Ohm's law is not V=IR

Discussion in 'Physics' started by Ratch, May 16, 2008.

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  1.  
    Ratch

    Ratch Thread Starter New Member

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    As promised, I would like to call your attention to a misnomer in electrical science of the formula V = I*R, which is wrongly called Ohm's law.

    First I will show a couple of links.

    http://maxwell.byu.edu/~spencerr/websumm122/node50.html#SECTION00066000000000000000

    http://www.launc.tased.edu.au/online/sciences/PhysSci/done/electric/resistnc/Resistance.htm

    Still not convinced? Don't blame you. One can publish anything the WWW.

    OK, let's look next at a couple of good physics texts.

    I will first quote from a college textbook called Physics, by Halliday & Resnick, 1967, page 780. It was written by David Halliday, Professor of Physics, University of Pittsburgh and Robert Resnick, Professor of Physics, Rensselaer Polytechnic Institute

    -----------------------------------------------------------------------
    "We stress the relationship V=I*R is NOT a statement of Ohm's law. A
    conductor obeys Ohm's law only if its V vs. I curve is linear, that is, if R
    is independent of V and I. The relationship R=V/I remains as the general
    definition of the resistance of a conductor whether or not the conductor
    obeys Ohm's law. . . . . . . . . Ohm's law is a specific property of
    certain materials and is NOT a general law of electromagnetism, for example,
    like Gauss's law."
    -----------------------------------------------------------------------

    Next a quote from another college textbook called Physics for Scientists & Engineers, by
    Raymond Serway, Third Edition, 1990, page 745. It was written by Raymond A. Serway of James Madison University

    -----------------------------------------------------------------------
    "A current density J and an electric field E are established in a
    conductor when a potential difference is maintained across the conductor.
    If the potential difference is constant, the current will also be constant.
    Very often, the current density in a conductor is proportional to the
    electric field in the conductor. that is J=sigma*E where sigma is called the
    conductivity of the conductor. Materials that obey the above equation are
    said to follow Ohm's law, named after Georg Simon Ohm (1787-1854). More
    specifically,

    Ohm's law states that for many materials (including most metals) the
    ratio of the current density and electric field is a constant, sigma, which is
    independent of the electric field producing the current.

    Materials that obey Ohm's law, and hence demonstrate this linear
    behavior between E and J, are said to be ohmic. The electrical behavior of
    most materials is quite linear for very small changes in the current.
    Experimentally, one finds that not all materials have this property.
    Materials that do not obey Ohm's law are said to be nonohmic. Ohm's law is
    not a fundamental law of nature, but an empirical relationship valid only
    for certain materials."
    -----------------------------------------------------------------------

    OK, what could be clearer? Ohm's law refers to the linearity between voltage and current, not relationship between voltage, current, and resistance. Yet the formula, V = I*R has been misnamed across countless classrooms, books and discussions. What does your textbook or school teach?

    To summarize, certain materials like conductive metals follow Ohm's law, in that their V vs.I curve is linear. Ohm's law is a property of a material, not a general law of nature. Other conductive entities like diode junctions or gas discharge bulbs do not have the Ohm's law property, because their conductivity changes depending on what voltage or current is applied, causing their V vs. I curve to be nonlinear. In all cases, V = I*R is always correct, but it should not be called Ohm's law. It should be called the resistance or impedance formula.

    No matter what you call V = I*R, circuits will still get designed and analyzed, and science will still progress.

    Ratch
  2.  
    triggernum5

    triggernum5 Active Member

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    Yet in any specific test conditions, Ohm's law must hold for conservational purposes.. ie. a material with a varying resistance, will have a certain resistance under the current conditions..
    Analogous to a thermistor in a circuit, except its resistance varies due to temperature, rather than potential..
    Ohm's law does hold for non-ohmic materials, but R is a function, not a constant..
  3.  
    Ratch

    Ratch Thread Starter New Member

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    triggernum5,

    The resistance/impedance formula will hold under any voltage and current condition. That formula is not Ohm's law.

    Contradictory statement. If a material is non-ohmic, then it does not follow Ohm's law. Ohm's law is a property of a material, not a method of determining resistance from the voltage and current. Ratch
  4.  
    triggernum5

    triggernum5 Active Member

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    Well thats a debate over language and semantics.. Those tend to kill quality technical discussion, but I won't disagree with that..
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