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Simple voltage drop / sign convention

  1. Jul 2, 2013 #1
    1. The problem statement, all variables and given/known data

    [Broken]

    Find Vad and Veb


    2. Relevant equations

    voltage drop, KVL


    3. The attempt at a solution

    I actually did:

    Vad

    -24V - 4V + 6V

    = -22V

    but this is wrong.


    I know going the other way

    Vad: 6V + 8V + 12V

    = 26V,

    because it is only drop, and plus to minus on drops means they are always positive

    and it's the correct answer.


    But it seems for Vab the real correct way counting the sources was

    Vad: 24V -4V + 6V

    = 26V


    I thought it was different for voltage sources and sign orientation?


    So when the drop direction is plus to minus (+ to -), the voltage drop is always positive, for both sources AND given sign convention?



    Sorry for asking this now. KCL is much easier.



    Veb would be

    8V - 6V + 24V

    = 10V
     
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Jul 3, 2013 #2

    lewando

    User Avatar
    Gold Member

    For the passive components this is true and for the voltage source you have shown, this is true.

    Sometimes you will encounter a "negative voltage source" (a voltage drop moving from + to - terminals which is negative). These are thrown at you, occasionally, to see how well you react.


    You have a simple math error there, I'm afraid.

    The drops encountered moving from e to b clockwise are:

    -8V (because the voltage increased resulting in a "negative drop")
    -6V (same reason)
    +24V (because the voltage dropped by 24V going from a to b)

    Summing the drops: -8V -6V +24V = 10 V
     
  4. Jul 3, 2013 #3
    Keep your notation correct
    V[itex]_{eb}[/itex] = -V[itex]_{be}[/itex]

    as an example
    V[itex]_{af}[/itex] = 6v
    V[itex]_{fa}[/itex] = -6v
     
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