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KVL Exercise - Basic

  1. Feb 6, 2013 #1
    1. The problem statement, all variables and given/known data
    Click on the link
    https://www.diigo.com/item/image/2sb3i/yjim [Broken]


    2. Relevant equations
    KVL Law
    Similar Example :
    https://www.diigo.com/item/image/2sb3i/orwa [Broken]

    3. The attempt at a solution
    (a)
    In the left loop :
    -8-12+VR2 = 0
    VR2 = -20 v

    (b)
    In the big loop :
    -8-12+7-9-V2-3+VR1 = 0
    -8-12+7-9-V2-3+1 = 0
    V2 = -24

    Why is my solution wrong?

    If I take the right loop for (b) using the right answer for (a) :
    -4+7-9-V2-3+1 = 0
    V2 = -8 which is true.

    Is the wrong with the negative voltage (-8) ??

    thanks.
     
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Feb 6, 2013 #2

    gneill

    User Avatar

    Staff: Mentor

    Yes, it looks like whoever created the circuit diagram created confusion when they labeled the leftmost resistor's potential drop. As it is marked, strictly speaking there should be a -8V potential drop across that resistor proceeding from bottom to top, which translates into a +8V potential rise as you've properly interpreted it. However, it would appear that they (confusingly) intended the "+ -" indicators to show the actual direction of the potential change of 8V.

    Bottom line: assume that the "-8V" is really "8V" oriented according to the "+ -".
     
  4. Feb 6, 2013 #3
    thank you
    but I see that there is no difference if it was "+8" instead of "-8" ???
     
  5. Feb 6, 2013 #4

    gneill

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    Staff: Mentor

    Um, how can it make no difference if you change the sign of a potential change?

    For part (a) your equation summing the potential drops would become: +8-12+VR2 = 0
     
  6. Feb 6, 2013 #5
    ok, how the solution of (a) will be if :
    https://www.diigo.com/item/image/2sb3i/y0us [Broken]
     
    Last edited by a moderator: May 6, 2017
  7. Feb 6, 2013 #6

    gneill

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    Staff: Mentor

    Just solve your equation that I presented in my previous post...
     
    Last edited by a moderator: May 6, 2017
  8. Feb 6, 2013 #7
    I'm sorry
    but can you tell me the equation of (a) if the potential change was "-8"?
    I thought that the equation +8-12+VR2 = 0 is used when the potential change = -8.
     
  9. Feb 6, 2013 #8

    gneill

    User Avatar

    Staff: Mentor

    Yes, that's right. The problem is not with your understanding, it's with the problem itself; the resistor is not labeled correctly for the given answers. The resistor should have its potential labeled as "8V", not "-8V", or alternatively, the + and - tags should be reversed.
     
  10. Feb 6, 2013 #9
    aha
    thank you Mr. gneill
     
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