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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 about as definitive a statement for Ohm's Law that has been
    presented so far, and if i could i would quote it about 20 more times.
    Thanks for the post eng, i wish you were here for the start of this
    debate.

    Key points:
    1. V=RI is *not* a statement of Ohm's Law.
    2. Obeys Ohm's Law only if R is constant.

    To me, this is self evident because we would not have any rule about
    anything if in fact it did not do anything. Also, we can not take the
    stand that "any R in real life is not constant" because that is not how
    we develop theoretical laws and formulas...if we did we wouldnt have
    any laws because nothing behaves like its law says it does except
    within some bounds of some kind. Theory comes before practicality.
    We *apply* the theory to the practical case and accept some small
    inaccuracies sometimes even though we dont explicitly define them.
    If we did try to get perfect answers every time we would have to
    reject all calculations done for anything we know of in real life.

    A duck is a duck is a duck is a duck, but that's for more casual
    conversation, not for a technically oriented debate where the
    species comes into question. I had many types of fish in my life
    and i learned the species of all of them. When i am talking to the
    neighbor about fish it might be ok to just call them 'fish', but when
    talking to experts in the field it makes the conversation more intelligent
    to include the species.

    So part of the background of how we talk about things in general is
    who we are talking with, and here, in this thread, we are talking with
    experts and we want expert opinion, not just casual conversation.
    This is partly so we can convey the best possible information to the
    newcomers in the field.

    Sceadwian, i sympathize with you because, of all reasons, i once took
    your view about this too. I have since learned the better view after
    taking a wider perspective on what any rule is in general and doing
    tons of lab work.


    PS
    The little cartoon was quite funny :) A nice addition if you ask me.
     
    Last edited: May 8, 2009
  2. Sceadwian

    Sceadwian Banned

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    3. R is under real world conditions never constant.
    We have officially entered the black holes horizon. Reality ceased to exist... well, does it ever?
    Please, scream on the way down we want to see what laws of physics we can break as we go!

    Georg Ohm will be giving cookies and suckers out at the end as he deems fit.




    If a duck is a duck is a duck is a duck is a more casual conversation, please again describe to me the exact point on the slope of a conductors VI curve that a device becomes or ceases to become ohmic. Because again not one single material in the known universe satisfies the requirements for Ohm's law if the slope is 1.
     
    Last edited: May 8, 2009
  3. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Just a little curious here...did you even read my previous post at all?
     
  4. dave

    Dave New Member

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  5. Sceadwian

    Sceadwian Banned

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    So then what is the limit of ohms law stability as a function of the slope of the VI curve of the conductor? The law itself does not exist at ALL unless that boundary is described, so you can't prove the 'law' itself even exists if you can't tell me the exact deviation from a slope of 1 in the VR curve where it becomes non-ohmic. Otherwise EVERYTHING you have said from this point and back is open to interpretation.
     
    Last edited: May 8, 2009
  6. unclejed613

    unclejed613 Well-Known Member

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    Ohm was the person who first discovered and defined on paper this basic physical principal. the scientific community does not use the term "Law" loosely. if a principle is a very basic one they call it a law. if there are laws underlying the principle, it's termed a "principal" or a "theorem". when i was young, Bernoulli's principle was called Bernoulli's Law. since then there have been laws of fluid dynamics discovered that are more basic, and Bernoulli's principle is no longer a law of physics, but an interaction of more basic laws of physics (hence it's a "principle" not a "law").

    also, using simple math one can take Ohm's law and define it 3 ways E/I=R, I*R=E, or E/R=I. it's the same mathematical relationship. the only thing that changes is the quantity you are solving for. to say that it's not Ohm's law because i'm not solving for R, is like saying that if i calculate the mass of matter turned into energy in a nuclear reaction using E/c²=m then i'm not using Einstein's equation, because it's not written in the classic form.
     
  7. Sceadwian

    Sceadwian Banned

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    Thank you so much unclejed that was very succinct. Very much at the core of what I feel is the point. Better so far by a lot than I have said at least.
     
    Last edited: May 8, 2009
  8. unclejed613

    unclejed613 Well-Known Member

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    Ohm's LAW is a basic law of physics. any of the 3 quantities are solveable using the other two. that one or more of them is nonlinear is irrelevant. the relationship between the QUANTITIES exists whether a device is linear or nonlinear. that's what makes Ohm's law basic enough to be a Law. it defines the relationship between the quantities without regard to the properties of the materials in a device or circuit.
     
  9. Sceadwian

    Sceadwian Banned

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    But as far as Ohm's law's experiments went, it didn't prove those laws, it only SUGGESTED them.
     
  10. Tesla23

    Tesla23 Member

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    So if I modify my summary to be:
    The true facts are:
    1. in most homogeneous materials (i.e. no junctions), for all practical purposes V = IR where R is a function of temperature only
    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. Once you have determined R in this way, then V=IR and I=V/R hold as long as you don't change V or I.

    Would anyone disagree with this?
     
    Last edited: May 8, 2009
  11. unclejed613

    unclejed613 Well-Known Member

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    as we used to say in high school "close enough for rock n roll".....
     
  12. Sceadwian

    Sceadwian Banned

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    Tesla 1. is again wrong. There are no materials that are perfectly linear in relation to Ohm's law.

    2 works for me.
     
  13. Tesla23

    Tesla23 Member

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    Sceadwian, 1 doesn't say 'perfectly linear' it says 'for all practical purposes'. I refer you to the reference below and the quote included. In particular the phrase There are very few (for example, some biological) materials to which the law is not usefully applied. Isn't this similar to 'most materials' and 'for all practical purposes'? Do you have examples in your work where deviations from V=IR are significant?


    "Georg Simon Ohm and Ohm's Law" by Gupta, Madhu Sudan
    IEEE Transactions on Education, vol. 23, issue 3, pp. 156-162

    from which I quote:

    VI. OHM'S LAW IN RETROSPECT
    The term "law" has been used in the sciences for a variety of
    types of results: from exact laws (like Coulomb's inverse
    square law in electrostatics) that are at present considered
    fundamental laws of nature, to approximate empirical relationships
    (like Boyle's law for gases) that apply under idealized
    conditions or over a limited range of parameter values. Ohm's
    law is also an empirical relationship, but it is applicable in a
    remarkably wide range of situations. If the qualifier "under
    isothermal condition" is added to the law, it is experimentally
    verifiable for metals from pA/cm2 to gA/cm2 [23], although
    some conflicting evidence is also available [24]. It is also possible
    to generalize Ohm's law to include the effect of temperature
    rise caused by current flow; the resulting current-voltage
    relationship then depends on the postulated mechanisms for
    heat loss and is nonlinear [251. There are very few (for example,
    some biological) materials to which the law is not usefully
    applied.
     
    Last edited: May 8, 2009
  14. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    Tesla, thanks for the quote.

    Hi,


    Sorry, that is not the same argument, but perhaps a good secondary 'law' to compare with what
    we are talking about.

    E=mc^2, m=E/c^2, c^2=E/m,

    are all usable whereas for a diode:

    R=E/I

    is the ONLY form that works, not the other two. Something that obeys Ohm's Law
    can use all three equations.


    You just disagreed with many universities and even disagreed with Prof. Saslow who indirectly added
    to the proof that for Ohm's Law 'R' has to be constant.

    You two guys still should tell us what your definition of 'ohmic resistance' is, and what other types
    of resistance there are! Please? Pretty please?? :)


    Ok sure...
    If a resistor quacks like a diode it is no longer ohmic.

    Ohmic is a relative term, relative to the significance of the circuit, but we never claim that
    a diode is ohmic because it deviates wayyyy too far from the resistor.

    BTW, a slope of 1 indicates the normalized slope. That is not the only slope that works unless you
    first normalize the VI curve. In other words, every resistors VI curve normalizes to a slope of 1.

    I am still waiting for YOUR definition of what an 'ohmic resistance' is and what it is not.
     
  15. suhasm

    suhasm New Member

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    I can find the resistance of a ohmic resistor when you give me V and I.
    I can do the same for a diode also.

    I can find the drop across a resistor if you give me I and R.
    I can do the same for a diode also.

    I can find the current in a resistor if you give me V and R.
    I can do the same for a diode also.

    If the equation R=V/I works for a diode , then naturally all the other two derivations must work too.
     
  16. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi suhasm,


    The problem is, i wont always be there to 'give' you the time of day,
    much less what the resistance or any other quantity is...and that
    is what Ohm's Law is for. Problem is, it doesnt work for diodes.
    If you dont believe that then you should read back some posts to
    the experiment i gave.

    The experiment is where we have a voltmeter and ammeter and
    we measure R of a resisitor and R of a diode at the same current.

    The problem starts when we drop the voltmeter and it breaks
    and we are no longer able to measure voltage, just current,
    and we want data points for other currents for both devices.
    The funny thing is, we can STILL calculate the voltage for
    the resistor, using guess what, Ohm's Law, but we are then
    unable to calculate the diodes voltage because we can no longer
    read voltage. Thus, Ohm's Law helps us with the resistor but
    it does not help us with the diode. This illustrates the 'ohmic'
    vs the 'non ohmic' device or material.


    Interesting screen name BTW :)
     
    Last edited: May 9, 2009
  17. Mikebits

    Mikebits Well-Known Member

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    The first time I hear a diode quack, I promise to go back to church. Or maybe Betty Fords clinic. :)
     
  18. suhasm

    suhasm New Member

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    interesting 'screen' name ?:D
    Thats my real name by the way :D - Suhas Mahesh

    I'm from India.
     
  19. Mikebits

    Mikebits Well-Known Member

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    Who's screen name are you referring to? By the way, nice blog site you have going.
     
  20. suhasm

    suhasm New Member

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    I'm referring to my own screen name...It was intended to be a reply to post #175 by Mr.AI....

    Thanks about the blog :)
     
  21. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi again,

    Oh ok, good. Nice site there too.

    Back on subject, i found another indisputable 'experiment' that illustrates
    Ohm's Law and what is not Ohm's Law with respect to diodes and resistors.

    Again we start with a resistor and diode, where the resistor is in series
    with the diode and the cathode of the diode is grounded and we have
    a current generator connected to the top of the resistor. Thus, we have a
    resistor biasing a diode into forward conduction.
    The resistor is marked R1 and the diode is marked D1 (unspecified diode
    but we know it is silicon) . We also have a current meter in series with
    the resistor (which also puts it in series with the diode).
    Now this time we want to find the voltage drop across the resistor and
    the voltage drop across the diode and come up with a formula for each
    that always works. Simple right?

    Now since we know the current through the resistor we can calculate the
    voltage across that device (VR) using Ohm's Law:
    VR=I*R1
    no problem, but we would also like to calculate the voltage across the diode
    (VD).
    Question is, if diodes obey Ohm's Law then how do we use Ohm's Law to solve
    for the voltage across the diode???
    An attempt like this:
    VD=I*RD
    fails because we dont know what RD is.

    Now one could say that although we dont know what RD is we also dont know
    what R1 is, so nothing works for either. The interesting thing however is that
    VR=I*R
    is a complete fomula, while
    VD=I*RD
    is not because we never know what RD is unless we already measured VD
    and that would defeat the whole purpose of having a formula to compute
    VD.
     
    Last edited: May 9, 2009

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