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Ohms Law Limits

by strokebow
Tags: limits, ohms
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sophiecentaur
#19
Dec3-11, 06:24 PM
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I don't see that any of this relates to Ohm's Law at all. There's nothing inherently non linear about any of these effects and that's all Ohm's Law tells you. It deals specifically with resistance and doesn't concern any transforming effects of distributed components. In all these examples, doubling the Volts will double the Current. So where's any violation?
vk6kro
#20
Dec3-11, 06:51 PM
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Quote Quote by sophiecentaur View Post
I don't see that any of this relates to Ohm's Law at all. There's nothing inherently non linear about any of these effects and that's all Ohm's Law tells you. It deals specifically with resistance and doesn't concern any transforming effects of distributed components. In all these examples, doubling the Volts will double the Current. So where's any violation?
That was the point I was making. You can get apparent anomalies, but Ohm's Law continues to apply.

If fact, I cringe when I hear talk of "non-ohmic" resistors. Just because the resistance of a resistor changes (due to temperature, for example) does not mean Ohm's Law no longer applies.
A small change in voltage still produces a small change in current and the ratio of these two still gives the resistance of the resistor.
sophiecentaur
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Dec3-11, 07:00 PM
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It would have been better if they had called it 'Ohm's Behaviour', perhaps.
Nevertamed
#22
Dec6-11, 11:20 AM
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ohms law fails when the resistance of the conductor increases as it gets hot by electron friction
phinds
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Dec6-11, 11:24 AM
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Quote Quote by Nevertamed View Post
ohms law fails when the resistance of the conductor increases as it gets hot by electron friction
That doesn't seem to make sense. All you are saying is that the resistance varies with temperature. Whatever the resistance is, Ohms Law should still hold.
sophiecentaur
#24
Dec6-11, 03:49 PM
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Quote Quote by Nevertamed View Post
ohms law fails when the resistance of the conductor increases as it gets hot by electron friction
You obviously haven't read what Ohm's Law says.
Nevertamed
#25
Dec6-11, 10:45 PM
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in Ohm's law as resistance is proportional to voltage, then: temperature is a limitation, resistance varies with temperature, and depending on materials and room temp, it can vary a lot. unless we state that temp is constant we can secure the lineality of the ecuation
vk6kro
#26
Dec7-11, 03:23 AM
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It depends on the definition of Ohm's Law.

There is the one we all saw on day 1 of Electricity at school. Graph voltage vs current and if it is a straight line then you can work out the resistance.

According to that version, even a rheostat would be regarded as non ohmic because it can change.

It leads to the strange situation where a lamp filament is "non-ohmic" if you measure its voltage and current curve slowly (and give it time for the temperature to change) but "ohmic" if you change the voltage rapidly. The current in a lamp filament with 60 Hz AC on it is close to sinusoidal, meaning the resistance is close to constant because the temperature is fairly constant and the resistance is also stable.

Much more useful is the incremental one which assumes that Ohm's Law always applies and you can work out the resistance at any point on a V vs I curve by taking a small change in voltage and observing the small change in current that results.

This small signal resistance is very real and it can be measured as the actual impedance of a circuit. It may be quite different to the ratio of DC voltage to DC current that applies at that point.
sophiecentaur
#27
Dec7-11, 08:39 AM
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Ohm's Law is its own definition. The condition for V/I being constant is for the temperature to be constant. Measurement with a 'probe', low level AC of a filaments lamp will reduce the temperature fluctuation and give a value of dV/dI that is not resistance. The resistance is still V/I for the particular point on the temperature dependent VI curve that you will get using DC.

The only time that Ohm's Law truly fails for a metal is for enormous current flux when there are just no more conduction electrons available and the conductor becomes .non linear.
es1
#28
Dec8-11, 01:25 AM
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I wonder though: It is Ohm's Law that is failing or that the device (or circuit) in question is no longer well modeled by an ideal resistor in the examples given? I would argue Ohm's law isn't breaking, the ideal resistor model is.

If one replaced the ideal model in Ohm's law with a new model that also took temp, time, etc., into consideration, wouldn't Ohm's law still hold? I think it does.

I agree with Sophie and the only thing that really breaks it is current density and possibly arc'ing across the geometry of the element. Because if the function for R had to take current or voltage as an independent variable then equation we would be left with would be self referencing and I think this would count as a new beast.
sophiecentaur
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Dec8-11, 04:02 AM
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Quote Quote by es1 View Post
I wonder though: It is Ohm's Law that is failing or that the device (or circuit) in question is no longer well modeled by an ideal resistor in the examples given? I would argue Ohm's law isn't breaking, the ideal resistor model is.

If one replaced the ideal model in Ohm's law with a new model that also took temp, time, etc., into consideration, wouldn't Ohm's law still hold? I think it does.

I agree with Sophie and the only thing that really breaks it is current density and possibly arc'ing across the geometry of the element. Because if the function for R had to take current or voltage as an independent variable then equation we would be left with would be self referencing and I think this would count as a new beast.
But, as with many so called laws of nature, Ohm's Law has no 'authority' to make things behave a certain way. It doesn't punish them if they don't conform. Ohm's law is merely a description of what happens under a particular set of circumstances. If one reads the wording carefully (actually, it's blindingly obvious that you should always do that) then there isn't much to disagree with and there's certainly no reason to hop about saying "Ohm got it wrong". Add some extra factors and metals will behave differently - so what?

I would say that the only time a metal stops obeying Ohm's law is when it is under such extreme conditions that it can almost not be classed as a metal because its very structure will have changed (metallic bonding won't even be the same). George O. can't really be blamed for not having put a codacil on his Law to take account of this.
Nevertamed
#30
Dec8-11, 06:32 AM
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Quote Quote by sophiecentaur View Post
But, as with many so called laws of nature, Ohm's Law has no 'authority' to make things behave a certain way. It doesn't punish them if they don't conform. Ohm's law is merely a description of what happens under a particular set of circumstances. If one reads the wording carefully (actually, it's blindingly obvious that you should always do that) then there isn't much to disagree with and there's certainly no reason to hop about saying "Ohm got it wrong". Add some extra factors and metals will behave differently - so what?

I would say that the only time a metal stops obeying Ohm's law is when it is under such extreme conditions that it can almost not be classed as a metal because its very structure will have changed (metallic bonding won't even be the same). George O. can't really be blamed for not having put a codacil on his Law to take account of this.
wonderful answer
Kholdstare
#31
Dec31-11, 08:33 AM
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Can anyone quote the exact statement given by Ohm with web reference. Just being curious.

Given a constant temperature current will be equal to voltage multiplied by resistance. - correct
Given a constant temperature current will be proportional to voltage. - wrong
vk6kro
#32
Dec31-11, 10:07 AM
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Quote Quote by Kholdstare View Post
Can anyone quote the exact statement given by Ohm with web reference. Just being curious.

Given a constant temperature current will be equal to voltage multiplied by resistance. - wrong
Given a constant temperature current will be proportional to voltage. - correct
The history of Ohm's Law is covered in the Wikipedia article:
http://en.wikipedia.org/wiki/Ohms_law
sophiecentaur
#33
Dec31-11, 12:57 PM
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Quote Quote by Kholdstare View Post
Can anyone quote the exact statement given by Ohm with web reference. Just being curious.

Given a constant temperature current will be equal to voltage multiplied by resistance. - correct
Given a constant temperature current will be proportional to voltage. - wrong
?????
If you get your history right, you will realise that Ohm only introduced Resistance as the constant of proportionality for a linear relationshipbetween two quantities that he could actually MEASURE. They didn't sell Ohmmeters at the time!
Nevertamed
#34
Dec31-11, 05:00 PM
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Quote Quote by Kholdstare View Post
Can anyone quote the exact statement given by Ohm with web reference. Just being curious.

Given a constant temperature current will be equal to voltage multiplied by resistance. - correct
Given a constant temperature current will be proportional to voltage. - wrong
dude could u explain wtf are u talking about? under that condition voltage follows current proportionally: CORRECT

I= V R: WRONG
Kholdstare
#35
Dec31-11, 09:57 PM
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Quote Quote by sophiecentaur View Post
?????
If you get your history right, you will realise that Ohm only introduced Resistance as the constant of proportionality for a linear relationshipbetween two quantities that he could actually MEASURE. They didn't sell Ohmmeters at the time!
So Ohm did introduce resistance as a constant of proportionality. I think the statement of Ohm's law will be something very similar to this:
"Given a constant temperature current through a material will be proportional to the voltage drop across it (in steady state)."

Now, Ohm's law is indeed true if we do not take into consideration the quantum effects.
In fact if you are able to measure I-V curve for a long conductor with very high accuracy or say you measured it exactly, you will discover that the I-V curve is never linear. The variation in resistance is laughably tiny, but it exists and due to QM effects. In nano-scale structures the variation is tremendous. In those cases the resistance depends on the applied bias.

Now, as Ohm's law is only a model to predict the behavior of real materials (just like QM is a model to predict the behavior of electrons - we don't even understand what electrons actually are, only know their mass, charge, observables etc.), the idea that Ohm's law only predicts the behavior of ideal conductors is bogus. If it did that we wont be needing it cause there is no ideal conductor.

My conclusion is that when it comes to accuracy, the I-V relationship derived from QM beats Ohm's law (both in large and small materials) proving Ohm's law does have its limits.
Kholdstare
#36
Dec31-11, 10:06 PM
P: 390
Quote Quote by Nevertamed View Post
dude could u explain wtf are u talking about? under that condition voltage follows current proportionally: CORRECT

I= V R: WRONG
You got me wrong there. I said,

I = [itex]\frac{V}{R(V)}[/itex]

But,

[itex]\frac{I_{2}}{I_{1}}[/itex] [itex]\neq[/itex] [itex]\frac{V_{2}}{V_{1}}[/itex]


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