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I'm confused about ohms law

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

  1. suhasm

    suhasm New Member

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    After seeing contradictory posts on many forums , and getting differing opinions from my teachers , i'm completely lost with the ohms law....

    I have a number of questions. Can someone please answer them for me?

    1) Do semiconductors obey Ohms law? and why do they obey/not obey?
    if they obey , then why is the drop across a ideal diode constant even when the current in it changes?

    2) I heard someone say in another thread "semiconductors obey ohms law , but semiconductor junctions dont". why is it so?

    3) A thread in another forum said that liquids dont obey ohms law as ions are the current carriers...:confused: true or false?

    4) Is ohm's law a special case of V=IR or is V=IR a special case of ohms law?

    5) Is Ohms law a law at all?

    6) Is there any theory in physics which explains why ohms law works the way it does?
     
  2. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Ohms law works in all cases, you need to know two of the variables and can work out the third.

    Your problem is that you're trying to apply it where you don't know two of the variables, or they don't have a fixed value - you can't apply 'R' to a semi-conductor.
     
  3. suhasm

    suhasm New Member

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    @ nigel
    What do you mean "do not have a fixed value"?

    Do you mean to say that the resistance of a diode automatically adjusts itself so that the drop across it is always 0.7V?
     
  4. dave

    Dave New Member

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  5. solis365

    solis365 New Member

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    i guess your questions about semiconductors are more a matter of semantics. it is too general to say "semiconductors obey ohms law" although it is technically true.

    all materials have an associated conductivity or resistivity. a chunk of silicon has some resistivity, and if you hook up some leads to just a plain block of silicon, it will have an associated resistance and will obey ohms law. however, semiconductor-based DEVICES may not obey this law in the context of some circuit. i think this was what was meant by "semiconductor junctions do not." due to advantageous properties of semiconductors when they are doped we can create nonlinear devices which do not obey ohms law on the macro scale.

    i am not sure about fluids, i have never encountered that question. there are some non-ohmic materials that do not behave that way.

    however i do believe ohms law can be derived from physical laws but it is a fairly advanced derivation, not taught in intro college classes...
     
    Last edited: Apr 27, 2009
  6. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Exactly what it says, it's not fixed - you can't say a semiconductor is x ohms, resistance doesn't apply.

    As far as ohms law is concerned it does, but it doesn't really have 'resistance' so you're mising one of the three variables for ohms law.
     
  7. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    First, read the Wiki.

    Ohm observed that in a "pure" resistor, current through the resistor is proportional to the voltage across the resistor. This is only true for what we call "resistors".

    Ohms law does not apply for two-terminal circuits which are "non-linear", like semiconductor devices, thermistors, etc. They have their own "laws" which describe the relationship between current and voltage.
     
  8. Willbe

    Willbe New Member

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    I 'spose.
    µΩΔΘΣΦ
     
  9. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    The formula V=I *R is not Ohms Law.

    Ohms Law states that the current flowing thru a conductor is directly proportional to the applied voltage
    and inversely proportion to the resistance of the conductor.

    So 'technically' if you apply a voltage to a semiconductor a current will flow which is inversely proportional to its resistance.

    Its important to note that the 'resistance' is only being measured at a finite point along the resistance 'curve/plot' of the semiconductor.

    Dont confuse a semi-conductor material with the action of a 'transistor or diode' which is combination of two dissimilar semiconductor materials.
     
    Last edited: Apr 27, 2009
  10. crutschow

    crutschow Well-Known Member Most Helpful Member

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    Ohms law simply quantifies the observed relationship between resistance, voltage, and current. If the resistance varies with operating point, then you can still say that the component obeys ohms law for the resistance at a particular operating point.

    For example, a thermistor's resistance varies with temperature. But for a given temperature ohms law can be used to calculate the current through the thermistor for the resistance at that temperature.

    And using ohms law you can calculate the equivalent resistance for a semiconductor junction at a particular current. But if the current changes, the calculated resistance will change, since the junction has a nonlinear relation between current and equivalent resistance.
     
  11. suhasm

    suhasm New Member

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    We can determine the resistance of a diode at some point in its operating point. However , that still doesnt mean that R*I = 0.7V
    :confused:

    Why dont semiconductor junctions obey ohms law?
    Or do semiconductor junctions obey ohms law at a microscopic scale?
     
  12. suhasm

    suhasm New Member

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    @willbe
    '


    If semiconductor junctions obey ohms law , then shouldnt the drop across a diode be proportional to the current flowing through it?
     
  13. Sceadwian

    Sceadwian Banned

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    No.
    I will crush this thread before it blossoms in the the behemoth abomination that was the basic transistor thread a few days ago!

    ALL MATERIALS obey ohms law.
    Semi conductors HOWEVER can change their EFFECTIVE resistance values when an electric field is present near the PN junction. So technically yes, they obey ohms law. They however are ACTIVE devices so the voltage/current traveling through them changes the static EM field that exists and hence the devices conductivity making them non-linear.
     
    Last edited: Apr 27, 2009
  14. suhasm

    suhasm New Member

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    @ Sciadwian

    I think that was the answer i was looking for..
    Can someone please explain that in a little bit more simple manner?
    What do you exactly mean by effective resistance?
     
  15. dknguyen

    dknguyen Well-Known Member

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    "Effective" in english means that you can treat something as something else, even though they are actually two different things.

    "Effective" in effective resistance just means that it is not actually a resistor, and does not behave like a resistor, but under a set of very specific conditions (temperature, voltage, current, frequency etc) you can model the component as a resistor.

    In the formula V=I*R you are assuming R is a constant value no matter what the temperature, voltages, or currents are in the component. THis is true for a resistor, but not for a semiconductor, capacitor, or an inductor). THe effective resistance of these components changes as the current in it changes, or as the voltage being applied to it's terminals changes (or both).

    It's more like V=I*R(x), where R(x) is the function for the resistance and 'x' is all the variables that the effective resistance is dependent (temperture, current, voltage, frequency, etc).

    THe thing to carry away from this is that R in V=IR only stays a constant value for resistors. FOr every other component R is a value that can change as the component's operating variables change. The resistance is like it's own little equation rather than just a fixed number (unless it's an ideal resistor, but even real resistors change their resistance slightly with things like temperature).
     
    Last edited: Apr 27, 2009
  16. Sceadwian

    Sceadwian Banned

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    I put it about as simply as you can get before you get deep into the math of semi-conductor theory (I think) Please someone help me out here if they can describe it more simply.
     
  17. dougy83

    dougy83 Well-Known Member

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    Resistors will change in value for all of the listed variables...
     
  18. dknguyen

    dknguyen Well-Known Member

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    Ideal resistors, let's make it easy for the newcomer.
     
    Last edited: Apr 27, 2009
  19. Sceadwian

    Sceadwian Banned

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    It would be best to mention resistors temperature non-linearity up front, it's a very important thing to know.
     
  20. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi there,


    Strictly speaking, semiconductors (like si diodes) do NOT obey Ohm's Law. This is
    because the current through the element is not proportional to the voltage across it.
    It's as simple as that. The voltage across a regular diode is around 0.7 volts and
    as the current varies widely the voltage across it does not change that much.
    It doesnt matter how small of a range you choose (a change of 1 amp, a change
    of 0.1 amp, a change of 0.000001 amp) it still does NOT obey Ohm's Law.

    An element that obeys Ohm's Law follows this equation exactly:
    V=I*R, or similar.

    A semiconductor diode does not follow this Law, therefore it does not obey
    Ohm's Law.

    A semiconductor diode equation can be approximated at a given bias point by
    saying that Rd=V/I at some point x when the range is relatively small, but that
    is merely an approximation and NOT a law of any kind. The resistance is held
    constant with the added constraint that the current does not change too much
    relative to the application, and thus V=I*Rd over a small range of I.
    This means we USE Ohm's Law once we know what Rd we will be working with,
    but it should be noted that the diode still is not obeying Ohm's Law, we are just
    pretending that it does over a short range of current change.

    A pure resistance obeys Ohm's Law, in that V=I*R (or similar algebraic form).
    That's about the only thing that does, because R is constant.
    A silicon diode follows this or similar law:
    V=0.026*ln(I/I0+1)
    Note the logarithmic relationship there that is certainly not proportional to current.

    At the level it sounds like you are (the OP) it would probably be better for you
    to take a good look at Ohm's Law for resistors alone for now, and get into
    semiconductors after you have a good feel for how resistors work in circuits.
     
    Last edited: Apr 28, 2009
  21. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    I cannot see why, when talking about the resistance of semiconductors materials
    its deemed necessary to explain in terms of semiconductor devices.

    A piece of semiconductor material is passive, a semiconductor device ie a transistor/diode is formed
    from the junction of two dissimilar semiconductor materials.

    Conductors have a temperature/resistance coefficient and their resistance changes in a linear way with respect to temperature,
    either in a negative or positive sense.
    Manufactured alloys can be made that have close to zero temperature coefficients.

    Manufactured compounds can be made that have a non linear resistance change with change in temperature,
    again with a negative or positive sense.

    Which ever type of material you choose, conductor or semi-conductor, it will obey Ohms Law at any given temperature.
    The same rule applies to thermistor compounds.
     
    Last edited: Apr 28, 2009

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