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Ohm's Law or not Ohm's Law... That is the question

Discussion in 'General Electronics Discussion' started by CDRIVE, Sep 19, 2012.

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  1. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Hey guys. I've not been reading this thread in great detail because I assumed it would remain technical.

    I've read one post (above) and now I'm not so sure.

    Please try to keep the math separate from emotion. If you can't do that, try to play the ball and not the man.

    You're all big boys, and if I have to clean this thread up it will be a real pain because there's a lot of information mixed with the invective.

    Incidentally, I have a foot in both (or is that all three?) camps regarding this. I don't think there's a perfect answer to this as we apply the same math to other things (F = ma) and yet we don't call that Ohms Law.

    When we arrive at (after various mathematical transformations) at a value that represents some form of voltage being proportional to some value representing current and we determine there is another factor which represents the constant of proportionality, which we may or may not call resistance, then we can say it is Ohm's Law, or we can say it's like Ohm's law.

    It strikes me that we are arguing over whether it IS or it is LIKE Ohm's law.

    And personally, the difference is so small that I can live with people having either view, or describing it in either way, or indeed simply describing the relationship in terms of proportionality.

    I don't know? Is anyone arguing that it is something different?
     
  2. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    I can't speak for others but as for me? Exactly as Laplace sees it.

    Chris
     
  3. BobK

    BobK

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    This is exactly my point. Proportional is key word. The current in an inductor or a capacitor is not proportional to the voltage.

    Here is the equation that fully describes the relationship of the voltage to the current in an indcutor:

    E = L dI/dt

    The voltage is proportional not to the current, but the first derivative of the current wrt time. And the constant of proporionality is L, the inductance.

    Here is the equation for a capacitor:

    dE/dt = 1/C I

    In this case the current is proportinal to the first derivative of the voltage wrt time. And the constant of propotionality is 1/C.

    These two equations describe the exact behavior of the ideal components, just as Ohm's law describes the exact behavior of the resistor.

    The impedance analog of Ohm's law, does not describe the exact relationship of the voltage and current in a capacitor or inductor, it describes the behavior of the RMS average of those quantities over a cycle. It it does this ONLY if the voltage (current) is a sine wave of a single frequency.

    Now let's look at the diode. Guess what, it has an equation also. It is called the Shockley equation:

    I = Is (e^(Vd/nVT) - 1)

    In this case, the current (I) and voltage (Vd) are NOT PROPORTIONAL, they are related by an exponetial.

    Bob
     
  4. BobK

    BobK

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    CDRIVE,

    I have not enaged in any personal attack, or at least none was intended. I have attacked only your statement that "all components obey Ohm's law." I did say that the statement is ridiculous, which might be a bit overstated, but please understand this is light of the fact that pretty much everyone on this board, including myself, has made a ridiculous statement or two in the sense I am using it. I have great respect for your knowledge and you have been very helpful to many people on this board. I just thought your statement was dead wrong, and did not want to let it go by, lest some less experienced person would think that every component obeys the law:

    E = IR where R is a constant.

    Which is the accepted meaning of Ohm's law.


    Bob
     
  5. Merlin3189

    Merlin3189

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    Well at least some good has come from this silly squabble!

    What do you think Mr Ohm would think of this?

    Did he claim his "law" applied to ALL components (most of which had not even been imagined at that time)?
    Did he claim that any relationship that looks like V=IR - like v=iZ, f=ma, s=ut, F=-kx, T=PV, etc. - was HIS law? (Especially since he said x = a/(b + l') rather than V=IR, as Steve pointed out.)

    Perhaps he just thought that it was interesting and useful to know that, when many materials are made into long, thin, straight pieces (like wire) and kept at a constant temperature, then the current (measured by his galvanometer deflection) was, as far as he could tell, proportional to the electric field (deduced from the temperature of his thermocouple) and inversely proportional to the length of the wire (plus a constant representing the rest of his circuit.)
    Being a mathematician he may have thought that this suggested a useful way of modelling the behaviour of many conductors (or resistors.) Since metals generally behave much as he described, it is, so we do.
    And when we wind them into coils, we model them as an Ohmic resistance in series with an ideal inductance, which is a useful approximation to how they behave. It seems a little odd to me, that anyone would describe this as an Ohmic resistance obeying Ohm's Law in series with an ideal inductance obeying Ohm's Law. And if we have to take interwinding capacitance into account, it seems some people would have us model it with an ideal Ohmic resistance, an ideal inductance and an ideal capacitance, ALL obeying Ohm's law (or should that be, Ohm's Laws?)

    If this insanity drives us to despair and we throw our choke out of the window, presumably they now add the property that it is also an ideal particle, which we model with Newton's version of Ohm's law!
    Or would I be so frustrated that I threw it at relativistic speed? No matter, no doubt Ohm's law would still serve!


    Oh, and completely BTW, I just noticed this new post,

    <Quote>Hay guys i am new to this forum ... my question is: is i would like to have an inferred/rgb camera that feeds into a chipset this chipset has to run a program like face recognition. The chip then displays the results on a screen as fast as possible effectively the feed has to have no lag whatsoever. The chipset preferable needs to have wifi capabilities</Quote>

    Meanwhile, the super members discuss such complex subjects as Ohm's Law and the behaviour of simple ideal components!
     
    Last edited: Sep 23, 2012
  6. BobK

    BobK

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    Yes, and if that is what everyone meant by Ohm's law, you would be correct. Of course the simplicity of this equation is hiding the fact that it indeed a differential equation, not a simple proportionality of I to V.

    Bob
     
  7. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Hi Merlin. You've certainly injected some welcomed levity into this thread. This quote certainly accomplishes that! ;)

    Chris
     
  8. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Perhaps, when Georg Ohm documented his findings that was the accepted definition. Today however, if you search the interned and text books for Ohms Law you will more than likely return an Ohms Law circle, which is greatly expanded over V = I R.

    If this is not considered Ohms Law, then the industry should have given it an appropriate name.

    Chris
     

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  9. Laplace

    Laplace

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    It is the miracle of modern mathematical transformation that will take something complex and make it obey the simplicity of Ohm's Law. E/I=R E/I=Z
     
  10. BobK

    BobK

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    Yep, you just take the Laplace transform of it and it gives you a differential equation that, when solved gives you a complex exponential. This is certainly just as simple as

    E = I R

    Bob
     
  11. Laplace

    Laplace

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    If you were to direct your studies to advanced calculus so as to better understand the Laplace Transform, and to advanced circuit analysis so as to better understand operations in the complex frequency domain, then you would know why circuit analysis is never done in the manner stated above. I have never seen a differential equation since back when we were forced to learn circuit analysis using them, presumably to better appreciate why they are not a useful method for doing circuit analysis.

    Simplicity has value. The Transform method takes the complex and makes it simple. Real circuit analysis doesn't require anything more than elementary algebra, a table of inverse transforms, and some facility with partial fraction expansion.

    E/I=Z
     
  12. davelectronic

    davelectronic

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    My maths is not excellent, and although a lot of this on paper goes over my head, ive followed this thread in the background very interesting debate, i would like to play it out on paper, but fear my maths is not up to it :( apart from the small squabble very interesting for a modest hobbyist. :)
     
  13. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Yes it has but in the end it will probably never be unanimously resolved. It will probably fall into the category of "We agree to disagree.". :)

    Chris
     
  14. BobK

    BobK

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    I agree that different people have different ideas about what Ohm's law is, and what components it applies to.

    Bob
     
  15. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I agree that the use of fancy math can reduce the complexity of certain things so that they can be calculated using simple proportionality.

    I am agnostic on whether you should/must call that Ohm's Law, but I think you can.
     
  16. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    In that case please see below for a name regarding the subject.

    Chris


    ?
     
  17. JMW

    JMW

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    A VIP member with over 1000 posts and can't apply Ohms law to an inductor? Oh boy, we're in a whole heap o' trouble. Bet we had some good answers there.
     
  18. BobK

    BobK

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    If you can show me how Ohm's Law:

    E = I R, where R is a constant, does not give the relationship of the voltage across and inductor and the current through it. Period. If you can show me otherwise, please do. Laplace demonstrated that a whole family of equations involving mathematical transformations will allow you to write an equation that looks like Ohm's law for an inductor, which has different meaning for different inputs, and actaully hides a differential equation. But I, for one, does not think that is the same thing as saying Ohms law applies to an inductor.

    Ohms law states that the voltage and current are proportional. They are not in an inductor, capacitor, transistor, diode, neon light, triac, diac SCR. So if you are agreeing with the statement that I challenged, "All components obey Ohm's law", you have a lot of work to do.

    Bob
     
  19. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    There were 56 posts prior to yours. Are you saying you didn't read them? If so, that would be a pity. Though I strongly believe that Reactive Ohms Law does apply I wouldn't trivialize any of the arguments to the contrary thus far. To paraphrase davelectronic, ..."Dispite its rather bumpy start, this has been a very interesting thread".

    Chris
     
  20. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Someone remind me about this thread when I next tell people they need to understand Ohms Law and that it's pretty trivial :D
     
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