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I 2 definitions for voltage, how are they equivalent?

  1. Sep 22, 2018 #1
    So I'm studying electrostatics and I came across to two different definitions of potential difference/voltage (because we're in stationary regimes) and I'm having trouble understanding how the expressions are equivalent.

    They are for a voltage between point A and point B

    $$U=V_a - V_b =\int_{a}^{b} \textbf{E} \cdot d\textbf{s}$$

    and, on the other hand,

    $$U= V_b - V_a = - \int_{a}^{b} \textbf{E} \cdot d\textbf{s}$$

    How can both of this expressions represent the potential difference between points A and B? Aren't they symmetric?
     
  2. jcsd
  3. Sep 22, 2018 #2

    Doc Al

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    Not quite sure I understand the question.
    The sign of the potential difference depends on which point you're measuring with respect to:
    ##V_a - V_b = - (V_b - V_a)##
     
  4. Sep 22, 2018 #3
    My question is that in both cases the expression is presented as "the potential difference between points a and b". However if I use both definitions will I obtain the same result or will I obtain symmetric results? I'm thinking the later is probably the correct one but now that leaves me with other question: shouldn't the second expression be the potential difference between points b and a?
     
  5. Sep 22, 2018 #4

    Mister T

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    What does ##U## stand for?

    ##V_b-V_a \neq V_a-V_b## except for the trivial case of ##V_a=V_b##.
     
  6. Sep 22, 2018 #5
    U is the voltage between points a and b.

    Exactly that's my doubt, are both expressions correct?
     
  7. Sep 22, 2018 #6

    Mister T

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    $$V_a - V_b =\int_{a}^{b} \textbf{E} \cdot d\textbf{s}$$

    and,

    $$V_b - V_a = - \int_{a}^{b} \textbf{E} \cdot d\textbf{s}$$

    But they cannot both be equal to ##U## at the same time, except when ##U=0##.
     
  8. Sep 23, 2018 #7

    Tom.G

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    Consider:
    If you a 1 meter long stick and a 1.5 meter long stick, isn't the absolute difference between them the same regardless of how you do the length comparison?
     
  9. Sep 23, 2018 #8

    Dale

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    They are both correct, but they are not equal to each other. The thing with voltages is that it is a directed difference. Strictly speaking you should never write just voltage ##U##. It should always be written more clearly ##U_{ab}=-U_{ba}##. You need to always specify which point is being considered to be the reference or the “ground”.

    There is no such thing as “the voltage between points a and b”. There is “the voltage at a with respect to b” and “the voltage at b with respect to a”. Often in context which point is used as reference/ground is clear, so it may not always be stated so precisely, but that is the meaning.
     
  10. Sep 23, 2018 #9

    Mister T

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    That's not a definition because it could refer to ##V_b-V_a## or ##V_a-V_b## or ##|{V_b-V_a}|##.

    It's like referring to an altitude between two points. For example, if you have an altitude difference of 10 meters between points a and b you don't know if the altitude at point b is 10 meters greater than the altitude at point a or the other way around. Saying that the altitude between them is 10 meters doesn't tell us which of those to choose. And saying that the absolute value of the difference is 10 meters doesn't tell us, either.
     
  11. Sep 23, 2018 #10

    Mister T

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    Yes, but knowing only that the difference is 0.5 meters doesn't tell us which of the two items is longer. And it certainly doesn't imply that ##1.5-1.0## is equal to ##1.0-1.5##. But that is indeed what the OP's two equations imply about the value of ##U##.
     
  12. Sep 23, 2018 #11

    Tom.G

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    Agreed.
    I also agree that the polarity is often, but not always important. (re: active devices vs resistive heater)

    However, the OPs question was:
    With no reference to polarity. That was the question I was trying to answer.
     
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