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.

 

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)

 

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.

 

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.

 

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

 

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

 

Better arrest rubber for breaking Hooke's law then

 

...ouch

 

Since when?

 

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 Please share

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Good riddance, ETO .

 

It doesn't? You mean rubber bands is just a big scam

I think the law holds as long as the elastic limit isn't exceeded.

I haven't studied Physics in a while either, since college. And I don't want to tell you how long ago that was!

 

Rubber bands are the spawn of an evil ploy to allow schoolchildren to propel rocks at high speeds towards unsuspecting panes of glass.

As for the law holding as long as the elastic limit isn't exceeded: I believe that applies for Hookean materials, but not for rubber, as it's a non-Hookean material.

Not meaning to rubber your face in it but wikipedia seems to agree with me:

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Good riddance, ETO .

 

Anyway I think the message to take away here is that like Hooke's Law, there are cases for which Ohm's Law does not account; such as a variable resistance as Chaerl and PhilDubya explained above. Ohm's Law makes the assumption that R is constant, and does not account for the variable resistance shown to a varying degree by all resistors.

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Good riddance, ETO .

 

I disagree. Ohm's law isn't violated in any materials that I'm aware of. The resistance of materials may vary depending on temp, E and M fileds, or other phenomina, but the law is consistant.

And just to split hairs, hooks law is based on elasticity, not materials. It holds as long as elasticity doesn't break down.

 

I suppose it depends on the definition of Ohm's law; two different definitions have been discussed in this thread. Some components, for example diodes or semiconductors, may show a non-linear relationship between voltage and resistance; most materials show an inversely-proportional relationship between current and resistance as suggested by ohm's law, but deviate slightly from this relationship.

If considering ohm's law to indicate a linear graph of V against R or I against R (as it seems to suggest from what I understand of the law), there are clearly cases where it is not true. In fact, similar to Hooke's Law, I believe there are components which are termed "ohmic" and those which are not, as they do not obey Ohm's Law.

Just throwing that idea out there

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Good riddance, ETO .