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Ohms Law

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

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  1. DMW

    DMW New Member

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    Removed,
    10 pages for formula and not a Thankyou ;)
     
    Last edited: Mar 1, 2008
  2. Ratchit

    Ratchit Well-Known Member

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    To the Ineffable All,
    There seems to be some confusion about what Ohm's law is. The formula V=IR or V=IZ is NOT Ohm's law. It is the resistance or impedance formula. Ohm's law is a property of a material, not a method of calculating current,impedance, or voltage. Read what the physics books say about this.

    "We stress that the relationship V=IR is not a statement of Ohm's law. A conductor obeys Ohm's law only if its V--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."
    The above snippet is from Physics, by Prof David Halliday, University of Pittsburgh & Prof Robert Resnick,Rensselaer Polytechnic Institute, 1967 , page 780.

    And the following.
    "Ohm's law states that for many materials (including most metals), the ratio of the current density and electric field is a constant, which is independent of the electric field producing the current.
    Materials that obey Ohm's law, and hence demonstrate this linear behavior 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 emperical relationship valid only for certain materials."
    The above is from Physics for Scientists and Engineers, Raymond A Serway, James Madison University, Third edition, 1990, page 745.

    There you have it. Ohm's law should not be confused with the always correct resistance or impedance formula. It is a property of a material, not a method of calculation. Materials like semiconductors with their bent V--I curves do not obey Ohm's law. Ratch
     
  3. Sheldon Richards

    Sheldon Richards New Member

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    very interesting post on ohms law
     
  4. dave

    Dave New Member

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  5. kansara_nitish

    kansara_nitish New Member

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    Observe carefully:
    1: R=V/I
    2. V=R*I
    3. I=V/R
    [​IMG]
     
  6. PhillDubya

    PhillDubya New Member

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    Basically, it only applies in metals, not in semiconductors, or more appropriate, devices that current and voltage are non-linear. But it is ABSOLUTELY Ohm's Law. Does it apply in every instance, with every material? No. But it is still Ohm's Law, and is ABSOLUTELY correct to refer to that relationship: V=IR, as Ohm's Law.

    Are you debating actual engineering concepts, or semantics?

    Most any university engineering (electrical) professor (Ph.D) will tell you, and explain that it is in fact Ohm's law, fundamentally, when applied to metals. Once you get into semiconductors, and other materials, with a non-linear relationship, you will in fact start to see that it does not apply, but it is still HIS LAW. It is applied through observation.

    "The greater the voltage, the greater the resulting current. For a large class of conductors, the current increases in direct proportion to the voltage. Physical experimentation leads to the following equation: i = v/R, or, v=Ri, which is know as Ohm's Law. "

    Source: Foundations of Electrical Engineering J.R. Cogdell

    So, is the law wrong when concerning metals, or as it has been applied here in this thread?

    Or, are you just debating semantics?

    Seriously, no one in this thread is wrong in stating that those equations are Ohm's Law.....:rolleyes: An Ohm, is in fact, a Volt/Ampere.
     
  7. BaCaRdi

    BaCaRdi Member

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    Ohm's "Law" see what I am saying, there is a law for each section of Electronics..

    If it were easy everyone would be doing it. You have found the perfect place here my friend. Everyone here is Excellent! Ask the proper questions get the proper answers.

    Your other questions need to be answered by:

    Lenz's "Law"

    No I can't forget Mr. Maxwell!

     
    Last edited: Dec 22, 2008
  8. drtom

    drtom New Member

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    hi i'm tom and i'm new here-- and i dont "tipe"too well!
    but i have fun learning from others and giving what i can to spread knowledgs.
    i have often joked that "laziness" is the mother of invention or creation of new ways of doing stuff!
    i dont like using ohms law bwcause it makes me do a bunch of math with sometimes odd numbers.
    i know that in elex we have some leeway and dont need to be too exact but my methode is usually exact or as they say "close enuf for govt. work.
    we often lite up leds with pic chips with 5v. and a max of 25 ma from the chip.
    i see that most leds get 2v at reasonable currents so if the resistor varies its always about 2v which is close enuf. soo, how to get the max of 25 ma just for fun?
    first what res. gives 25 ma? well 25 v on 1000ohms is 25 ma without thinking , right?
    that means 5v on 200 ohms is the same since 200 is 1/5 of 1000.
    now how to get 25 ma with the led using 2v, we have 3v on res. 5v on 200 is like 1v on 40 ohms in our head right? that means the 2 v is on 80 ohms of the led and 3v is on 3x40 or 120 ohms . so 120 ohm +led = 25 ma. or we could say 3v on 120 is like 6v on 240 or then 12 v on 480 and 24 v on 960, theres 40 ohms left with 1 v so its the same as 25v on 1000 where we started .
    10 v on 100k ohms is the same as 100v on 1million ohms si we have 100 microamps!

    how about 16v on 4700 ohms . thats almost like 32 v on 10000 (9400) so call it 33 v on 10000 or 3.3v on 1000 or 3.3 ma.
    with some practice it becomes so ez you dont need to use a calculator or even your head!
    it all starts with iv,iohm, and 1 amp--1v on 10ohm=10v on 100 or 100 on 1000 or 100 ma

    6v on 17 ohm=12v on 34=36v on 100=360v on 100 or 360 ma or 1/3 amp appx, since the "old way is 6 over 17 or appx 1 over 3 or appx 1/3 amp
    now if your'e a purist and fussy about being exact you should realize that this is probably as close as the tolerance will be anyway!!!
    hope you have fun with this

    tom
     
    Last edited: Dec 22, 2008
  9. PhillDubya

    PhillDubya New Member

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    Learn Ohm's Law, along with a little English, spelling, grammar, and vocabulary. :rolleyes:
     
  10. russ_hensel

    russ_hensel New Member

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    Ohms law is true for resistors sort of by definition. If it does not follow the law it is not a resistor. ( at least in some definitions ) Semiconductors do follow ohms law, but semiconductor junctions do not. ( which is probably what was meant by saying semiconductors do not follow it ) It is not worth arguing much over these issues, you can move definition and laws around to variously make things true or false. Someone can probably find something wrong in almost any statement unless the statement is made unbarebly tedious.
     
  11. Chaerl

    Chaerl New Member

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    Semiconductor do obey the ohm's law. That's why it is a law, not theory. I guess everyone knows the difference between the two. But, there are additional parameters that is needed to consider in using ohm's law with the semiconductors. This is due to the fact of the non-linearity of semicon with respect to certain parameters (say, temperature). Example, a diode, if 100mA flows through it, it will have 0.7V drop. If we slightly change the source current, the voltage drop also change. This is normally affected by the ambient temperature as well as self-heating. But then, if you're interested this with the resistance of the diode at 100mA current and 0.7V voltage drop, Of course, you can use V=IR. The point here is, if I increase the current, it doesn't mean the voltage stays at 0.7V.

    The voltage across the resistor is directly proportional to its resistance. True. so you need a constant. In the case of ohm's law, V=IR, I is the constant. But in reality, is it constant? Even the ideal resistor has a slight non-linearity. That is why there is a tolerance. I hope this clears up things.

    If say something wrong, please do comment. BTW, I'm new in this forum. And I have shared my ideas in other threads. I hope its okay.
     
  12. PhillDubya

    PhillDubya New Member

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    Pardon?

    In V=IR, the equation for the voltage across a resistor, the constant is R, not I.

    From your fundemental equations for Ohm's Law:
    For resistance:
    V= IR = RI

    For inductance:
    V=L d/dt I

    For capacitance:
    V=C^-1 ∫ i(t) d(t)


    If you are going to incorporate a varying temperature into R, thus making R variable, then you would need a different equation.
     
    Last edited: Feb 16, 2009
  13. Chaerl

    Chaerl New Member

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    Theoretically, you're right. My mistake, R should be the constant. BUt, V=IR is an equation. Algebraically, I can make my I as constant and vary R to know the variation of V. This is what I was thinking when I wrote earlier. I could have a constant current source and a variable resistor, test what is the voltage. I would say in application, you can have constant I (if its possible in the real world).

    Again in theory, which is ideally correct, if you consider the temperature into the R. But not, its not just the ambient temperature, there is self heating, thermal cycle and shocks during manufacturing or actual application, humidity, and even aging should be considered. We can derive the formula considering this stuff or we could simply assumed what is the guranted tolerance from the supplier. In design, you may consider the worst case, I prefer to use monte carlo (its more realistic)
     
  14. Chad38

    Chad38 New Member

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    Ohm's Law

    Ohm's law cannot be directly applied to semiconductors due to the fact that the resistance of a semiconductor can be affected two different ways when temperature is applied to the semiconductor material. Intrinsic semiconductors have less resistance as the temperature increases, while extrinsic semiconductors have the opposite effect, and their resistance increases as the temperature increases.
     
  15. smanches

    smanches New Member

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    Ohm's law does not have anything to do with temperature though. You would use the resistance of the material at a specific temperature, but resistance is still a single discreet variable in the equation. The equation itself does not change, just the value of one of it's variables which is dependent on another external variable.
     
  16. BrownOut

    BrownOut Banned

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    Huh! It's a law. Breaking a law is against the law.
     
  17. BrownOut

    BrownOut Banned

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    Oh sorry, sometimes I get so excited, I forget to see if the thread is more than one page.
     
  18. giftiger_wunsch

    giftiger_wunsch New Member

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    Better arrest rubber for breaking Hooke's law then :D
     
  19. giftiger_wunsch

    giftiger_wunsch New Member

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    ...ouch :D
     
  20. BrownOut

    BrownOut Banned

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    Since when?
     
  21. giftiger_wunsch

    giftiger_wunsch New Member

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    I admit that I haven't studied physics for a while, but it was my understanding that Hooke's law states that the extension of an object (such as a steel spring, a piece of rubber, etc.) was directly proportional to the force applied to it, and that materials for which Hooke's law applies are termed Hookean materials; rubber does not demonstrate a linear distance of extension when increasing force is applied. If you know better, then I bow before your knowledge :p Please share :)
     
    Last edited: Jun 16, 2009
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