circuit analysis voltage

Discussion in 'Homework Help' started by fran1942, May 5, 2011.

  1. fran1942

    Thread Starter Member

    Jul 26, 2010
    58
    0
    Hello, on the attached circuit we have been told that V1 = -1V-(14v) = -15V.

    I am trying to understand how this can be.
    I would have thought you would make a loop with the 1V power source and the 7 and 5 ohm resistors eg.
    current = 1/13ohm = .00769A
    .00769A * 5ohm = .3846V answer for V1 ?

    Obviously I am wrong, but can someone please explain how it is that V1 = -1V-(14v) = -15V.

    Thanks kindly in advance.

    nb. also on the second image I attached, is my volt calculation correct at 1V ?
    Thanks for answering these newbie questions. I am just trying to get a grasp on electronic circuits.
     
    Last edited: May 5, 2011
  2. jegues

    Well-Known Member

    Sep 13, 2010
    735
    43
    If you desire the voltage across an element you simply take the difference between the voltages at each of its respective nodes.

    See the figure attached.
     
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  3. jegues

    Well-Known Member

    Sep 13, 2010
    735
    43
    Simply write a KVL in the bottom loop.

    See figure attached.
     
    • Q2.JPG
      Q2.JPG
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  4. fran1942

    Thread Starter Member

    Jul 26, 2010
    58
    0
    thanks kindly for that.

    If anyone can help with the first question in this post that would be much appreciated.
     
    Last edited: May 6, 2011
  5. jegues

    Well-Known Member

    Sep 13, 2010
    735
    43
    Actually, I think I made an algebra mistake in this one, it should be -7V.
     
  6. hobbyist

    Distinguished Member

    Aug 10, 2008
    764
    56
    Just look carefully at the drawing, the voltage loop can be seen if you look past the other components that are being a distraction, for this analysis.

    V1 is the voltage drop across the 5 ohm resistor.
    Starting at the left side of this resistor you have +14v. on the right side of the resistor you have neg. 1v., so the loop to follow is starting at the left side of the resistor and going counterclockwise, you write the equation as you go through the voltage source, so you have
    (-14v. + -1v.), that is across the rersistor.

    so in other words, the right side of ther resistor is NEG. 15v. with respect to the left side of the resitor. (using the arrow there showing)

    If the arrow was in the other direction, then the left side would be POS. 15v. with respect to the right side of the resistor.
     
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