confusion about ground reference

Discussion in 'Homework Help' started by grudd, Jun 26, 2014.

  1. grudd

    Thread Starter New Member

    Jun 26, 2014
    Hi everyone,

    Im new here, trying to teach myself about electronics using this website, "Practical electronics for Inventors", and what I remember from the physics classes I took back in college!

    My question has to do with relative ground points on schematics and combining batteries. I have attached a crude drawing of part of the example from the book I am having trouble understanding.

    I understand that points B and D are our reference, so our point voltage there is 0v. Point A has a voltage of 12V, and point C has a voltage of 9V. So, Vab=12V and Vcd=9V.

    What I am having trouble grasping is what the process of finding the voltage from point A to C, or any other voltages where we have to find it across the two schematics. Could someone please enlighten me as to the process one would use in determining these voltages?

  2. paulktreg

    Distinguished Member

    Jun 2, 2008
    You aren't really going "across" two schematics.

    Connect points B and D together (with a thick dark line if necessary) and that should make things clearer?
  3. grudd

    Thread Starter New Member

    Jun 26, 2014
    That does clear things up a bit. Im still a little confused on finding the voltage on the open side of the circuit (between points A and C). I understand in this problem, the current is moving in contrary motion, so the resulting voltage between the points is 3V. what trips me up is the next section, where the ground reference is changed to point C. I redrew the schematic to a more rectangular form.

    I am thinking because the current is now moving in the same direction, the resulting voltage between points A and D (again, the "open" side of the circuit) would be 21V, but the answer in the book is -21V. Am i missing a sign convention here?
  4. grudd

    Thread Starter New Member

    Jun 26, 2014
    here is how i redrew the second part of the problem
  5. MaxHeadRoom


    Jul 18, 2013
    In measuring the points A & D which are essentially two supplies in series when measured at these points, it depends on which side you use as reference, it will be 21 or -21 depending on the reference point.
  6. grudd

    Thread Starter New Member

    Jun 26, 2014
    Thank you! thats what i figured, but the book only gave the negative answer.
  7. Jony130

    AAC Fanatic!

    Feb 17, 2009
    The Book ask about voltage between point A and C.
    Va voltage is reference to ground is equal to Va - Vb = 12V - 0V = 12V.
    Vc voltage is also measure in reference to ground. So Vc = Vc - Vd = 9V - 0V = 9V.
    And from this we can find Vac = 12V - 9V = 3V or Vca = 9V - 12V = -3V.
    But if we change the reference point from D to C. We have this situation.
    Vd = Vd - Vc = -9V - 0V = -9V and now Vad = Va - Vd = 12V - (-9V) = 21V.
    But maybe the book ask you to find Vda voltage.
    Vda = Vd - Va = -9V - 12V = -21V
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  8. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    I did not use the simulator to find the answers, but it was the best way to illustrate them.
    Last edited: Jun 28, 2014
  9. grudd

    Thread Starter New Member

    Jun 26, 2014
    great! all of that helps make it much clearer in my mind. Jony, I think the book made an error with the sign because the answer is reported as Vad, not Vda. The math tells us the voltage should be positive, but the given answer is negative, hence my confusion.

    Thanks for all your help, ill probably be back with more very basic questions!