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Ohms Law

Discussion in 'Electronic Theory' started by mechie, Jan 15, 2003.

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  1. BrownOut

    BrownOut Banned

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    I appreciate that you have an open mind on these subjects. However, the laws of Physics cannot be broken. Keep this in mind as you go forward and it will help you to learn new things.
     
  2. Tesla23

    Tesla23 Member

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    This has all been discussed before, see I'm confused about ohms law.

    The bottom line is that what the term Ohm's Law means is a matter of semantics. If you want to confirm to the definition you will find in the mainstream literature then it only relates to materials that have a linear relationship between current and voltage. See, for example:

    "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:

    What is Ohm's Law?
    In modem teminology, Ohm's law states that the current I flowing through a conductor is proportional to the voltage V applied across it:

    I = V/R.

    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.


    You can, of course, define it to be whatever you like, but if you want to be understood by engineers and physicists then you need to restrict it to V=IR where R is a function of temperature (not voltage). Just because you found something on a web page doesn't make it generally accepted or true, you need to look at the source of the information or if it has been reviewed by people you trust.

    The introduction of small signal resistances is completely spurious, as mentioned in the previous thread.

    Ohm did not suddenly realise that you can divide V by I and get something called R that you can define everywhere, he discovered that in the conductors that he tested, that V and I were linearily related.
     
    Last edited: Jun 20, 2009
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  3. giftiger_wunsch

    giftiger_wunsch New Member

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    Couldn't have put it better myself.
     
  4. dave

    Dave New Member

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  5. Hero999

    Hero999 Banned

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    The resistance of something can change depending on the current.

    Here's an example:

    A red LED is connected to a 9V battery in series with a 470R resistor, a the voltage across the LED is 1.8V and the current through it is 7.2mA. The resistance of the LED is V/I = 1.8/0.0072 = 250R.

    The series reesistor is reduced to 390R. The current trough the LED increases to 17.7mA and the voltage across the LED increases to 2.1V. The resistance of the LED is still V/I which is now 2.1/0.0177 = 118.7R.

    The LED's resistance is non-linear, it depends on the current through the LED. It's also dependant on the temperature, if you conducted the above experiment at different temperatures the resistance would vary.
     
  6. BrownOut

    BrownOut Banned

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    Boloney! Spurious you say? Engineers have to deal with ss resistance all the time, and yes ohm's law is valid for these cases. How else you gonna analyze a circuit at it's operating point?

    That's the difference between those who work with science and those who talk phylosophically about science.
     
    Last edited: Jun 20, 2009
  7. nike6

    nike6 Banned

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    I don't think the barrier voltage of the Gallium metal counts as Ohm-type resistance.
    though for a given external current, it can be seen equivalent.
    as we know barrier voltage is not constant but a function of current.

    LEDs also have some fixed Ohm-type properties:
    -the bonding wire, some 10s to 100 mOhm, can not carry high currents (i.e. the LED will explode immediately due to vaporization).
    -the connection wires, some mOhm, neglible.
    -the semiconductor channel, some 20 to 30 Ohms for small LEDs. this resistance is independent from the external effective resistance relating to the junction voltage drop (what you get when you reversly apply Ohms law), so it should be subtracted.

    i think approximately it is correct to subtract the junction voltage from the external voltage, and then to apply Ohms law.
    the internal resistance also could be subtracted but in most cases is neglible.
    the variation of the junction barrier voltage due to current also will be neglible in most cases.

    for laser diodes it's not so neglible, far more critical, they are usually operated near the margin.

    I have always remembered ohms law by LED resistor: for 12 volts, a 1K resistor will result in a 10mA current.

    people can find these basic laws in a (professional) book about energy electronics, or even in an introductionary teaching book.
    the crux with these books is that concepts like transistor and AM/FM will not be explained with sufficient details. such a book is just a starting point.
     
    Last edited: Jun 20, 2009
  8. Hero999

    Hero999 Banned

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    You've completely missed the point, just because the resistance of something is dependant on the current, Ohm's law still applies.

    Nothing is a perfect resistor, even the resistance of metal wire varies slightly depending on the current.
     
  9. j.friend

    j.friend New Member

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    Isn't it true that there are many resistors that are non-ohmmic. I believe that is what we were taught in grade 10 science
     
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