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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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    Oh come on, we would break out our LED flashlights and argue over who's light has more lumens. Of course the hangover would follow :)
     
  2. flat5

    flat5 Member

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    I would say the current increases because more work is being done. More current is needed to do the work.
    W=E*I
     
  3. wschroeder

    wschroeder New Member

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    Nice, but why does adding more resistance in parallel result in less total resistance? :rolleyes:
     
  4. dave

    Dave New Member

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  5. Mikebits

    Mikebits Well-Known Member

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    It is like yo mamma trying to paddle your butt, and your siblings butt at the same time. :)
     
    Last edited: May 3, 2009
  6. wschroeder

    wschroeder New Member

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    My point is that a discussion of the algebraic math formula is not a real discussion of what Ohm's Law is about. Math is important, but a broader and better understanding of electrical circuits is gained when the description and relationship of the parts is grasped.
     
  7. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi again,


    You ask a question that you already know the answer to :)

    I'm saying that semantics isnt an issue because Ohm's Law has been
    established long ago. People would rather make up their own definition
    then take the time to try to understand why all the universities and
    people with real experience are telling them how Ohm's Law really
    works.
    For those people who wont accept the true definition, do you even know
    who first discovered Ohm's Law??

    Here is another proof that Ohm's Law doesnt work for a resistance taken
    to a power:

    V=R*I
    and for example with a 2 ohm resistor:
    2v=2 ohms * 1 ampere
    and changed around:
    2 ohms=2v/1A
    and the units are:
    ohms=volts/amperes

    Now if R is not a constant but allowed to vary as R^2 we have:
    2v=(2 ohms)^2 * 1 amp
    and changed around:
    (2 ohms)^2=2v/1A
    and this is clearly NOT Ohm's Law with Ohm's Law the resistance (of
    course) comes out in units of ohms squared:
    ohms^2=volts/amperes
    which is clearly not coming out in units of ohms!!

    Also, to add to the now big pile of proofs the electrical circuit analysis
    book that i had for quite a few years now declares Ohm's Law as requiring
    CONSTANT RESISTANCE and even states that law as:

    "v=R*i"

    with the case of each letter v,R,i, exactly like that, and NOT like this:

    V=R*I

    The book, as i have posted several days ago, also states that it is
    a linear relationship. This is most likely a consequence of requiring
    zero volts for zero amps.

    Stating it as v=R*i means that v and i vary as R remains constant.
    Another problem with saying that because you can calculate the ratio
    r=v/i for a diode is that if you hold R constant r=R then you also have
    to hold BOTH v and i constant:
    R=V/I
    which is again not truely Ohm's Law.

    This is not about semantics. It's about believing what other people tell
    you for once in your life and, even if holding this in reserve for a while,
    trying to find out exactly why many people, many websites, several
    books, and many universities are saying this too. That's what this is all
    about.

    So far we have a stack of evidence supporting Ohm's Law as requiring
    constant resistance and being linear, and absolutely NO proof at all
    that states otherwise.

    Challenges:
    1. Who first discovered Ohm's Law?
    2. Find proof that Ohm's Law doesnt require constant resistance as in R=v/i or similar.
     
    Last edited: May 3, 2009
  8. wschroeder

    wschroeder New Member

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    Ohm's Law still holds true as Current in the circuit/device would move inversely to the variable Resistance. You didn't demonstrate that in your math above.
     
  9. Sceadwian

    Sceadwian Banned

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    MrAl, regardless of the fact that the resistance of a semiconductor device changes with respect to the voltage/currents involved, at ANY given point in time the voltage current resistance equation has real values that follow the ohms law equation, that is a simple fact. How you can keep arguing against that is beyond me, you keep mentioning linearity and time, the problem with that is that at no point in the equation is time mentioned because ohms law is about instances not changing values. There is no requirement for linearity in ohms law equation if R changes V and I will change as well it doesn't matter what is effecting those changes or if they're non-linear. The fact that universities teach that semi-conductors are non-ohmic devices is simply a trick of language nothing more.

    Simply looking at the dictionary definition of ohms law reads
    Do you see any mention of linearity or considerations for changing over time? Again your ENTIRE argument is semantic, not real. Just because for you personally think of things that way doesn't change the fact that there is no mention or requirement in the math for linearity over time. It's simply not there.
     
  10. Krumlink

    Krumlink New Member

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    Ohms law can be seen as a triangle equation; You can build a triangle out of given parts to find the answer.

    V=IR
    [​IMG]

    Therefore you can find Voltage with the other 2 parts, Amps with the other two parts, etc.
     
  11. Miles Prower

    Miles Prower Member

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    It is all about semantics because that is all you've been arguing, and doing it with patently false analogies. It doesn't matter that Ohm's Law was originally described for linear resistances. That was only a happenstance of Georg Ohm's choice of linear materials to do his experiments, from which derives the linear relationship:

    R= V / I

    To say you can't generalize this particular static relationship to a more general dynamic case (which does include the more specific linear relationship, since the derivative of any linear relationship is a constant):

    [​IMG]

    is just as illegitimate as saying that you can't generalize the static Q= CV into the dynamic I= C(dV/dT).

    Show me where any of those universities, books, etc you appeal to say otherwise? If you say you can't use the derivative form of Ohm's Law, then you can't explain how it is that an ideal voltage source has zero ohms of output impedance, or why an ideal current source has infinite output impedance. Furthermore, you render meaningless such quantities as dynamic plate/drain/collector resistance, all of which are vital to doing circuit design with vacuum tubes and transistors of all sorts.

    Keep on flogging that horse if you must, but I've said all I need to say on the matter.

    PS: "It's about believing what other people tell you for once in your life..."

    Not gonna happen. I prefer to think for myself, thank you so very much. [​IMG]
     
    Last edited: May 3, 2009
  12. MrAl

    MrAl Well-Known Member Most Helpful Member

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    I want to see some references here...not one so far, yet you argue that
    the references I and others have found are somehow "faulty".

    After several proofs and several reliable references, some of you guys still
    dont seem to believe in Ohm's Law that 'R' has to be constant, even though
    a reference has been found that even stated linearity. That's just arrogant
    now, especially since you havent stated even ONE SINGLE reference in your
    arguments favor yet!

    If you want to believe that an ordinary ratio is the same as Ohm's Law that's
    up to you, but i wont even believe that you truely believe that anymore if you
    have read the posts in this thread carefully.

    The proofs that i and many others have given clearly shows that a diode does not
    obey Ohm's Law, and if you want to say otherwise, then at least try to show some
    proof and gee cant find even one reference on the WHOLE web?

    If you work with different devices enough five years from now you will come back
    here with enough experience to agree with the proofs and references presented
    here. Until then, good luck!

    If you still dont agree after five years then i suggest you start writing to some
    universities to explain how you have disproved Ohm's Law, or how it is no longer
    needed because we already have a 'ratio'. See what some of the professors have
    to say about that. Good luck with that too :)

    Since you are now just wasting everyones time, including mine, i will just ignore any
    more arguments without at least one single reference, or at least agree to take some
    courses, or gee, at the very least go ask a professor of engineering.

    Here is another reference with good notes on Ohm's Law from Boston University:
    http://physics.bu.edu/py106/notes/Ohm.html
    with quote:
    "The connection between voltage and resistance can be more complicated
    in some materials.These materials are called non-ohmic. We'll focus mainly
    on ohmic materials for now, those obeying Ohm's Law."
     
    Last edited: May 3, 2009
  13. wschroeder

    wschroeder New Member

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    Fascinating, that term non-ohmic. If you look at a piece of electrical wire you will see the insulation rating of 600V or 1000V, etc, which suggests that it's not just an insulator but also a conductor under the right conditions of difference of potential between the conductor and outside world. Lightning jumping between the cloud and earth through the conductor/insulator, AIR, is another example.

    OHM's LAW is certainly a useful tool to describe the interaction of EMF, R, and I of that instant when the current flowed.

    Still relating to these non-ohmic materials, designing with MOSFET's will teach the importance of applying the ratios described by Ohm's Law. They have RON ratings across the Source-Drain in OHMs and mOhms.
     
  14. Sceadwian

    Sceadwian Banned

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    ESR is a common rating for capacitors, that's frequency dependent. Nothing static there.

    MrAl, have you looked up what the word semantic means? Just because the voltage/current relationship is more complicated than a purely linear resistance doesn't mean it doesn't follow ohms law. Even if they are called non-ohmic devices it doesn't mean they don't follow ohms law, it's just a term that's used NOTHING more. There is not one single thing in the VIR dependancy that includes time or describes any one of the three values as being forced to be static or linear in realation to anything other than themselves.
     
  15. Tesla23

    Tesla23 Member

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    I guess that by now the OP has moved elsewhere for definitive information, if not to a new area of study.

    For what it's worth, attached is a precis of what I understand Ohm discovered and has been named in his honour. (from Electricity, Magnetism, and Light By Wayne M. Saslow)

    Also, a short piece that shows that it caused him a lot of grief as well.
     

    Attached Files:

  16. suhasm

    suhasm New Member

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    The OP is right here...
    I have been following this discussion quite closely, I did not respond because i did not want to get in the way of more knowledgeable ppl expressing their opinions.

    Both Mr.AL and Sceadwian are offering excellent arguments and i dont know which argument to take.

    I feel that Sceadwian's argument is correct, although i dont know why. (actually Sceadwian's argument was what i had in mind when i posted this topic).

    But now after seeing Mr.AL's argument , even that is also very hard to ignore.

    I was thinking about a thought experiment yesterday night , and let me describe it to you. Maybe it will throw some light on why i am not understanding ohms law.

    Let us a take a piece of pure silicon. At 25 degrees , let us assume that the number of mobile charges is 1 billion (i have just chosen an arbitrary number). Suppose i apply a potential difference across its ends , then a current will begin to flow and that current will be V/R . Now suppose , i apply a voltage large enough , such that this ratio V/R exceeds 2 , then the current flowing will be 2 amperes. A current of 2 A is 2C/s.But the amount of charge in the piece of silicon is less than 2 coulombs . So what will happen? Will the current be limited to the maximum supported by the semiconductor? That implies that the resistance of the semiconductor will dynamically change to suit the voltage applied.

    I think this example shows that ohms law works for only a few range of values and not more.

    EDIT : @Tesla , the first image says that the VI curve of a semiconductor is non ohmic. Is that true? I thought that the VI curve of semiC is linear when temp. is held constant.
     
    Last edited: May 3, 2009
  17. Miles Prower

    Miles Prower Member

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    Current is a rate. If 2C pass in one second, that's a current of 2.0A. If 0.5C pass in 250mS, that's also 2.0A. If 6.74nC pass at a rate of 3.37nS, that, too, is 2.0A. In the case above where there aren't 2C of charge available, what is available will try to increase the drift velocity. Of course, the motion will be retarded by collisions with the crystalline lattice, causing temperature rise. Trying to pass too much current will eventually lead to melting.
     
  18. wschroeder

    wschroeder New Member

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    I guess taking sides with red herrings on the left and straw men on the right is how this subject is settled ...:eek:

    To be clear, OHM's LAW is still applicable to dynamic and AC circuits which includes non-ohmic materials such as semiconductors.

    If you don't think Ohm's Law is applicable then someone will have to explain away the term IMPEDANCE, juxtaposed to the term RESISTANCE.

    Inductive and Capacitive Reactance in a dynamic circuit is always quantitatively expressed in OHMS. That's not a mistake.

    MOSFETs, a popular nonohmic semiconductor device, have ratings in ON Resistance which is identified in the datasheets in terms of OHMS.

    So what do you do with those OHMS, ignore them because OHMS LAW doesn't apply to dynamic or non-ohmic materials. If this is where the discussion finally landed and where some may have landed with it, you are more lost than you can imagine.
     
  19. Leftyretro

    Leftyretro New Member

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    That's a very clear and logical summation. Not that I'm taking sides to this somewhat very wierd thread ;)

    Lefty
     
  20. Mikebits

    Mikebits Well-Known Member

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    The way I see it is as such. See accompanying diagram. You will note a non linear graph representing a diode curve. If we take the graph and pick a point on the graph we can find tangent of line where slope = 0. At this point we can apply ohms law and it works. At the linear moment R will equal E/I.
    Does that help to see the counter argument? :)
     

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    Last edited: May 4, 2009
  21. Tesla23

    Tesla23 Member

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    suhasm,

    you seem to have a reasonable grasp of the physics, that is that for many materials when you take a lump of it and connect it into a circuit, and hold the temperature constant, then V = IR over a wide range of V and I.

    Everything else then is semantics, and all that matters then is whether you can understand others and they can understand you. You cannot decide whether your teacher thinks that a diode satisfies ohms law by doing a thought experiment, you need to find out his interpretation of ohms law. This is important as he is marking your exam!

    I only stuck my nose in here as strange things were being said, like if something didn't satisfy ohms law them this would violate conservation of energy. I think that what I have posted reflects the usage of ohms law in the literature and the texts I have read, others may disagree but they haven't posted any supporting material. Really I don't care!

    I suspect that the problem is that some circuit theory texts refer to ohms law as being a property of a circuit, but even then it usually refers to the VI characteristics of a resistor.

    About your question on semiconductors, I suspect that this is a slip up, and he intends to refer to semiconductor devices. It could be intentional as semiconductors are reasonably linear, but they do become non-linear when the drift velocity saturates, and this occurs much earlier than for metals as, with far fewer carriers those that can move have to move faster to carry the same current as a conductor. GaAs shows a negative dynamic impedance at high enough electric fields.

    The reference to impedance and reactance is a complete furfy. Linear reactances have a constant ratio between voltage and current for sinusoidal waveforms. This is not related to Ohm's observation in conductors, but in circuit theory there is something, often called the Complex Form of Ohm's Law, which states that V = IZ, where Z is the complex impedance. V and I are phasor amplitudes and Z is complex.
     

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