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... 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?

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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.

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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?

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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...

 

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.

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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.

 

I 'spose.
µΩΔΘΣΦ

 

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.

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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.

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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


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

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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?

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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.

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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?

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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).

 

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.

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