10. Apr 3, 2015 #9
    now what if point A was after (to the left of) the 8 volt emf? the voltage will always increase across an emf right? it doesn't depend on the direction of current?
     
  11. Apr 3, 2015 #10

    mfb

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    It does not depend on the direction of current then, but it does depend on the orientation of the voltage source.
     
  12. Apr 3, 2015 #11
    do you mean the orientation of the negative and positive end? if you go from negative to positive it would be an increase in voltage?
     
  13. Apr 4, 2015 #12

    mfb

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    Right.
     
  14. Apr 4, 2015 #13
    thank you all for your help!
     
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