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I'm confused about ohms law

Discussion in 'General Electronics Chat' started by suhasm, Apr 27, 2009.

  1. MrAl

    MrAl Well-Known Member Most Helpful Member

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    That's what i have been saying all along, and you have been disagreeing.
    And if you dont like the phrase "ohmic resistance", then you should
    write to the publishers or writers of the "Standard Handbook for
    Electrical Engineers" and talk to them, NOT me. You seem to want to
    make up ALL your own definitions, so go ahead, but dont talk to me
    about it, talk to them, and too all the universities you dont seem to
    agree with either.

    Just to recap:
    A resistor obeys Ohm's Law, an si diode does NOT. If you dont like it,
    write to the universities and argue with THEM from now on.


    You have to realize that Websters is not an authority on electicity and can therefore only offer the simplest
    of all possible definitions while words often have more than one definition. You also have to realize that
    if you disagree, you are disagreeing not only with me, but with several universities especially the one
    that that was quoted from.
     
    Last edited: May 8, 2009
  2. Mikebits

    Mikebits Well-Known Member

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    I said I was done but...
    Do you really think they just pull this stuff out of their behind? Surely the definitions are researched at length, and experts are consulted. What use would a dictionary be if it was just sorta kinda but not really, or maybe?

    And Websters offers all possible definitions. Oh, look up ohmic at Oxford.
     
    Last edited: May 8, 2009
  3. Tesla23

    Tesla23 Member

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    Oh Sceadwian, the irony is quite exquisite! In a thread all about semantics you contact the professor for the definitive answer, but unknown to you (I suspect) you ask the wrong question!

    You asked:
    No-one in this thread has been suggesting that semiconductor material is significantly non-ohmic, I said as much a few posts back. So of course when you ask the professor whether a semiconductor material is non-ohmic he tells you that you are worrying about a trifling. Why don't you ask him whether a diode is ohmic?

    So until the good professor agrees that a diode is ohmic, or that it is useful to apply ohms law to a diode, the cheer squad had better quietly munch their coleslaw and that black king should be moved a few squares away.
     
  4. dave

    Dave New Member

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  5. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi Mike,

    Hey you must be online now too :)

    Yes, but that is a very simple definition and the universities offer better
    definitions, unless you just think that the universities just pull stuff out
    of their bottom? So who is the real bottom dweller there? :)

    Also, why would they call it "ohmic resistance" if there was not any other kind?

    Oh i see what the problem is:
    When they say 'ohmic' they say that it is an adjective pertaining to an ohm, but
    that part of the definition is not disputed...in R=V/I we can call R in units of ohms
    but that's not the end of the story. What we are talking about here is a different
    kind of 'ohmic'. The kind we are talking about is the kind that characterizes a
    whole device as having a certain property. In other words, we are talking about
    and "ohmic device" or "ohmic element", and the difference between these type of
    devices and other types of devices (non ohmic devices).
    In other words, an ohmic device behaves like a resistor.
    You know what else is interesting, when some universities teach about light bulbs
    in circuits they make the effort to ensure that they say that the light bulb is
    a theoretical light bulb that has a linear characteristic, rather than let the reader
    think that the light bulb is a real life light bulb where the resistance varies
    with voltage.

    I almost forgot to mention that i got a kick out of some of the jokes stuck in this
    thread too, and it's nice to see a little levity along with some serious technical
    talk.
     
    Last edited: May 8, 2009
  6. Mikebits

    Mikebits Well-Known Member

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    Who ever said Universities were never wrong? :)
     
  7. Mikebits

    Mikebits Well-Known Member

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    We should just chill :) I like an argument just like my ex wife does:) but we should always keep it on friendly terms. Can I make a quote?

    "Don't take it as personal"

    I generally like all you folks, and I like a good debate, but I can also see when the heat gets raised too high on the burner. Too much heat denatures a topic into a pee pee contest.

    This debate has been good and I think I like both sides of the argument, but I think it is round 10 and we should shake and make like kissey face and call it even, as this discussion will never end.

    BTW, I got last word in... yaahh

    And! might I add I thought both sides of this argument have been brilliant.
     
    Last edited: May 8, 2009
  8. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Yes, you have a point there, but then you are taking the EXTREMIST
    view again just as Sceanwian does all the time. It's not just 1
    university, it appears to be ALL of them.
    You have to realize that taking the extremist view can ream all logic
    out of anything, and does not add to the order of the system (the
    system here being the law or rule we want to talk about).

    For example, there is such a thing as a straight line in geometry,
    but in real life there is no real straight line. Does this mean that i
    can never call anything on earth straight? I dont think so! :)
    If i take the extremist view, i would say that there is nothing on
    the earth that is straight so the concept of 'straight line' is flawed.
    Thus, i would not be able to advance to lines that intersect and
    whatever because i dont believe in a straight line.

    It's when we can find a relationship within bounds that we can truely
    say something interesting about nature, but without those bounds
    there is no reason in the whole of the universe.
     
    Last edited: May 8, 2009
  9. suhasm

    suhasm New Member

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    Now that the titans have got their semantics all sorted , is this the final verdict?:

    1) Everything obeys Voltage = Current * Impedence at every instantaneous point on its operating curve.

    2)If the VI curve of any substance is somewhat linear in its practical operating region , then it is said to follow ohms law which is V=IR where R is constant.

    3)If the VI curve is not linear , then it is said to be non-ohmic and it does not follow ohms law. But it does still follows E=IZ where Z is not constant.
     
  10. Mikebits

    Mikebits Well-Known Member

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    Hmmm, good point, but we are not talking in such simplistic terms as a straight line. A law fulfills the law if it meets the laws criteria. This is irregardless of it's function over time, rather the law implies time has stopped at the instant the law is tested. If at Time(X) the law is satisfied, then the law is obeyed and thus the law is not violated.

    This implies that a device at a given time, at a given environment meets the criterion for V to equal IR then it must meet the requirement for ohms law.
     
  11. Mikebits

    Mikebits Well-Known Member

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    Impedance is not the same thing as resistance.

    Better discuss these questions with your teacher. His way is the way that matters for the grade.
     
  12. suhasm

    suhasm New Member

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    I'm not thinking about the grade. All we have in our textbook is the definition about of ohms law and V=IR. I am asking coz i want to know what the true fact is.

    I know that impedance is not the same thing as resistance. But a circuit which has a reactive component along with the resistive component will not have a straight graph for VI (i think). Thats why i mentioned Z in the equation. It was to state that E=IZ would work even if the curve was non linear.
     
  13. eng1

    eng1 New Member

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    How is it stated exactly?

    A physics textbook by Halliday,Resnick that is used in colleges and has been translated to different languages, clearly says that the relationship V=RI is not a statement of Ohm's law and a conductor obeys Ohm's law only if its I/V plot is linear or, in other words, if R is constant.

    The authors also say that the relationship R=V/I remains as the general definition of the resistance of any device. ohms = volts / amperes with no other implications; you can always measure a voltage and a current through a device, calculate their ratio and say that it is a resistance.
     
    Last edited: May 8, 2009
  14. Tesla23

    Tesla23 Member

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    I think we can agree on the facts if we don't use the name Ohm.

    The true facts are:
    1. in most homogeneous materials (i.e. no junctions), V = IR and R is constant (at constant temperature)
    2. For any two terminal device with a voltage V across it and a current I flowing through it, you can calculate a quantity R = V/I that has the dimensions of resistance.

    everything else is semantics. For optimum performance, you need to use the same semantics as your teacher for your exam.
     
  15. JimB

    JimB Super Moderator Most Helpful Member

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    For a circuit with only R, L and C, a graph of V and I will be linear, provided the frequenct is constant.

    JimB
     
  16. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi Al,

    I think this short smilie sums up this discussion, please excuse the levity.

    [​IMG]
     
  17. Sceadwian

    Sceadwian Banned

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    The average Voltage maybe. Like I said the real world is a whole lot more complicated, in real world materials any resistor above absolute zero has thermal noise associated with it. MrAl is being quiet unyielding in his interpretation of Ohm's law but fails to note if you are unyielding in your definition of Ohm's law then not one single material on earth can be proven to be ohmic if you look at it close enough. It's a GENERAL approximation not holy writ handed down from on high. On a quantum level like most macro laws it breaks down completely.

    I have been erratic in my statements, however that's because this is all semantic in the first place. The equations that were derived from Ohm's work outside of Ohm's Law, so for expediancy sake I refer to them general has Ohm's law even when not applied to fixed resistors, because say "The equations derived from Ohm's law" is a mouthful.

    I'm a pretty simple guy when you get right down to it. To me it looks like a duck, and quacks like a duck. I'll call it a duck =)
     
  18. dknguyen

    dknguyen Well-Known Member

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    Obviously you have never run into a robot duck before.
     
    Last edited: May 8, 2009
  19. Sceadwian

    Sceadwian Banned

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    I like Robot chickens way better DK =)

    Oh to get my feelings across better let me rephrase the duck comment.

    If it looks like a duck and quacks like a duck I'll call it a duck. Even if someone from a University tells me it's an Aix sponsa, Anas superciliosa, or even any of the other 20+ species of ducks.
     
  20. Tesla23

    Tesla23 Member

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    If you are a simple guy then why make simple things complicated? I write down what every engineer assumes and uses regularly:

    1. in most homogeneous materials (i.e. no junctions), V = IR and R is constant (at constant temperature)

    and you try to make things more complicated - "what about noise?" you say. This thread spurred me on to find out the range of validity of this statement (notice I am avoiding the use of the name we have been discussing) and I gave you a reference that it appears to be valid in conductors from pA/cm^2 to gA/cm^2. I think you can safely assume V=IR for most conductors you are working with.

    I'd really like to see your analysis of a circuit with a resistor in it, do you actually include all these 'real world effects' that you keep inflicting us with, or do you just write down V=IR?
     
  21. Sceadwian

    Sceadwian Banned

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    1. Is blatantly false. Current causes heating which is empirically unavoidable linked to the material itself. Simply put you can't just discount temperature like that it doesn't work. Ohm's results were on a VERY low scale relative to modern current and voltages, so the current heating may be over looked under very specific conditions. His results however weren't perfectly linear, as they were real world experiments. The equations were derived from the closest match to his results, and the exact definition of linearity for an ohmic material was never defined.
     

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