Ohms Law

[BrownOut]

rubber does not demonstrate a linear distance of extension when increasing force is applied

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!

 

[giftiger_wunsch]

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:

Hooke's law only holds for some materials under certain loading conditions.

Rubber is generally regarded as a "non-hookean" material because its elasticity is stress dependent and sensitive to temperature and loading rate.

 

[giftiger_wunsch]

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.

 

[BrownOut]

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.

 

[giftiger_wunsch]

I disagree. Ohm's law isn't violated in any materials that I'm aware of.

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

 

[BrownOut]

But who said ohm's law has to graph to a linear function? For example, in a diode curve, the I-V characterists obey ohm's law for the properties at that particular point of the curve.

Having a non-linear function doesn't mean the law isn't holding.

 

[giftiger_wunsch]

Well ohm's law states that V is directly proportional to R, and I is inversely proportional to R. This means that the relationship is linear. V = IR holds for every situation of which I am aware, but as indicated by another member who quoted ohm's law earlier in this thread, V = IR is not ohm's law, it is simply the equation for resistance.

 

[BrownOut]

Don't be confused by the linear relationship AND the nonlinear resistance of materials. That would be like saying that the law of gravity doesn't hold in space, since you can float around.

As I've said, the linear ohm's law holds for any point on a non-linear resistence relationship.

And V = IR is ohm's law.

 

[giftiger_wunsch]

Don't be confused by the linear relationship AND the nonlinear resistance of materials. That would be like saying that the law of gravity doesn't hold in space, since you can float around.

As I've said, the linear ohm's law holds for any point on a non-linear resistence relationship.

And V = IR is ohm's law.

According to my impromptu research, ohm's law describes the relationship between voltage and resistance, and current and resistance; i.e. it describes these linear relationships and does not apply to components which do not have a linear relationship between these values.

Again, I disagree; V = IR is not the law, it is merely the formula for resistance. By definition, resistance is V/I Ω, so V = IR cannot be disputed; however ohm's law applies to ohmic components (i.e. the majority of components), and sums up the linear relationships between V and R and between I and R.

In short, I agree with EricGibbs and Ratchit.


As for the gravity analogy, that was a poor example. The ability to overcome a force doesn't show that the force doesn't exist, and I see no way in which that example was actually analogous to ohm's law...

 

[BrownOut]

Well, if we use your favorite wikipedia, it states:

Ohm's law applies to electrical circuits; it states[1] that the current through a conductor between two points is directly proportional to the potential difference or voltage across the two points, and inversely proportional to the resistance between them.

The mathematical equation that describes this relationship is:[2]

V = IR

That's ohms law. It doesn't get any more clear than that. Yes, it sums up the relationship of V,I and R, and the expression is that summation.

Of course the ability to overcome a force doesn't show the force doesn't exist. But in space, one doesn't overcome the force, one remains subject to the force of gravity, just as a material with nonlinear resistance remains subject to ohm's law.

 

[giftiger_wunsch]

By saying that two variables are directly proportional, it is saying that the two variables have a positive, linear relationship. Just as Hooke's Law approximates that the extension of a material is directly proportional to the force applied to it, which is not true for non-hookean materials, Ohm's law approximates that voltage is directly proportional to resistance, which is not true for non-ohmic components.

V = IR and variants are used to calculate V, I, or R from a single set of data; proportionality refers to a continuous function.

What is really being stated is that V(x) = IR(x) for a point x. This implies that current, I, is a constant, which we know is not necessarily the case.

 

[BrownOut]

Ohm's law in not an approximation. It describes the exact relationship for voltage, current and resisitance and is summed by the V=IR equation, just like the famous E=MC^2 relationsip sums up another famous law. Ohm's law is true for all materials, although the law is not material dependent.

proportionality refers to a continuous function

Show me a material that doesn't exhibit continuity.

 

[BrownOut]

Look Georg Simon Ohm: The Discovery of Ohm's Law

The relation V / I = R even holds also for non-ohmic devices, but then the resistance R depends on V and is no longer a constant

And Here:

Georg Ohm

The equation I = V/R is known as "Ohm’s Law".

I guess if you want to go around denying the laws of physics, then nobody can stop you. [ET: REMOVED SENTENCE]

 

[giftiger_wunsch]

Clearly we're not going to come to an agreement, so I'm just going to leave it at that.

[ET: REMOVED SENTENCE]

 

[BrownOut]

[et: Removed sentence]

 

[timsvb]

Been havin a look at your discussion.I'm a newbie too.Have a look at this site.
Ohm's law calculation calculator calculate magic triangle equation tip online voltage volts resitor resistance amps amperes audio engineering - sengpielaudio Sengpiel Berlin Seems you are both right,but pretty tight fitting to your ideas.S'pose the key word is "Non-Ohmic" components.Looks like others have sorted this out before.They have defined deals as non compliant with a very sound law.Ohms law is a very powerful tool,and if we forget our laws and "Rules of Thumb",we also dispense with our first port of call as far as any diagnostics are concerned.My two cents.

 

[BrownOut]

That's a mis-statement. Ohm's law holds for non-ohmic materials. I refer you again to this: Georg Simon Ohm: The Discovery of Ohm's Law

This is important. Later in your technical career, you'll have to understand concepts like Small-Signal Resistance. That concept demands that ohm's law holds even for non-linerar resistance materials, aka so called non-ohmic.

 

[timsvb]

This is from your link above."As stated above, this work included “Ohm’s Law” theory: The relationship of a current passing through most materials is directly proportional to the potential difference applied across the material."
Not out for a stouche, but have an open mind.Don't know whether "most" refers to what you guys were talkin about but "most" doesn't include everything.I'll keep it in mind as I further my career, and leave well enough alone for now. Thanks for the tip.
Regards tim from oz.

 

[BrownOut]

As I already said; that's a mis-statement. Ohm's law works for all materials. The law relates physical quantities, and is not dependent on material.

The author made a nice calculator applet, but he doesn't really understand the law.

 

[timsvb]

me either.Just as well only lost two cents.