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I'm confused about ohms law

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

  1. Mikebits

    Mikebits Well-Known Member

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    Here is another thought, if we take the argument that semiconductors do not follow ohms law based on the fact that their resistance changes based on current and temp rise, then can we not apply this thinking to a piece of wire?

    As wire temp increases, wire R increases, as in a temp coefficient, yet we say the wire follows ohms law. Of course the coefficient of temp in the wire is much smaller and affects the linearity of the wire in a much lesser degree, but does the smaller coefficient qualify the wire to meet the ohms law? Is the degree of the coefficient the deciding pass/fail criteria for meeting ohms law?
     
    Last edited: May 4, 2009
  2. Sceadwian

    Sceadwian Banned

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    Tesla what I meant about the whole violating the conservation of energy thing (poor choice of words on my part) was that at any given point a device has a V I and R equivalent that will satisfy V=IR or one of it's derivatives, it has to or energy is otherwise being fed into or drawn out of the circuit in question at some point.
     
  3. Sceadwian

    Sceadwian Banned

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    Mike, from that perspective there is no real world substance that actually obey's Ohm's law, but again it's semantics. Ohm's law was derived from real word tests on conductors at fixed temperatures, he admitted that things were more complex than that but the general equations are so useful it can be gleefully overlooked.
     
  4. dave

    Dave New Member

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  5. MrAl

    MrAl Well-Known Member Most Helpful Member

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    First off, i should say that it is very nice to see so many people interested in this subject.
    It's not everybody who cares if we are using Ohm's Law or Piddlie's Rule for Rinoceros Tounges :)

    Secondly (and this is really second), i still am waiting for one reference that supports the
    idea that Ohm's Rule includes diodes. If you cant find one, maybe you should ask yourself
    why.

    Anyway, i think i see part of why there is so much confusion here. The thing is, we call
    this thing we are talking about here "Ohm's Law", but really it is not a law at all. It should
    be called "Ohm's Rule", because laws are usually more applicable and rules can be more confined.
    Calling it a 'Law' leads people to believe that it is somehow applicable to everything in the
    universe, while really it is certainly not.

    For example, let me define a so called 'law' right now:

    Tree Law:
    A tree is something that has a main trunk, at least three branches, and leaves.

    Now you will find yourself asking yourself, "Is this really a Law?"

    We know that in the fall there are many trees that dont have any leaves at all, yet we would
    be hard pressed to NOT call them trees. This cant be a law then. But what if we call it a
    'rule' instead?

    Tree Rule:
    A tree is something that has a main trunk, at least three branches, and leaves.

    Anything wrong now? Well, we havent given a context for this rule yet so there could be
    arguments, but this so called 'rule' is perfectly acceptable. But why on earth would we
    present such a silly sounding rule like that, which is clearly incorrect?

    This is where the context comes into play. Now i will state the context...
    We are animal conservationists working in a remote location in Africa and we need to
    define what trees can support some of the animals that roam in the wilderness, so we
    define a rule that allows us to more quickly count up the trees that will provide at
    least some nurishment to them at any given time. We can then quickly distinguish
    between good trees and not so good trees simply by counting the number of branches and
    seeing if they have leaves.


    Having said that, let me now define Ohm's Rule:
    Any element that has constant R for any v or i follows Ohm's Rule.
    The context here is that v and i are allowed to vary, but R is not.

    Now, there is no getting away from this fact, that R must be constant, because i had
    just declared that if you want it to follow Ohm's Rule then you must insist that
    R be constant.
    Note that now if you want to change R by saying that the temperature changes, then
    that's entirely up to you, but it will no longer be following Ohm's Rule, just like
    if you want to follow the Tree Rule above then you can not consider a growth with
    only two limbs a real tree.

    Note also that now if you do anything to change R, you have violated Ohm's Rule,
    so that device does not follow that rule at least not exactly.
    This also means that if something changes resistance even the smallest amount,
    it does not follow the rule. The criterion is zero change in resistance.
    Yes, this means that nothing that we know of follows Ohm's Rule, but certain
    things are said to follow the rule if they follow it closely without being perfect.
    Other things are said to not follow the rule because they vary way too much.
    This is much like the way we define a square: a figure with four sides that are
    EXACTLY the same, and the criterion here is also zero variation in any of the
    four sides (from being equal that is), although we accept some figures that are
    draw slightly imperfect to be squares when really they are not exactly.
    Note also that if the figure of the square is too much different than the
    ideal we classify that as something else altogether.

    Still havent heard from anyone about who first discovered Ohm's "Law" either.
     
  6. wschroeder

    wschroeder New Member

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    Really now, What does it have to do with the price of eggs in China.
    Wasn't OHM's RULE about the same time as Queen Victoria's RULE? Maybe it was around the RULE of THUMB.

    In my world Ohms Law is idispensible for calculating bias resistors for transistors in my apps. When R is unknown what explains its determination?
     
  7. Sceadwian

    Sceadwian Banned

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    I hate to back track but I'm getting tired of this thread. This entire thread is being misconstrued as a technical discussion when it is in fact linguistic in nature. So I downloaded a translated copy of "The Galvanic Circuit Investigated" Which is the article where Ohm stated all his observations that in fact became Ohm's law. It's a rather dry long winded read but there was an excellent quote in it that unfortunately if you want to be meticulously attentive to language makes MrAl correct.

    This is a quote from Maxwell listed in the book not one from Ohm directly however I think I'll leave it in Maxwell's hands.


    Forgive typos the PDF file downloaded from Google (I love google) is graphical in nature so I had to transpose it real fast.

    This quote was based on the fact that the ONLY thing Ohm tested was conductors at a fixed temperature. (I think he did some tests on solution as well but I didn't read all the details on that) Which would make Ohms law probably one of the most seriously mis-quoted scientific findings I have EVER read about =\ It does not however change the fact that the meaning of things change over time and that the general consensus I have come across in almost every website and person I have ever talked to about electronics is that "Ohms Law" Is the ratio. If you want to get technical it's not, however it's a moot point you're fighting over words which mean nothing because it's "Ohm's law" has been quoted so many times in refrence to the ratio it's now better known as that ratio than the study of conductors it was based on. Being that language and meaning are dynamic and not fixed entities enough people use the term Ohm's Law to refer to the general ratio rather than his actual experiments that any perceived previous preconception of it as a 'law' is irrelevent.

    Just my two cents though.

     
  8. wschroeder

    wschroeder New Member

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    OHMS LAW is not a "unified field theory" of electricity. All electrical circuits have characteristics which are not wholly resistive (should I say ohmic for the sake of this discussion), all also have capacitance and inductance.

    To demonstrate to my class these effects I would get a 500' reel of #12AWG electrical wire. In front of the class I would poke both ends of the wire on the reel directly into a 120V receptacle in the room. To their surprise nothing blew up. If I had stretched that same wire out singly and ran it all around the room and performed the same stunt the power panel circuit breaker would surely trip after a very nasty spark.

    The characteristics of the same insulated copper wire changed dramatically depending on it's physical shape. OHMS LAW explains the relationship of E I and R, but not all the nuances. But we have the tools to explain those nuances. And guess what, OHMS LAW still works great to explain the circuit.
     
  9. Sceadwian

    Sceadwian Banned

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    Again Schroeder, it's all semantics at this point. Technically MrAL is right, however based on the number of users (myself included) who have a more flexible definition of Ohms law which we consider to be the ratio where resistance isn't fixed. Ohm's law in fact all of Ohm's experiments used a fixed R value at a stable temperature.

    The fact that Ohm's law has become the ratio we use to describe all this stuff is what I'm commenting on. As MrAl said, the ratio is not Ohm's law. It's just what was derived from his experiments. His experiments in fact were all based on a fixed resistance at a known temperature and the voltage and currents associated with it. The fact that this flexibility is coming into common terminology and being miss-quoted so much I find rather funny, as we're effectively arguing about the usage of a term that is being used out of context.

    I will keep this in mind when describing "Ohm's law" in the future but V=IR and it's derivatives is not in it's essence "Ohm's law" even though they were inferred from it. Mind you Ohm was not a mathematician, his results were from meticulous experimentation of fixed conductors at fixed temperatures in the real world.

    If you still doubt after this point no further gain can be had from this thread until you read more about it.
    The galvanic circuit investigated ... - Google Book Search

    I hope MrAl will forgive me for yelling at him, and I hope people will read the provided link and I hope also that MrAL takes something away from this in the flexibility of the definition of a term or phrase in language as they are NOT static, and due to the number of people that think Ohm's Law is in fact not ohms law the masses win, always have always will.
     
    Last edited: May 4, 2009
  10. wschroeder

    wschroeder New Member

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    This is why drugs should be legalized.... :p At some point engineers have to design and build something, don't they?

    Maybe it's better that students pay $900/credit to NYU to spend time debating the truth about OHM, and not in the workplace. :rolleyes:
     
  11. Sceadwian

    Sceadwian Banned

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    You have to respect the essence of it though Schroeder. I'm all about the practice of things, I'm also about learning theory though, they do not exist outside of each other and everyone has to learn to get along. The name calling and finger point gets old. I just try to look from the other persons point of view, in this case I learned something. Albiet nothing practically usable, aside from perhaps not getting called an ass by saying dynamic loads are Ohmic in front of a physics professor.

    I was fine before, I'm fine after, just gotta learn to watch the tongue =)
     
    Last edited: May 4, 2009
  12. wschroeder

    wschroeder New Member

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    yes, the discussion of what OHM didn't say is a far cry different than what OHM did say. Are they adding Al's questions to the TRIVIAL PURSUIT game?

    That philosophical treatise of his about TREES still has me wondering if I suffered a massive brain fart.

    Thanks for the Maxwell quotes.... worked for me.
     
  13. Sceadwian

    Sceadwian Banned

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    I think my real argument is that Ohm actually knew just how chaotic and deterministic the real world was and so limited his experiments to something he knew at the time he could control and demonstrate reliably. The actual essence of his theory extends FAR outside the realm of what at the time was acceptable knowledge. He KNEW ionic flow in fluids (basically anything non metal) was very difficult to describe. We're talking about determining the actual intentions and thoughts of a person from over 150 years ago though. And in a language other than English no less.
     
    Last edited: May 4, 2009
  14. suhasm

    suhasm New Member

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    Here are my 2 cents on this topic:

    I agree with Sceadwian's concept of effective resistance. It makes complete sense. wschroeder said that he demonstrated the concept of inductive reactance by plugging in a roll of copper wire into the mains and having nothing blow up. I'm sure that in 1700s , this behavior , if it had been noticed , would have been branded as a case of disobeying of ohms law. However , now that we know about inductive reactance , we can also include the effects of inductance by adding another (frequency dependent)resistance in our mathematical models.

    My point is that , in a semiconductor junction , there are effects of electric fields which may impede/promote current flow. Just like , the definition of resistance was broadened to impedance when inductive reactance was discovered , why dont we change the definition again to include interactions of electric fields on the charge carriers at the microscopic level?
    Surely wont that make things easier to analyse?

    I am not debating on the 'ohm's law' here. I am only referring to V=IR.
    Suppose we somehow include the 'resistance' offered by the electric fields inside the diode + all the other resistances + parasitic inductances,capacitances , then wont the diode still follow V=IR at every instantaneous point of time?
     
    Last edited: May 4, 2009
  15. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    Yes, at the instant in time the measurement was taken.:)

    It should be remembered that when using a meter to measure the resistance of a material, that really we are measuring the current thru the material caused by the meters internal battery [voltage] and that the 'resistance' is V/I.

    The resistance of a piece of 'p' OR 'n' semiconductor material can be measured by using a standard meter.

    If the test meter voltage causes the material to become 'active' [ a semiconductor 'p:n' JUNCTION say] then thats what the 'measured resistance' is at that voltage/current at that instant.

    Suggesting that changing Ohms Law to Ohms Rule is just fudging the explanation.
     
  16. Tesla23

    Tesla23 Member

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    In the interest of posting references to this thread for posterity:

    Another very useful paper (in English) is

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

    unfortunately it is not freely available on the web, you need access to IEEE Explore.

    It makes you realise how different things were then. Ohm had to make his own ammeter (a magnetized needle suspended near the conductor and he measured the torsion on it) and his own wet cells. He didn't have a voltmeter and had to remove the effect of the internal resistance of the battery (without the benefit of ohms law).

    Also the following paragraph is interesting:

    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 low is not usefully
    applied.
    Another characteristic of Ohm's law, uncommon among experimental
    laws, is the fact that under most circumstances, it
    has not required any refmements or correction factors for 150
    years.


    One of the reasons I dug up that paper is I went looking for some data on the range of validity of ohms law. I have used current sense resistors for ever, but never thought to ask how accurately ohm's law might hold (I knew there was no practical deviation, but what was THE limit?). I went to the NIST site looking for conductivity data, I found some for metals to 5 or 6 significant figures with no reference to the current density (that I could see). The Gupta paper suggests that it may be valid in metals over 21 orders of magnitude - pretty impressive.
     
  17. Tesla23

    Tesla23 Member

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    Hi suhasm, this is my last comment on the semantic side of this debate. It is good (great actually) that you are thinking about analyzing circuits that are non-resistive, and by all means develop your own techniques (you will learn lots), but then go and read how others have done it and learn what they call things. Believe me there are established techniques for analyzing everything mentioned on this thread to date.
     
  18. suhasm

    suhasm New Member

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    @ericgibbs
    If V=IR hold goods for every instantaneous point of time , then whats the problem? Doesnt that imply that semiconductor junctions also obey v=IR?
    V=IR no where mentions anything about time at all.

    And why does ohms law assume that the only type of resistance is the 'ohmic resistance'? Isnt resistance just anything that impedes current ? So logically , all components that impede current (L.reactance , C.reactance , drift velocity saturation (?) , electric field influence etc..) must be
     
  19. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    IF you read my post carefully, you will see I am agreeing with you.!:)

    The TIME I am referring too, is the real time the measurement was taken.

    For example:
    I pick up a diode for testing , measure the fwd and reverse resistance values, then come back 1 hour later, the resistance values could be different if the ambient conditions have changed, ie: temperature.

    That wouldnt worry me one bit, providing the values were within a 'range' of what I would be expecting.
    The actual resistive value would be of no value to me.
    BUT.
    If I was using the diode as a temperature sensing element in a circuit then the resistive values
    at different temperatures would be important to me.

    However, carrying out the 1 hour test when measuring a precision resistor, the 'range' of values
    I would be expecting would be much tighter.

    As always the accuracy/resolution of a measurement required depends upon the application.
     
    Last edited: May 5, 2009
  20. suhasm

    suhasm New Member

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    @ericgibbs , does that imply that diodes indeed do obey ohms law?
     
  21. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    Yes,
    Resistance is a passive measurement when measured using a 'dc' current source.

    The current will be directly proportional to the applied voltage.

    Over a range of applied voltages, the proportionality may not be linear, but at at each finite point of the V versus I plot,
    Ohms Law must apply, otherwise you couldnt calculate the resistance.!

    So the resistance is:R = V/I at every instant of V/I.
     
    Last edited: May 5, 2009

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