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I'm confused about ohms law

Discussion in 'General Electronics Chat' started by suhasm, Apr 27, 2009.

  1. Sceadwian

    Sceadwian Banned

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    MrAl you have failed utterly to prove anything, just because more complex math is required to define how ohms law relates to an 'active' circuit does NOT mean that ohms law fails in an active circuit. Again, I will say for ANY given moment in time a circuit has a V I R equivalent that will work out. To say ohms law is an ultimate ratio with no other dependancies is ignorant, because even the most commone resistors are non linear with both voltage (very slightly) and with current and temperature (symbiotically) as well.
     
  2. Mikebits

    Mikebits Well-Known Member

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    I think it is time to have a funeral for the horse.
     
  3. Sceadwian

    Sceadwian Banned

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    I'll play taps.
     
  4. dave

    Dave New Member

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  5. Tesla23

    Tesla23 Member

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    To summarise:

    Ohms Law: an empirical observation that for a range of materials or devices (now called ohmic materials or devices) V = IxR where R is the (approximately constant) resistance. The fact that R is approximately constant over a range of V and I makes this relationship useful. Common usage is that devices that deviate from Ohms Law are called non-ohmic.

    Sceadwian's Law*: all devices satisfy V = IxRs where Rs is the Sceadwian Resistance. Rs varies in an unknown way with I. A diode example:
    V I Rs
    .5V 1uA 500k
    .6V 1mA 600
    .62V 100mA 6.2

    No-one uses numbers like these, they are not useful in any sort of circuit analysis and Sceadwian has not been able to offer any reference to anyone else using them. Not only that, but knowing the Sceadwian resistance at one value of I does not help predict the operation of the device at any other point. The value of Ohm's law is that it characterises the device or material and allows it's operation to be predicted.

    I suggest we bury Sceadwian's law with the horse. (with apologies to Sceadwian).

    * proposed by Sceadwian and others.
     
    Last edited: May 1, 2009
  6. Sceadwian

    Sceadwian Banned

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    I didn't say anyone used numbers like that Tesla, they make sense though, the 'law' the mathematical equivalent WORKS. Period. There is NO saying 'you're wrong' to that.
     
    Last edited: May 1, 2009
  7. Mikebits

    Mikebits Well-Known Member

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    Well, no need to be insulting.

    I have read on some web sites that support your claims that a device must be linear in order to satisfy ohms law. If this is the requirement then I capitulate to you, but I must admit I do not agree with this requirement.

    I still say that ohms law is satisfied based on Rdiode=ΔE/ΔI at t.
    I do see your argument that you cannot sit down with pencil and paper and draw a series R and diode circuit and compute Vr drop on Resistor.
    To this point I agree since other factors come into play.

    But, what I do argue is this; If you create a circuit as follows:

    Voltage source with series resistor in series with diode in series with current meter, measure Vr drop of resistor one can use ohms law to compute R of diode.

    I defy you to show me how series R of diode could violate ohms law at above given measurement.

    Kirkoff's law will demonstrate that above circuit obeys ohms law.

    I have read this topic of non-ohmic and I think it is a matter of semantics, and I am not so sure I agree with classifying a diode as violating ohms law just because it is non linear.

    At this point, I wave my hands in the air and say, lets just agree that we disagree :)
     
  8. Mikebits

    Mikebits Well-Known Member

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    Suhasm, Are you not now glad you asked? :)
     
  9. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi again,


    First off, if more complex math is required then we dont have Ohm's Law
    anymore. Ohm's Law is a very simple and elegant rule that allows us to
    calculate a third quantity knowing two others. If you have to complicate
    that rule then it can not be Ohm's Law.
    For example, i think it is very clever to think of adding a series resistor to
    a diode and then calculating the diodes dynamic resistance based partly
    on the drop across the resistor, but it is not Ohm's Law because Ohm's
    Law does not say that we have to connect other elements in series with
    anything else, and it deals with one element not two, and furthermore
    we dont have to subtract anything (as we do with a series resistor and
    ANY other circuit element, including another resistor).

    Second, if we say that resistors dont obey Ohm's Law because they
    vary slightly in resistance (hence non constant resistance) with any
    other variable such as current (self heating) then we have just reduced
    Ohm's Law to nothing at all (again). In other words, we would eliminate
    Ohm's Law from reality and gee i just wonder maybe we should tell
    everybody that we no longer need Ohm's Law because we found out that
    no element on earth truely obeys it 'perfectly'. Good idea? I dont think
    so. I dont think so because Ohm's Law is still a very useful tool when
    used appropriately.

    Third, if you can say that a diode obeys Ohm's Law because for one
    value of current I we can say that we know what R is by measuring V
    and because V=I*R holds for that one value, then i can say an ordinary
    resistor follows the ideal diode law:
    v=N*Vt*ln(i/IS+1)
    just by finding a new N for every new current i that I measure.
    So what do we say now, that every law for different elements is really
    the same? Gee, i think from now on i will only use the law of gravity
    for everything including circuits :)

    How about this...
    We have a rotating resistor rotating laterally, so that it's leads come into
    contact with two brushes mounted on opposite sides of rotation. When
    the leads are rotated such that they come into contact with the brushes
    we measure the resistors value, very quickly, and note the result.
    We rotate it very fast...30,000 rpm. Now at some point in time it comes
    into contact with the brushes and we measure its resistance, so i guess
    we know V=I*R at that one instant in time (or over a short interval).
    I guess that means we can say that this apparatus between the two
    brushes follows Ohm's Law, because for that one instant we can measure
    its resistance? I dont think so :) Also, what is its resistance when
    there is an open circuit between the two brushes?

    Note that when we use Ohm's Law properly we can calculate a third
    quantity from knowing two others, and that we only have to calculate
    the resistance once.

    Again, Ohm's Law is expressed as a ratio because Ohm's Law is calculated
    as a ratio, but a ratio is not the same as Ohm's Law.
    Elements that obey Ohm's Law have certain properties, and elements that
    do not obey do not possess these properties...that's how we can tell the
    difference between the two types of elements.

    It's not really about semantics either, because the two types of elements
    are very clearly different in nature.

    What some of you guys need is some lab time. Take a few resistors and
    do a few experiments using Ohm's Law. Measure the voltage, calculate
    the current...see how easy it is to do. Then, take a few diodes and
    try to calculate the current...see how much harder it is to do, and not
    once can you use Ohm's Law to do so.
    If i hand you a 100 ohm resistor to do the experimenting with, you can
    quickly calculate the current knowing the voltage across the device,
    but if i hand you a 1N4148 diode, it's not going to happen because
    the diode is a very very different kind of device, and it is often
    characterized by saying that it does not follow Ohm's Law while
    the resistor does.

    Another challenge for you is to find one notable reference that shows
    Ohm's Law being used for a diode (not a piecewise linearization however
    because we already know we can use Ohm's Law to approximate a
    diodes behavior over a very short range of current).
    We have presented several references that clearly show that a diode
    does not follow Ohm's Law, so it's up to you now to find one that shows
    different.
     
    Last edited: May 2, 2009
  10. Miles Prower

    Miles Prower Member

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    Four pages and growing. Oy vey!

    1) Calling the relationship, R= V/I "Ohm's Law" was a misnomer from the get-go. "Law" implies universality, like the Law of Conservation of Energy. It's not universal since not all resistances are linear.

    2) "Non-ohmic" simply means nonlinear. Another poor choice of words here.

    3) There is nothing wrong with taking a static relationship like R= V/I and extending it to dynamic cases by converting it from a ratio into a partial derivative. If such extensions weren't allowed, we'd forever be stuck with DC only analysis. Q=CV is the static case, whereas I= C(dV/dT) is dynamic.

    Arguing over semantics is such a dead end cause. [​IMG]
     
  11. Sceadwian

    Sceadwian Banned

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    Yeah, I hadn't posted in a while, I was gonna let it go but I made the mistake of posting again yesterday =)
     
  12. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello Miles,


    Thanks for adding to the discussion.
    It's not a matter of semantics however. The ratio V/I existed long
    before Ohm, but Ohm showed that some elements exhibit special
    properties which brought on the result of Ohm's Law.

    Guys: do the lab work, then make the judgment...not before.

    1000 universities cant be wrong.
     
  13. Miles Prower

    Miles Prower Member

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    Not even when Europe's finest universities taught that the Sun revolved around Earth, and that comets were harbingers of God's wrath? [​IMG] [​IMG]
     
  14. Sceadwian

    Sceadwian Banned

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    I agree with Miles, it's about semantics nothing else at this point.
     
  15. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    I has become like arguing about religion
     
  16. Sceadwian

    Sceadwian Banned

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  17. wschroeder

    wschroeder New Member

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    When I taught electrical theory, I attempted to create literate as opposed to functionally illiterate students when it comes to Ohm's Law... you know, plug in 2 known values to get the third. Ohm's Law did not begin as a math formula, but a physics statement. Engineers long ago converted the words to a math statement to help them specify proper values in their designs. Students always tend to remember math formulas as the important part without any understanding of how that formula came into existence.

    Ohm's Law basically states that Current (i) is directly proportional to Voltage (E) for a given resistance and inversely proportional to the Resistance for a given voltage (R) in a circuit. In math terms: I = E / R, or E= I*R By the way, very linear.

    Here's a test of functional illliteracy I would throw at my students. Most know that adding resistance to a circuit of given voltage creates higher total resistance and lower current.

    The question is: Why after adding more 100 watt incandescent lamps (resistors), paralleled, to a power electrical circuit does the total current become higher with each additional lamp?
     
    Last edited: May 2, 2009
  18. Sceadwian

    Sceadwian Banned

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    Because resistors in parallel decrease total resistance. Is that a trick question somehow? Did I fail? =)
     
    Last edited: May 2, 2009
  19. Leftyretro

    Leftyretro New Member

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    Well you probably have the correct answer, but that doesn't mean you aren't a failure ;)

    Sorry, just kidding, couldn't help myself, left handed you know. :p

    Lefty

    PS: and yea, if this thread had started in a bar the fight would have long been broken up and we all would be recovering with hangovers and wondering WTF.
     
    Last edited: May 2, 2009
  20. Mikebits

    Mikebits Well-Known Member

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    For some reason, I have failed to see your point.
     
  21. Sceadwian

    Sceadwian Banned

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    My wife and stepson are left handed. I'm outnumbered! It's a conspiracy I tell ya!
     

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