Comparing voltages

Discussion in 'Homework Help' started by tempneff, Nov 24, 2013.

  1. tempneff

    Thread Starter New Member

    Aug 11, 2012
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    Hi,

    I am solving a E&M problem and it led me to a conceptual question about comparing voltages. If I measure from ground to,say, three points. I get the values 1v, 5v, and -5v. Which do I consider the minimum, am I taking the absolute value?

    Thanks in advance.
     
  2. donpetru

    Active Member

    Nov 14, 2008
    186
    25
    In mathematics, the module or the absolute value of a real number x, denoted |x| is the actual number taken without sign (thus, for example, number 5 is the absolute value of 5 and -5). Making an analogy with the electrical, things are the same. And if I were to take into account your example, the minimum value will be 1V (because I taking the absolute value).
     
  3. tempneff

    Thread Starter New Member

    Aug 11, 2012
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    I am not questioning which would be minimum if I used absolute values, but rather whether one should use absolute values.

    In other words, when we compare voltages do we use the magnitude of the potential or the number itself?
     
  4. MrChips

    Moderator

    Oct 2, 2009
    12,421
    3,356
    It's all relative, like north and south.
    In the mathematical world, -5 is less than 5.
     
  5. WBahn

    Moderator

    Mar 31, 2012
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    It depends on the purpose of what you are doing. If you are asking which has the lowest potential, then you use the algebraic minimum. One way to get at this is to consider that you could have measurement them with respect to any other node -- you didn't have to use ground. So what would you have gotten if you had measurement them relative to something that was at -10V? What would you have gotten had you measured them relative to +10V?

    Then ask if, for your purpose with regards to the question you are answering, if you should get the same answer for which has the minimum voltage regardless of which node you choose to measure them?
     
  6. JMW

    Member

    Nov 21, 2011
    88
    8
    While mathematically -5 is less than +1, -500 volts can be fatal, +20 not likely. I would go with the -5
     
  7. WBahn

    Moderator

    Mar 31, 2012
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    Which is more appropriate depends entirely on what you are trying to determine.
     
  8. tempneff

    Thread Starter New Member

    Aug 11, 2012
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    I am trying to determine the point of minimum potential between to fixed charges. So I used the E field created by both at some point r on the shared axis with the origin placed at the first charge E_{total}=\frac{q_1}{r^2}k+\frac{q_2}{(d-r)^2}k where d is the distance between the charges. Next I took the derivative of E with respect to r. Finally I set equal to 0 to find the point of maximum Efield,then I figured that since E and V are related with opposing signs the max E is the min V??..
     
  9. WBahn

    Moderator

    Mar 31, 2012
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    If you are looking for a point of minimum potential between two charges, then you are (almost certainly) looking for the minimum of the absolute value.
     
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