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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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    You talk and talk and talk, but i havent seen a *single* academic proof
    or a *single* reference to support your own 'theory', while i have offered
    several of each. You cant knock something you dont understand :)
    I never said V=R*I doesnt work out, just that it is not always Ohm's Law.
    Also, show me one way in which you *DO* use Ohm's Law, yourself.

    You should really find ONE reference if you want to be taken seriously
    on this matter.


    Electrician:
    Very good idea there. I would have said either a capacitor or an inductor.
     
    Last edited: May 7, 2009
  2. Sceadwian

    Sceadwian Banned

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    Then the onus is on you MrAl, shut the hell up or tell me what V=IR is if it isn't ohms law, because that's the 'law' that they described from his results, it does NOT NEED A CONSTANT R. EVERY SINGLE LAST CIRCUIT IN EXISTENCE WILL ALWAYS HAVE AN R equal to V\I. Ohm's own measurements confirm this, his R was however always static, this in no way shape or form states that R being dynamic doesn't meet the law! R is not a real number assigned to a substance, it is inferred ONLY from the results that are measured from real world materials, and not one single material on this planet meets Ohm's law requirements for linearity at every voltage and current state, period.

    Again let me state for the record, if Ohm's law is what you say it is. R=V/I for every value of V and I for any given material this will NEVER result in a static R. Again let me repeat that. NOT ONE SINGLE KNOWN PHYSICAL MATERIAL IS OHMIC BY YOUR DESCRIPTION. Take temperature out of the equation and you HAVE to limit I. I asked this of Wayne Saslow when I tried to ask him what the slope variation was allowed to be to declare a material ohmic. Never got an answer, because if the slope is required to be 1 it fails instantly for ANY material outside of Ohm's very narrow limits. Nothing in the V=IR equation or it's derivatives infer a limit of any kind for any value or describe linearity in any way. Which is smart not because he was trying to be sly about it, but because at the time he knew R related under those conditions, but he KNEW they didn't under some other conditions and he had no theory for those other conditions.

    Ohm's experiments ONLY showed that R was constant under the physical constraints HE placed on the systems he tested, NOT that they couldn't make sense in system where R wasn't constant. I don't have to prove anything the ratio that describes Ohm's law is used outside tuse of perfectly linear resistors in just about every field related to electrical discharge, and it's most of the time still called Ohm's law.

    I'll state this one more time. Just because Ohm's inferences were based on a static R does NOT IN ANY WAY SHAPE OR FORM mean that it can't apply to a non static R. Ohm was just a smart scientist, he only tested things he knew would give him good results. Not to say he was trying to be sly, he KNEW the results become skewed with temperature and with various other materials, he wasn't trying to deal with that because at the time nothing was known about the why. If he included those 'facts' in his results he'd be laughed out of academia, because he couldn't explain them.
     
    Last edited: May 7, 2009
  3. Tesla23

    Tesla23 Member

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    Now Scaedwian I think you have it wrong again. There has been ample material posted here (some by yourself) to support the assertion that Ohm's Law is a law of physics, not maths. But I'm not going to argue semantics, that's futile, if you want to say that 'it's always possible to divide V by I (presumably I non zero) and get a quantity called R' then that's fine. No-one will argue with you. I'd just like you to post some examples to show where this is useful, or indeed examples of anyone else apart from yourself doing this. Surely if we are going to give this result the exalted classification of a 'Law' it should be useful or others must know/use it?

    I know I am a relative newcomer and so am careful of forum etiquette, but I did see you criticise Colin for providing advice that didn't live up to your standards, so in the same spirit, if you don't know what you are talking about, don't give advice. Today's post was a fairly incoherent rant, for example:
    - saying that a device didn't need to have a constant R to satisfy Ohms Law, when you yourself posted material describing Ohms law as the discovery that R was essentially constant
    - saying that Ohm's Law only applies over very narrow limits, I assume you have a reference for this, I posted a link that suggested that in conductors it applies over 20 orders of magnitude (at constant temperature).
     
  4. dave

    Dave New Member

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  5. unclejed613

    unclejed613 Well-Known Member

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    give the man a prize.....
    i saw this in Radio Electronics or Popular Electronics back in 1970 or so. they had a column where they had puzzles like black box problems and the infinite grid of 1 ohm resistors. i always thought it was one of the most interesting black box puzzles i have ever seen. the actual answer they gave was a set of vibrator contacts (an electromechanical device used in automobile power supplies for tube type equipment, the predecessor of the switchmode power supply) and yes they had a 50% duty cycle and the instruments were standard average reading meters (the kind with needles), so the voltmeter read 5 volts and the ammeter read 5 A, and the wattmeter read 0, because when there was voltage, there was no current, and when there was current, there was no voltage.

    the whole point with a black box problem is to have sufficient data to deduce what's in the box. the vibrator contact problem has enough data to ascertain what is happening. for more information, you would connect a scope across the black box and you would find out what frequency the chopping is taking place at, and by observing the contact bounce, you would see that they are mechanical contacts and not a transistor. in MrAL's experiment you had a complete data point set of current and voltage at a single operating point, and incomplete data sets for other operating points. without a complete data set at other operating points, you can't make any clear assumptions about what's inside. what if the black box contained an NIC which read 1K at 1mA, but was 500 ohms at 2mA? without a voltage measurement at the 2mA operating point, you have no clue what the voltage is at the terminals, and therefore no clue what's inside the box. these principals used for solving black box problems are also used in SPICE for simulating devices, and also by engineers for designing circuits. the most common "black box" theorems for circuit design are Thevenin and Norton equivalents, where every component is treated as a black box (Thevenin equivalents use voltage sources, Norton equivalents use current sources) and it all hinges on Ohm's law applying to EVERYTHING, no matter how nonlinear (or backward) it's behavior.

    when Ohm published his paper, it wasn't well received, there wasn't much to apply it to at the time, so it got ignored by the scientific community. Ohm was primarily a mathematician, not a physicist. so when his paper was published, it was seen as a nice mathematical solution to a physical phenomenon, but not much more than that. yet that simple mathematical relationship is the basis of all modern technology. and as a mathematical relationship, it doesn't matter which of the 3 possible forms of the equation is written, it's the same equation, and in any of the 3 forms it is Ohm's law. we might not mention it much here but it does have applications outside of electronics. the same relationship exists in fluid dynamics.
     
    Last edited: May 7, 2009
  6. unclejed613

    unclejed613 Well-Known Member

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    when designing amplifiers, i regularly have to calculate the impedance of a constant current source. i also need to calculate the output impedance and the output resistance (two different things, as the output impedance is determined by open loop gain, feedback ratio and output resistance, and the output resistance is the actual physical output resistance which includes resistive AND active components (which are acting like resistors)). if i attempt to "violate" Ohm's law (or just ignore it)with active components, i end up with an expensive pile of carbon and silicon and lots of smoke... therefore, i need to measure voltage and current at many different operating points in the devices i'm using. that's what characteristic curves for transistors and other devices are for. want to know what the resistance of a C-E junction is at a certain operating point? you can look it up. actually it's a voltage vs current curve, but it's easy to figure out the resistance from there. so i use Ohm's law with highly nonlinear devices to find a stable operating point, and one with the least amount of nonlinearity in the portion of the curve i'm allowing the transistor to operate in. an amplifier is in it's simplest state a large voltage or current controlled voltage divider or current summer.
     
  7. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello again,


    Actually, introducing more elements other than resistor and diode at this
    point is going to just obscure what we have been talking about.

    The resistor follows Ohm's Law because for most of the normal operating
    region v=i*R is valid. For a diode, you can't calculate v=i*R because R
    isnt constant and so you never know what it is beforehand.

    Ohm's Law isnt for switching circuits either, which also require other math
    of which sometimes the law of averages applies, but that's not Ohm's Law.
    Ohm's Law doesnt apply to impedances other than purely resistive either.
    E=I*Z is called an "Ohm-like" relationship.

    You can map out a semiconductors VI curve and calculate resistance at any point.
    That's not Ohm's Law either. We already said that you can linearize a diodes
    curve at some point and approximate it's behavior over a short range...no big deal.

    Another point here about ohmic devices:
    You can take an ohmic device out of the circuit and replace it with another
    ohmic device with the same resistance and it will still work. Try replacing it
    with a diode and wow you get big problems unless you are lucky. That's because
    there are MAJOR differences between an ohmic device and a diode.
    Also, try replacing a diode with a resistor and see what happens. There is something
    VERY different about a diode and one way to describe this is to say that it is
    non ohmic.
    In other words, you can replace a device that obeys Ohm's Law with another device
    that obeys Ohm's Law, but you can not replace a device that obeys Ohm's Law with
    a device that does not obey Ohm's Law unless you get lucky...ie the VI operating point
    is very stable.
     
    Last edited: May 7, 2009
  8. Sceadwian

    Sceadwian Banned

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    MrAl... you can calculate R=V/I for a diode, it's real simple, you calculate the voltage drop across the diode, you measure the current going through it with the voltage and the current known you can calculate the resistance, just because it doesn't change linearly with current/voltage doesn't mean the math suddenly stops working. So if when applied in that example it's not Ohm's law then what do you call it cause the math works just fine when I do it, and it dissipates power.
     
    Last edited: May 7, 2009
  9. Sceadwian

    Sceadwian Banned

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    The technicality of Ohm's law requiring a fixed R is _ONLY_ because that's what Ohm actually tested, his theory isn't actually based on math it's based on real world physical measurements he did with conductors of a fixed size at a fixed temperature. To state that his 'law' only applies to fixed resistors is short sighted though, technically it's correct,

    Also, for the second bit EVERY single last known material on this planet will deviate from a purely linear resistance at different voltage and currents, the temperature will go up, at higher potentials materials start to change their basic structures, at higher currents weird magnetic effects can start to take place all sorts of things get in the way.
     
  10. Leftyretro

    Leftyretro New Member

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    Well my tunnel diode didn't win me any free drinks. :(

    How about some ohmic material which at some low temperature that becomes a super conductor. Does Ohm's law apply or not? Where's my drink ;)

    Lefty
     
  11. Sceadwian

    Sceadwian Banned

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    Here's a nice frost one Lefty =)
     
  12. Mikebits

    Mikebits Well-Known Member

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    Maybe we can call it Ohm Slaw :)

    [​IMG]
     
  13. Sceadwian

    Sceadwian Banned

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    Who's what?

    My original e-mail
    The followup
    I think we all need to take that last line very seriously at this point =) He repeated the intent twice, and everyone here seems to be going over the SAME points over and over again to no avail. I'll still use the term "Ohm's law" flexibly. I should hope MrAl will try to be more flexible with it's use and I hope anyone else wandering through this train wreck of a thread learned something, I did =)
     
  14. Mikebits

    Mikebits Well-Known Member

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    Very cool, and very good reference. He confirmed pretty much everything pro- Sceadwians law supporters have been saying... :)
     
  15. Mikebits

    Mikebits Well-Known Member

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    The neat part SC, is that he spent an hour to research before responding. How cool is that? Nice guy...

    Most professors will not reply to email, even when your enrolled in their class. He is one of the good ones.
     
    Last edited: May 8, 2009
  16. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello again,


    You say that Ohm's Law should be more flexible, even after the person you
    wrote to said the same thing we have been saying all along here that you have
    been disagreeing with. I really dont understand how you are comprehending this
    so diversely.
    Perhaps you should re-read his own words:

    -You can always write down I=V/R, but it's really only useful when R is pretty nearly independent of V.
    -Of course it has a real resistance, but R=V/I is only a definition. The fact that for many materials it is independent of V is what makes it a law; Ohm's Law.

    THE FACT THAT FOR MANY MATERIALS IT IS INDEPENDENT OF V IS WHAT MAKES IT A LAW,
    OHM'S LAW

    Read that about 100 times to yourself ok? Thanks :)
    And remember this: you asked a professor, and he told you too!

    Here's a quote from the University of Guelph, Department of Physics:
    2. Material that obeys Ohm's Law is called "ohmic" or "linear" because
    the potential difference across it varies linearly with the current.


    You are also saying that "No materials obey Ohm's Law if you want to take
    that (linear requirement) view", but that is a very sarcastic view and almost
    bordering on a disestablishmentarian view. You must realize that if there are
    no bounds for any experiment no component would follow ANY law of any kind
    and you would end up with the conclusion that, "Everything is nothing".

    If you still dont agree (which im sure you dont) then perhaps you would take
    the time to DEFINE what exactly the word 'ohmic' means in this sentence:

    21. Joule's Law
    When the conductor contains ohmic resistance only and no counter emfs we have
    e=r*i, so that the power is:
    P=i^2*R=e^2/R

    That is a quote from my "Standard Handbook for Electrical Engineers, 12th Edition"
    book in the chapter on "Electric and Magnetic Circuits".
    In that paragraph (21.) tell me what the word 'ohmic' means to you and what the
    phrase 'ohmic resistance' means to you.

    PS
    If you think the professor was agreeing with you then you better give me his email
    so i can talk to him directly. I doubt very much he would reply saying that there
    is no linearity requirement because every OTHER university exclaims it to be so.

    Before i forget, i must tell you that one definition of linearity does not require
    the line to pass through the point (0,0) although the more strict definition does.
    For example, when we talk about the diodes 'ohmic' region we talk about a line
    that does not go through (0,0).
     
    Last edited: May 8, 2009
  17. Mikebits

    Mikebits Well-Known Member

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    ohmic

    One entry found.

    Main Entry: ohm [​IMG] Pronunciation: \ˈōm\ Function: noun Etymology: Georg Simon Ohm Date: 1867 : the practical meter-kilogram-second unit of electric resistance equal to the resistance of a circuit in which a potential difference of one volt produces a current of one ampere
    — ohm·ic \ˈō-mik\ adjective

    From Websters. Where does this definition state linearity ? According to Websters, Ohmic is just a adjective of Ohm.


     
    Last edited: May 8, 2009
  18. Sceadwian

    Sceadwian Banned

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    The equations that were derived from ohm's results do NOT require linearity to function they work just fine in non-linear circuits the only difference being it's not called ohms law if you do, and it's not called 'ohmic resistance' it's called static, chordal, or DC resistance. Again it's a whole lot of dancing around words for absolutely no reason.

    Electronics is supposed to be a science. Questioning the degree of linearity required to meat the definition of 'ohmic' is a VERY pertinent question as if you say it just has to be 'close' you throw science and logic right out the window.

    I never said the professor was agreeing with me. He did however say "You really have the idea" which is good enough for me. Mike said he was agreeing with me, probably to try to end this wreck of a thread =)

    The fact that the slope required to meet 'ohmic' requirements is not explicitly stated is fact 1 undeniable proof that Ohm's law is not in fact a 'law' at all. It's just a commonly used approximation for the sake of simplicity. The real world is much more complicated. So we are in fact arguing about something that at the end of the day doesn't mean squat. It in fact has a meaning so ambiguous that we've spent the last 136 posts arguing about this and neither one of us is truly satisfied with the others viewpoint and never will be. Suits me, I'll still use that equation that is not Ohm's law when I apply it to non-linear circuits all the time =)
     
    Last edited: May 8, 2009
  19. Mikebits

    Mikebits Well-Known Member

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    Sometimes a picture helps.

    [​IMG]
     
    Last edited: May 8, 2009
  20. Sceadwian

    Sceadwian Banned

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    I found this image helpful as well.
    [​IMG]
     
  21. Mikebits

    Mikebits Well-Known Member

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    There come a time when the game becomes futile, and a kind gesture of conceding a stalemate keeps the peace :)

    Pride only carries you so far, at some point it becomes a blockade. Me gives up :)

    And I just don't care that much, My upstairs neighbors shower is now leaking into my condo, and she has no insurance, so WTF do I care what a damn diode is. Any lawyers out there?

    The leak is real, but in other words, I have more important issues to worry about. We are getting nowhere, better off pissing up a rope.
     
    Last edited: May 8, 2009

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