# I'm confused about ohms law

Discussion in 'General Electronics Chat' started by suhasm, Apr 27, 2009.

1. ### suhasmNew Member

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Ah...thanks for the explanation. Now i get it....

2. ### ericgibbsWell-Known MemberMost Helpful Member

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Hi Al,
Interesting argument, but again your logic is flawed, you are making an 'assumption' that you know the value of the resistor beforehand.!

You cannot base a premiss on an assumption.

If I gave you an umarked resistor for 'your experiment', how would you determine the resistors value using only a current meter.?, you cannot.

If I used your assumtion process, I could say, if you know the value of the resistor, I know the diode is a 1N4148 for example,

From the datasheet for the 1N4148, I could tell the resistance of the diode at a know current.

You are taking a simple equation, derived from Ohms law and trying to disprove it, why.?

The 'mathematical' equation by definition must be true for R = V/I , if you know the values of V and I

3. ### unclejed613Well-Known Member

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one of the three quantities is not enough to solve for the other two unknowns. peeking into the black box before the experiment is cheating..... sorry,but if you have a black box problem, you MUST measure the only two quantities measureable from the outside of the box, voltage and current. knowing the resistance of the resistor, or even knowing the contents of the box before the experiment is cheating. if you were in my classroom and saw that your deduction was based on foreknowledge of the box contents, i would fail you on the spot..... with an unknown set of circuit elements in a black box and given an ammeter, voltmeter, and current or voltage source, you can know the resistance at any given operating point, and plot the results. you could deduce from those results that the behavior of the black box isn't linear, but without foreknowlwdge of the contents of the box, you could not tell me with certainty that there's even more than one component in the box, but you would be able to tell me the resistance at any given data point.

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5. ### ericgibbsWell-Known MemberMost Helpful Member

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hi jed,
If I understand you correctly, you are agreeing with me.?

6. ### RatchitWell-Known Member

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To the Ineffable All,

I have published this on other threads previously.

I would like to call your attention to a misnomer in electrical science of the formula V = I*R, which is wrongly called Ohm's law.

First I will show a couple of links.

http://maxwell.byu.edu/~spencerr/web...00000000000000

http://www.launc.tased.edu.au/online...Resistance.htm

Still not convinced? Don't blame you. One can publish anything the WWW.

OK, let's look next at a couple of good physics texts.

I will first quote from a college textbook called Physics, by Halliday & Resnick, 1967, page 780. It was written by David Halliday, Professor of Physics, University of Pittsburgh and Robert Resnick, Professor of Physics, Rensselaer Polytechnic Institute

-----------------------------------------------------------------------
"We stress the relationship V=I*R is NOT a statement of Ohm's law. A
conductor obeys Ohm's law only if its V vs. I curve is linear, that is, if R
is independent of V and I. The relationship R=V/I remains as the general
definition of the resistance of a conductor whether or not the conductor
obeys Ohm's law. . . . . . . . . Ohm's law is a specific property of
certain materials and is NOT a general law of electromagnetism, for example,
like Gauss's law."
-----------------------------------------------------------------------

Next a quote from another college textbook called Physics for Scientists & Engineers, by
Raymond Serway, Third Edition, 1990, page 745. It was written by Raymond A. Serway of James Madison University

-----------------------------------------------------------------------
"A current density J and an electric field E are established in a
conductor when a potential difference is maintained across the conductor.
If the potential difference is constant, the current will also be constant.
Very often, the current density in a conductor is proportional to the
electric field in the conductor. that is J=sigma*E where sigma is called the
conductivity of the conductor. Materials that obey the above equation are
said to follow Ohm's law, named after Georg Simon Ohm (1787-1854). More
specifically,

Ohm's law states that for many materials (including most metals) the
ratio of the current density and electric field is a constant, sigma, which is
independent of the electric field producing the current.

Materials that obey Ohm's law, and hence demonstrate this linear
behavior between E and J, are said to be ohmic. The electrical behavior of
most materials is quite linear for very small changes in the current.
Experimentally, one finds that not all materials have this property.
Materials that do not obey Ohm's law are said to be nonohmic. Ohm's law is
not a fundamental law of nature, but an empirical relationship valid only
for certain materials."
-----------------------------------------------------------------------

OK, what could be clearer? Ohm's law refers to the linearity between voltage and current, not relationship between voltage, current, and resistance. Yet the formula, V = I*R has been misnamed across countless classrooms, books and discussions. What does your textbook or school teach?

To summarize, certain materials like conductive metals follow Ohm's law, in that their V vs.I curve is linear. Ohm's law is a property of a material, not a general law of nature. Other conductive entities like diode junctions or gas discharge bulbs do not have the Ohm's law property, because their conductivity changes depending on what voltage or current is applied, causing their V vs. I curve to be nonlinear. In all cases, V = I*R is always correct, but it should not be called Ohm's law. It should be called the resistance or impedance formula.

No matter what you call V = I*R, circuits will still get designed and analyzed, and science will still progress.

Ratchit

7. ### MrAlWell-Known MemberMost Helpful Member

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Thanks for the post Ratchit. I was going to quote your quote here too soon so thanks for doing it now.

For the resistor that follows Ohm's Law
VR=I*R
and once we know the resistance R by ONE SINGLE MEASUREMENT we
can then calculate VR for NEW CURRENTS 'I' using OHMS LAW.

For the diode, even saying that
VD=I*RD
doesnt help in the same way because even if we know RD after
ONE SINGLE MEASUREMENT we still can not calculate a new VD
like WE CAN WITH THE RESISTOR.

THIS IS ONE REASON WHY THE RESISTOR IS SAID TO FOLLOW OHMS LAW
AND THE DIODE IS SAID TO NOT FOLLOW OHMS LAW.

For the diode, EVERY TIME you go to calculate the new VD you can
NOT DO IT every time, even after a single measurement.

For the resistor, you can calculate a new VR for any I after taking just
ONE SINGLE MEASUREMENT of both V and I.

Thus, with the resistor Ohm's Law allows us to calculate something that
YOU CAN NOT CALCULATE FOR THE DIODE KNOWING THE SAME
INFORMATION.

Who cares if we can find RD by measuring V and I for the diode? If
we can not calculate new V for new I then we are not using Ohm's Law.

Here is another way of looking at it...
If you had to store the formula for a resistor in the computer, it would
take three variables and you could calculate V for ANY resistor
for any I by simply replacing R with the resistors value.
With the diode, you need to take up more disk space because the
formula is much more complex involving logarithms.
Thus, the entropy of the two systems is MUCH DIFFERENT where the
required to completely characterize the diode than the resistor.

I am waiting for someone to tell me what a 'non ohmic' device is
(from one of the people that believe that a diode is ohmic).

Actually, if you look at the equation for the ideal diode and combine
it with a contact resistance you can see that there is no way in
heaven that the diode could be following Ohm's Law, because very
simply there is another more complicated term in the diode equation.
The equation for an idea diode is:
V=N*Vt*ln(i/IS+1)
but the non ideal diode looks like this:
V=N*Vt*ln(i/IS+1)+R*i

Now there is NO WAY you can say that:
V=R*i
is the same as:
V=R*i+N*Vt*ln(i/IS+1)

because after all there is added another COMPLETELY DIFFERENT term in addition to R*i.

Now if the diode follows Ohm's Law, how can the equation for the voltage contain another
completely different term? Simple: it does not follow Ohm's Law.

Try to graph a resistor using Ohm's Law, then try to graph a diode using the same equation
and you CAN NOT DO IT!!!

Now find some reference that agrees that a diode is ohmic or dont
keep trying to argue this point because there are no experts in the
field that agree with this point.

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I'm with Ratchit at this point, there is simply put no reason to continue this discussion. There's nothing left to be said.

9. ### MrAlWell-Known MemberMost Helpful Member

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Hello again,

Well, if you could find a couple of experts in the field that agree with your
but i dont see that ever happening.

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I'm not looking for experts in the field to agree with me MrAl. The equations related to Ohm's law are very useful in non-ohmic materials. The equations are often incorrectly associated with the term "Ohm's law" even if they are not in fact "Ohm's law". Again the entire 180+ post thread is based on simply a matter of language. That's why I did the duck thing =)

The entire Ohm's law conundrum as we've created it should only ever be mentioned in passing in text books, leniency should be given to people that quote Ohm's law out of context in relation to the equations as there is no point in beating this technicality into people, anyone curious enough to question it will look it up themself (as I did) and understand it, does it REALLY matter what we call it?

As Ratchit said, science and progress will go on regardless.

11. ### RatchitWell-Known Member

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MrAl,

That is what I am avering is not true. Ohm's Law is NOT V=IR . V=IR or V=IZ is the resistance formula or impedance formula, not Ohm's law. Ohm's law is a property of a material (current vs voltage linearity), not a method of calculation or definition of resistance or impedance of a component or circuit. Saying that V=IR is Ohm's law is propagating an entrenched misnomer.

Isn't that two measurements? Anyway it all boils down to linearity.

Well, you can calculate the diode current vs voltage using the diode equation. It won't be linear, and have the Ohm's law property, but it can be done.

The ease of calculation using a computer has nothing to do with Ohm's law. It is a property of resistance linearity and has nothing to do with how something is calculated.

That's right. The current vs. voltage plot of a diode proves it is non ohmic, because its plot is not straight line.

Ratchit

Last edited: May 9, 2009
12. ### MrAlWell-Known MemberMost Helpful Member

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Hello again,

Ratchit, you've taken my statements out of context and used them to argue
against some point which my statements never intended in the first place.
In fact you take one statement, disagree with it, then agree later in your own
post! Doesnt make sense.

To the other people who disagree that a diode is not ohmic:

My final words on this are that if anyone wants to take Ohm's Law for whatever
purpose they want to, go right ahead. Just dont try to convince me of YOUR
interpretation, and i cant recommend you wasting your time trying to convince
any professors in any universities either, but if you do want to do that that is
up to you. But, dont tell me, tell THEM. Also, you may wish to write to the
authors of most books that describe Ohm's Law and tell them of your theory.

The point is, if you truely believe this then please, by all means, write to the
various institutions that you disagree with and tell them about your findings.

Also, when you talk to newbies and you tell them your story about Ohm's Law,
you should also point out that there are no known institutions that agree with
you.

After presenting the huge amount of reference material and providing several
experiments for you to follow along with, you still disagree, so i wont talk
fine. But remember, when it comes to Ohm's Law, talk to the institutions from
now on. Thank you very much!

<MrAl has left the building>

Last edited: May 9, 2009
13. ### RatchitWell-Known Member

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MrAl,

Could you be a little more specific in regard to what statements? Especially the one where I disagree and later agree.

Ratchit

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No one at any point suggested a diode was ohmic MrAL, only that the equations which are derivative of Ohm's law CAN be successfully applied to a diode. I'll be using that particular turn of phrase for the rest of this thread to avoid confusion. The flexibility of calling the equations derived from Ohm's law "Ohm's law" is the actual problem, and it's no one in this thread that started that first, there are a lot of web sites that uses the derived ohm's law equations as if they were 'ohm's law' indscriminatly.

15. ### LeftyretroNew Member

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"I'll be using that particular turn of phrase for the rest of this thread to avoid confusion."

If this thread was a battery it would be the breakthrough we have all been waiting for.

Lefty

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For about the last week everytime I see this thread has a new addition a song pops into my head, for anyone that remembers it, it's from a Flinstones chewable vitamin commercial. "10 million strong, and growing"

17. ### MikebitsWell-Known Member

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I think the confusion arises by throwing in the term ohmic, which does not represent ohms law. From what I understand, ohmic refers to something that responds to E and I like a resistor. Of course a PN junction does not respond in this way. It is not a resistor.

Saying a resistor is ohmic and a diode is not is fine by me, but yet a diode still obeys ohms law, only the parameters for satisfying ohms law requires a bit more information. In the end the vdrop of the diode and current through the diode at a given time will still obey ohms law and give R.

18. ### LeftyretroNew Member

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Heck sometimes even I don't obey the law to the letter, why should Mr. Ohm be any different? Are we being too harsh on Mr. Ohm? I'm sure he was satisfied with just being able to share his great discovery and had no idea that the world would go and make it a law and all. If something else breaks his law now and then, I'm betting that he would be OK with it.

Ohm well, time to move on.

Lefty

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No Mike, all the 'authorative' sources I've read concur, a diode does not follow Ohm's law technically, Ohm's law as MrAL keeps repeating ad-infinim is not the ratio. Ohm's law only applies to metallic conductors at a fixed temperature, if it's not linear it's not Ohmic. There are however many websites out that refer to the equations directly as Ohm's law without putting those important catches in. The whole thing is it's a big argument over what do you call the equations themselves if not Ohm's Law. However the equations derived from Ohm's law when applied to non-linear devices will still calculate properly.

It's kind of like calling a tomato a vegetable (Technically it's a fruit)

Or calling a fork lift a tow motor, not sure anyone's gonna understand that one, but I think it's more pertinent to this confusion.

The 'tow motor' is a brand name for fork lift, I know many people that still call forklifts tow motors even if they're not tow motor branded. As a knowledgeable lift salesmen told me once it's kind of like walking down the road and calling every car you see a Ford or <insert your favorite car brand here>

Just look in the dictionary long enough and you'll find words that have more than one spelling and way more than one meaning. That's the way I look at the term "Ohm's law" however due to this thread I will always have to put that caveat (equations derived from Ohm's law) in any post I ever refer to Ohm's law ever again just so that MrAl there doesn't jump down my throat =\

Last edited: May 11, 2009
20. ### MikebitsWell-Known Member

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My argument is that not meeting the requirement to be ordained ohmic, does not preclude a material from satisfying ohms law. Ohmic is some obscure term invented after ohm law was written. And as such these two terms should be treated accordingly.

I should expect to see a ohmic term being inserted into an ohms law equation. Is there a numerical value for ohmic? Or is it simply a numberless term?

Can you find a ohmic meter anywhere?

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Actually yeah Mike it does, because Ohm didn't test anything 'non ohmic' his results were based off of directly measured values from his experiments, his math directly requires that linearity. Remember this was done over 150 years ago. I've linked the PDF for the translation I read, I didn't follow all the math you might have better luck with the math directly than me. The term 'ohmic' simply comes from what Ohm showed in his studies that at a fixed temperature a resistor of a known value will have a current linear with the voltage applied across it. Obviously it's a whole lot of 'fuzzy' in the definition though because no material is ohmic under all conditions, Ohm has been dead a long time so there's not exactly anyone to authoritatively answer the question anymore, and Academia doesn't always do the sensible thing.

Ohm wasn't even the first person to figure this out. Henry Cavendish came to the same conclusions himself but never published anything on it. It's also worthy to note that Ohm's law is only true for average current, thermal noise exists if the sampling period is extremely short.

If you want the short version just read the Wikiedia entry (the WHOLE thing) the requirements for linearity is about 3/4's the way down the page under "Linearity, nonlinearity, and small-signal ohmic approximations to linearity"

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