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ohms law

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aquamon

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Sceadwian

Banned
aquamon, it's not as black and white as you might think, there were points of contention on every side, and a lot of misunderstanding and arguing as well that didn't help matters in the slightest.

Just a question, did you read all 68 posts?
 

Hero999

Banned
I'll give it one last try.

I don't see the big deal.

Ohm's law is
V = IR

V = voltage
I = current
R = resistance

Or any other transposition or the above.

R may vary as a function of the voltage and current (e.g. a non-linear load, such as a diode) but Ohm's law still applies.

Sorry but anyone who doesn't understand that, after an hour's reading about on the topic, doesn't have the intelligence required to be an engineer.

The people arguing against Ohm's law missed the point about R varying as a function of voltage and current which is the case in non-linear loads.

Going back to the diode example discussed above.

Suppose the voltage is 700mV when the current is 700mA, the diode's resistance will be R = V/I = 1Ω

Suppose the voltage increases to 1.4V when the current is increased to 2.8A, the diode's resistance will still be R = V/R which is now equal to 1.4/2.8 = 0.5Ω. The current has increased fourfold and the voltage has only doubled (which is what diodes do) so the resistance has halved.

Now the voltage across the diode is reversed, the voltage is -50V, the current is -50µA, the diode's resistance is R = V/I = -50/-50µA = 1MΩ. The signs changed to negative because the voltage and current are reversed, but the result and formula is the same.

The only confusing thing is sometimes people call non-linear loads non-Ohmic which implies they don't follow Ohm's law, despite the fact that they do. I suspect the reason why some people call linear loads non-Ohmic is because they can't simply measure their resistance with a DVM and use Ohm's law to calculate the current for a given voltage or vice versa.
 

Sceadwian

Banned
Hero999 said:
I don't see the big deal.

Ohm's law is
V = IR

V = voltage
I = current
R = resistance
Nope, that's not true =) That's what just about 99.99% of all people believe though. Ohm's law only has to do with the specific case of fixed pure metal resistors at known unchanging temperatures and low currents. Ohm's law is that V=IR but his experimentation only dealt with a very very limited data set. Because experimentally the law fails under various conditions because of things like ionic flow, eddy currents inside conductors and the changing state of a materials conductance under voltage and temperature changes. The V=IR and all it's derivatives are perfectly true for a great number of things, but the equations are not actually "Ohm's law"

In the real world those three little letters fail abysmally to describe electric and current flow in real materials, but on a macro scale it's generally true.
 
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Sceadwian

Banned
Sorry but anyone who doesn't understand that, after an hour's reading about on the topic, doesn't have the intelligence required to be an engineer.
No, anyone that believes that didn't actually read the original papers that Ohm wrote on the subject, I did because I was trying to prove MrAL wrong over this very same linguistic misunderstanding. Believe me, I had my foot in my mouth for a week over that one, though the arguement you bring up goes to show exactly my problem. Even though I was wrong so many people have such a misinformed understanding of what Ohm's Law actually is that in common language it has actually become more prevalent for people to be incorrect. Kind of the same way words change over time, they were used out of context so many times they garnered the new meaning.

The people arguing against Ohm's law missed the point about R varying as a function of voltage and current which is the case in non-linear loads.
Which again proves the point. The original paper that Ohm published that had the equations dealt ONLY with highly controlled linear loads. Non-linear loads do not obey Ohm's law, the equations that were derived from the papers that Ohm wrote are often referred to as 'the law' out of context of the original manuscript (which is available online by the way) I spent HOURS researching this. I even contacted a science professor which agreed with the details I'm ascribing to and said that anyone trying to argue about this too much was wasting their time and being petty basically. His reply really made me put my foot in my mouth =) S'a hard pill to swallow.
 
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Mikebits

Well-Known Member
original manuscript (which is available online by the way) I spent HOURS researching this.
Have the link still? The only version I found was in German.
 

Sceadwian

Banned
Hrrmm, you're gonna make me hunt it down again aren't you =)
I can do better than link it, I can post it (legally)
 

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Mikebits

Well-Known Member
Thanks, I will certainly read it. How did you find this?
 

Sceadwian

Banned
I'm not sure, read the original Ohms law thread linked in the first post here, I'm not sure how much is left in there, but there are so many posts it's hard to follow. You can spot the portion of the thread that I realized I was wrong pretty aptly I think =) Mind you it's only about 2/3rds from the end of the thread I think =)
 

Hero999

Banned
Ohm's law is that V=IR but his experimentation only dealt with a very very limited data set. Because experimentally the law fails under various conditions because of things like ionic flow, eddy currents inside conductors and the changing state of a materials conductance under voltage and temperature changes.
Sorry, you're wrong, the resistance of a material may be dependant on other factors but Ohm's law still applies, regardless of whether the load is linear or not, noise, eddy currents, temperature changes, semiconductor junctions all change their resistance in response to the current, environmental factors or often just randomly but Ohm's law still applies: the current and voltage are still proportional to the resistance.
 
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Mikebits

Well-Known Member
lol, ohms law thread, take 2.

 
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Hero999

Banned
Which again proves the point. The original paper that Ohm published that had the equations dealt ONLY with highly controlled linear loads. Non-linear loads do not obey Ohm's law, the equations that were derived from the papers that Ohm wrote are often referred to as 'the law' out of context of the original manuscript (which is available online by the way) I spent HOURS researching this. I even contacted a science professor which agreed with the details I'm ascribing to and said that anyone trying to argue about this too much was wasting their time and being petty basically. His reply really made me put my foot in my mouth =) S'a hard pill to swallow.
It appears you're arguing about semantics.

If you're talking about a non-linear load then of course you can't use Ohm's formula to calculate a resistance value, that can be used to calculate the current at different voltage because the resistance will change, which is what I think you're saying.

This doesn't mean the Ohm's law doesn't apply, it just means that R can be dependant on other factors: voltage and current being two of them.
 
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mneary

New Member
It appears you're arguing about semantics.

If you're talking about a non-linear load then of course you can't use Ohm's formula to calculate the resistance which can be used to calculate the current at different voltage because the resistance will change, which is what I think you're saying.

This doesn't mean the Ohm's law doesn't apply, it just means that R can be dependant on other factor: voltage and current being two of them.
I haven't finished reading Dr. Ohm's journal, but so far all I can find excludes nonlinear resistances. If you have completed reading it, perhaps you can direct us to the part where he includes nonlinear effects. I wouldn't think that ignoring Dr. Ohm would be constructive.
 

Hero999

Banned
I've just looked at the attachment, sorry I'm not interested enough in the subject to read a 270 page book on it so I'm leaving the discussion.
 

ericgibbs

Well-Known Member
Most Helpful Member

Hero999

Banned
hi,
This links section covering Linear Approximations explains the non ohmic situation.
Ohm's law - Wikipedia, the free encyclopedia
Yes, I've read and understood that but I've learned nothing new. It doesn't mean that Ohm's law doesn't apply at all V still equals IR, it's just that the resistance of a non-linear element just changes as a function of voltage or current as I explained a few posts ago.
http://www.electro-tech-online.com/threads/ohms-law.106871/#post875134

The part of the Wiki article discussimg Johnson–Nyquist noise doen't debunk Ohm's law any more than the random movement of particles in a liquid or gas above absolute zero violates the first and second laws of thermodynamics.
 
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