# Ohms Law

Discussion in 'Electronic Theory' started by mechie, Jan 15, 2003.

Not open for further replies.
1. ### BrownOutBanned

Joined:
Jun 10, 2009
Messages:
4,630
Likes:
60
Location:
Huntsville, Alabama USA
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!

Joined:
Jun 15, 2009
Messages:
813
Likes:
3
Location:
Kent, England
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:

Joined:
Jun 15, 2009
Messages:
813
Likes:
3
Location:
Kent, England
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.

Joined:
Jan 12, 1997
Messages:
-
Likes:
0

5. ### BrownOutBanned

Joined:
Jun 10, 2009
Messages:
4,630
Likes:
60
Location:
Huntsville, Alabama USA

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.

Joined:
Jun 15, 2009
Messages:
813
Likes:
3
Location:
Kent, England
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

Last edited: Jun 16, 2009
7. ### BrownOutBanned

Joined:
Jun 10, 2009
Messages:
4,630
Likes:
60
Location:
Huntsville, Alabama USA
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.

Joined:
Jun 15, 2009
Messages:
813
Likes:
3
Location:
Kent, England
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.

9. ### BrownOutBanned

Joined:
Jun 10, 2009
Messages:
4,630
Likes:
60
Location:
Huntsville, Alabama USA
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.

Last edited: Jun 16, 2009

Joined:
Jun 15, 2009
Messages:
813
Likes:
3
Location:
Kent, England

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...

Last edited: Jun 16, 2009
11. ### BrownOutBanned

Joined:
Jun 10, 2009
Messages:
4,630
Likes:
60
Location:
Huntsville, Alabama USA
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.

Last edited: Jun 16, 2009

Joined:
Jun 15, 2009
Messages:
813
Likes:
3
Location:
Kent, England
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.

Last edited: Jun 16, 2009
13. ### BrownOutBanned

Joined:
Jun 10, 2009
Messages:
4,630
Likes:
60
Location:
Huntsville, Alabama USA
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.

Show me a material that doesn't exhibit continuity.

14. ### BrownOutBanned

Joined:
Jun 10, 2009
Messages:
4,630
Likes:
60
Location:
Huntsville, Alabama USA
Look Georg Simon Ohm: The Discovery of Ohm's Law

And Here:

Georg Ohm

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

Last edited by a moderator: Jun 16, 2009

Joined:
Jun 15, 2009
Messages:
813
Likes:
3
Location:
Kent, England
Clearly we're not going to come to an agreement, so I'm just going to leave it at that.

[ET: REMOVED SENTENCE]

Last edited by a moderator: Jun 16, 2009
16. ### BrownOutBanned

Joined:
Jun 10, 2009
Messages:
4,630
Likes:
60
Location:
Huntsville, Alabama USA
[et: Removed sentence]

Last edited by a moderator: Jun 16, 2009
17. ### timsvbNew Member

Joined:
Jun 9, 2009
Messages:
26
Likes:
1
Location:
australia
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.

18. ### BrownOutBanned

Joined:
Jun 10, 2009
Messages:
4,630
Likes:
60
Location:
Huntsville, Alabama USA
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.

Last edited: Jun 16, 2009
19. ### timsvbNew Member

Joined:
Jun 9, 2009
Messages:
26
Likes:
1
Location:
australia
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.

20. ### BrownOutBanned

Joined:
Jun 10, 2009
Messages:
4,630
Likes:
60
Location:
Huntsville, Alabama USA
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.

Last edited: Jun 16, 2009
21. ### timsvbNew Member

Joined:
Jun 9, 2009
Messages:
26
Likes:
1
Location:
australia
me either.Just as well only lost two cents.