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Polarity reversal

  1. Mar 10, 2017 #1
    1. The problem statement, all variables and given/known data
    Why is the voltage of the right source negative? 2OglMH4.png

    2. Relevant equations

    3. The attempt at a solution

    I learned that a circuit element is positive or negative, because of the terminal with respectively the highest or lowest voltage. So, if the positive terminal has a lower voltage/ potential than the negative terminal, then the element has a negative voltage. But this would mean that the polarity of the voltage source has changed.


    I guess I found a rare situation where the polarity of a source will change: "if you have a wet cell battery are filling it for the first time, and are using an old style, non smart charger, and short the terminals while you are filling it, yes it is possible to hook up the charger backward and reverse charge it."
    But I don't think that the practice problem meant this and therefore I'm wrong for a reason I can't figure out.
     
  2. jcsd
  3. Mar 10, 2017 #2

    gneill

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    Staff: Mentor

    Hi MeneerM, Welcome to Physics Forums!

    The polarity labels on circuit components are there for one of two reasons: to tell you how to orient and apply given data to a particular component (such as the polarity of a voltage source), or to tell you how to interpret a given measurement in terms of polarity (such as the potential drop across a resistor due to current flow).

    In the given circuit the +/- inside the 8 V voltage supply symbol tells you how the polarity of the source is to be interpreted. The -8 V beside the symbol tells you what potential difference to assign to the source. Since it's negative, that means the "+" end of the voltage source symbol will be at -8V with respect to the "-" end of the voltage source.

    upload_2017-3-10_15-53-51.png
     
  4. Mar 11, 2017 #3
    Do I understand it correctly if I say that the positive or negative voltage of the voltage source (or any other sources) just indicate the polarity and therefore which direction the positive charge want to flow?
    And is it correct if I conclude that the positive or negative voltage and polarity of the resistor dictates the potential drop?

    So the negative voltage just mean that the positive terminal is 8V less than the negative terminal and therefore the positive charge wants to flow to the positive terminal. Right?
     
  5. Mar 11, 2017 #4

    gneill

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    Staff: Mentor

    If the source value (the value written beside the symbol) is positive then that is true. The value tells you the potential difference between the "+" and "-" leads of the source. If the value is negative, it means that the "+" lead actually has a potential that is lower then the "-" lead. Remember, the "+" and "-" designations don't actually do anything other than tell you how to interpret values.
    upload_2017-3-11_10-34-0.png
    It depends on how and why they were created. If they were created before you know the actual current direction then they are meant to tell you how to interpret any potential change across the component. If you specified the current direction first, then they indicate the potential drop that occurs due to THAT current flowing through the component. Of course, if it turns out that the actual current flow was in the opposite direction of your "guess", you'll end up with that potential change having a negative value when interpreted through those symbols.
    Right.
     
  6. Mar 11, 2017 #5
    You're awesome, I finally start to understand it! I still got one last question: I concluded in my last post
    In which situation does this occur? I have learned that the positives terminal of a source is always the high potential side and it therefore "pushes" the positive charge AWAY.
    Or are two sources in this picture just a way of expressing the exact same voltage source and isn't there a negative side which sending positive charges away?

    upload_2017-3-11_10-34-0.png

    Thanks in advance.
     
  7. Mar 11, 2017 #6

    gneill

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    Staff: Mentor

    It's a matter of labeling versus "reality". Schematic symbols are just that: symbols. They need interpretation in order to know what they "really" represent. If you had a black box with a source inside and all you could see were the two leads, you could label the box with a schematic symbol that is oriented in either direction so long as you gave it a suitable value label that results in the actual potential difference orientation presented by the leads.

    Again, the labels on the symbol are just telling you how to interpret or apply the given information. Once you apply the conventions dictated by the labeling you can then interpret what "actually" exists. If it turns out that the "positive" lead on the voltage source is "actually" the negative pole of some "true" source, then that's fine. You can always erase the source symbol from the schematic and replace it with one that shows the "true" orientation of a source with its positive lead actually positive with respect to its negative lead, and having a positive value label (as I showed in the picture).

    The source symbol just represents a device that maintains a specified potential difference between its leads. That potential difference can be any value, positive or negative. By convention we tend to label such symbols such that we interpret the potential change with a certain orientation that makes rudimentary sense: The positively labeled lead is at the higher potential. But this is not a strict rule for circuit drawings. We can give the source "value label" a negative value and that reverses the actual polarity. A controlled source, for example, may have a value that depends upon some voltage measurement elsewhere in the circuit. That measured value might be be positive or negative at any given time, and the controlled source would output a value that depends in some way on that measured value. So it could supply a positive or negative potential difference at different times. The labeling on the symbol (the symbol orientation) lets you apply the results of the defining source function to correctly orient the result of the defining function.

    Another example is an AC source, where you know that the polarity changes every half cycle of the AC waveform. Yet we still mark those sources with an orientation, because it informs us how to interpret the phase of the source with respect to other sources in the circuit.
     
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