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Homework Help: The sign of the current and voltage

  1. Feb 16, 2016 #1
    • moved from general forum so homework template is missing
    I was doing the following problem:


    And I was asked to find Iy.

    I found Iy to be 2.64 using KCL. However, the right answer was negative 2.64.

    Is it negative only because there is a dependant voltage source with "+ -" ? And why must it be negative? Does "-" in the final answer play a significant role? If I put positive 2.64 instead of negative, would that be wrong?

    Also, here is another question


    I found V1 to be 11.9. However, the right answer was - 11.9.

    Can someone clarify for me when do you put negative sign ?
  2. jcsd
  3. Feb 16, 2016 #2
    Yes, because the problem has chosen which direction the current flows for you. If the current is actually flowing in the other direction, you will get a negative value for the current because of the chosen reference direction.
  4. Feb 17, 2016 #3


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    Staff: Mentor

    You made a mistake, but by not showing your working you aren't allowing anyone to point it out.

    Quite possibly you didn't solve for Iy, but instead solved for current in the direction opposite to the direction indicated for Iy. If so, that is quite okay, but you must remember before stating your answer to change its sign to correspond with the direction for Iy as specified in the given figure. Achieving the correct polarity is as essential as determining the correct magnitude; direction determines whether the designer needs to provide a power source or a power sink.
  5. Feb 19, 2016 #4


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    Science Advisor
    Homework Helper
    Gold Member

    +1 Show your working. Sign errors are very common when using KVL. I find it helps to write equations that sum to zero eg write them in the form..

    V1 + V2 ....+ Vn = 0
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