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Voltage confusion

  1. Oct 2, 2016 #1
    So whats the smplest illustration for voltage ,current and resistance and whats ohms law ?
     
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  3. Oct 2, 2016 #2

    anorlunda

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    virginia-route-29-copper-circuit.png

    The picture shows the simplest case when the material for the resistor R is ohmic. Ohmic means that the ratio V/I is constant.

    :welcome:
     
  4. Oct 2, 2016 #3

    Simon Bridge

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  5. Oct 2, 2016 #4

    rbelli1

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    I think that is that moment of disorientation after coming into contact with mains voltage. (PS: don't try this at home)

    BoB
     
  6. Oct 4, 2016 #5
    One problem that can be confusing is the formula on paper fails to point out that voltage has to be there first for current to flow. People sometimes make the mistake of assuming a voltage is present simply because IR says it should be there.
     
  7. Oct 4, 2016 #6

    anorlunda

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    Sorry, but I disagree. In circuit analysis, for Kirchoff's laws to apply, we say that voltages and currents apply simultaneously. There is no first/second.

    It is also perfectly valid to say "voltage is present simply because IR says it should be there". If you prefer, think of a resistor connected to an ideal current source. The voltage appears because the current flows (and the current flows because the voltage appears). No first and second, but simultaneously.

    If you really want to find out what happens first, then you need time domain solutions to Maxwell's equations, but then that it no longer circuit analysis. You will find that the fields propagate at near light speed.

    This recent thread talked about the assumptions of circuit analysis in some detail.
    https://www.physicsforums.com/threads/why-does-a-voltmeter-measure-a-voltage-across-inductor.880100
     
  8. Oct 4, 2016 #7

    LvW

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    Yes - it is, certainly, correct for the purpose of circuit analysis to assume that both - voltage and current - do exist at the same time and that - for example - a current is able to produce a voltage across a resistor (V=I*R).
    However, from the physical point of view it is clear that no current can exist without driving voltage - hence, voltage first and current next. This basic rule applies, of course, also to circuits with "current sources" which, in reality, always are voltage sources eqipped with a very large (dynamic) source resistance.

    Remember: Current is movement of electrical charges - and the force which can cause such a movement is provided by the electrical field created within the conducting material by the applied voltage.
     
    Last edited: Oct 4, 2016
  9. Oct 4, 2016 #8
    With respect to time, yes that's the case. I was not trying to say voltage is first in instance, but that voltage is a prerequisite for current to flow. V=IR does not make that distinction. If you assume a current you also have to assume a voltage which may or may not be there. That can give a person trouble conceptually looking at what's happening in a real circuit.
     
  10. Oct 4, 2016 #9
    You'd agree that Maxwell's equations describe your physical point of view? If so, then voltage does not come before current or vice versa - there's no causal relationship between them.

    @LvW, can you give an example of something you'd consider a voltage source? A battery perhaps, do you consider that a voltage source?
     
  11. Oct 4, 2016 #10

    anorlunda

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    The physical equations that describe the behavior are Maxwells Equations. To solve the time evolution is a study of fields, not circuits. With fields, you need the exact geometry to describe the sequence of events because fields are 3D. If you have just a resistor, and no geometry information, you can't solve the field equations, so the only tool you have left is circuit analysis.

    You may be thinking of the Drude Model. Maxwells Equations are a much better foundation.

    Edit: Also, consider that the OP was very much a beginner's question. We (including me) should not be injecting this esoteric stuff to this thread.
     
    Last edited: Oct 4, 2016
  12. Oct 5, 2016 #11

    LvW

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    OK - I must admit that I didn`t express myself clear enough. It was not correct to say "voltage first and current next" because this involves a time sequence.
    It was my only intention to answer the question: Which force causes the charges to move within the conductive material. Is it not true that we need an electric field as a precondition? And - yes - I would consider a battery as a (of course non-ideal) voltage source.
     
  13. Oct 5, 2016 #12
    Okay, let's use this as a starting point. In the following, in the questions I might pose, I'm not trying to trick you or anything, I genuinely just want to present you an alternative viewpoint. Your line of reasoning is shared by many.

    Let's assume non-ideal components, no hocus pocus. How do you know a battery is a voltage source? I just mean in practical terms - is it something you can measure?
     
  14. Oct 5, 2016 #13

    LvW

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    milesyoung - I am not sure if it makes much sense to discuss here the properties of a battery.
    On the other hand - it would be interesting to learn more about your "alternative viewpoint".
    Perhaps you consider the moving/separation of charges within the device - prior to using it as a usable voltage source - also as a kind of current ?
    In this case - my answer would be: Obviously, the OP was asking for some clarification regarding the terms "voltage", "current" and "resistor" - as they appear as electrical quantities in electrical circuits. And for such circuits the basic rule applies: Each current needs a driving voltage.
    Do you see my point? I think - in particular for beginners - it is important to know that relations like R=V/I may be used also in other forms (like I=V/R or V=I*R) - however, it is not always (per se) allowed to automatically derive from such forms any information about cause and effect.
    (Have you seen a similar discussion about the relation Ic=B*Ib?)
     
  15. Oct 5, 2016 #14
    I'm not interested either in a discussion on the specifics of batteries, but it's one of the simplest examples I could think of of something you associate with a voltage source.

    I'm not even talking about circuit theory, I just want to pose you the basic question: what is it about the behavior of a battery that makes you sure it's a voltage source? To put it another way, if I gave you a 2-terminal black box with a battery inside and asked you to probe the terminals, what would make you conclude that it's a voltage source?
     
  16. Oct 5, 2016 #15

    LvW

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    Any electrostatic measurement device would react upon the potential difference, would it not?
     
  17. Oct 5, 2016 #16
    Sure, but that would also be the case for a nonideal current source.

    What if I, in my EE studies, had a very difficult teacher. He/she had a preference for current sources and would almost never show a voltage source - practically everything would be modeled as current sources. I'd later come to this board, engage in this same discussion with you, but I'd be adamant about there being a current source in the box, not a voltage source. Who of us is right?
     
  18. Oct 5, 2016 #17

    LvW

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    Yes - because each non-ideal current source is (in reality) a non-ideal voltage source.
    Remember: Which effect (which force) causes the charges to move (forming the quantity we call "current") ?
     
  19. Oct 5, 2016 #18
    Why is my black box not part of your reality? :cry:

    The force can be either electric or magnetic, depending on your choice of reference frame. I assume you're analyzing this from the rest frame of the conductor, but that frame is no more fundamental than any other.

    https://en.wikipedia.org/wiki/Classical_electromagnetism_and_special_relativity

    I can try to find an example that shows the effect for a charged particle within a conductor that's not in electrostatic equilibrium, but even the simplest of cases tend to become very complicated very fast, and I'm afraid I never studied special relativity much beyond the simple cases I saw in classical EM.
     
  20. Oct 5, 2016 #19

    Averagesupernova

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    Doesn't the discussion come down to can we have a circulating current with no voltage? Is there not current in a copper pipe when we drop a magnet through it? In the real world there is a finite resistance in the copper. What about a super conducting pipe?
     
  21. Oct 5, 2016 #20

    LvW

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    The core of our discussion is: What is "electrical current" and where does it come from - correct?
    More than that, I think, here we speak about classical electric/electronic circuits and applications (and not about "exotic" arrangements based on superconducting material).
    In this context: Is there any real technical device which we call "current source" (labor slang), that is not energized by a voltage?
    And - we shouldn`t forget that the start of the discussion was the question: Is it correct to say "a current produces a voltage according V=I*R" ?
     
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