# I'm confused about ohms law

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

1. ### MrAlWell-Known MemberMost Helpful Member

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That's about as definitive a statement for Ohm's Law that has been
presented so far, and if i could i would quote it about 20 more times.
Thanks for the post eng, i wish you were here for the start of this
debate.

Key points:
1. V=RI is *not* a statement of Ohm's Law.
2. Obeys Ohm's Law only if R is constant.

To me, this is self evident because we would not have any rule about
anything if in fact it did not do anything. Also, we can not take the
stand that "any R in real life is not constant" because that is not how
we develop theoretical laws and formulas...if we did we wouldnt have
any laws because nothing behaves like its law says it does except
within some bounds of some kind. Theory comes before practicality.
We *apply* the theory to the practical case and accept some small
inaccuracies sometimes even though we dont explicitly define them.
If we did try to get perfect answers every time we would have to
reject all calculations done for anything we know of in real life.

A duck is a duck is a duck is a duck, but that's for more casual
conversation, not for a technically oriented debate where the
species comes into question. I had many types of fish in my life
and i learned the species of all of them. When i am talking to the
neighbor about fish it might be ok to just call them 'fish', but when
talking to experts in the field it makes the conversation more intelligent
to include the species.

So part of the background of how we talk about things in general is
who we are talking with, and here, in this thread, we are talking with
experts and we want expert opinion, not just casual conversation.
This is partly so we can convey the best possible information to the
newcomers in the field.

Sceadwian, i sympathize with you because, of all reasons, i once took
taking a wider perspective on what any rule is in general and doing
tons of lab work.

PS
The little cartoon was quite funny A nice addition if you ask me.

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3. R is under real world conditions never constant.
We have officially entered the black holes horizon. Reality ceased to exist... well, does it ever?
Please, scream on the way down we want to see what laws of physics we can break as we go!

Georg Ohm will be giving cookies and suckers out at the end as he deems fit.

If a duck is a duck is a duck is a duck is a more casual conversation, please again describe to me the exact point on the slope of a conductors VI curve that a device becomes or ceases to become ohmic. Because again not one single material in the known universe satisfies the requirements for Ohm's law if the slope is 1.

Last edited: May 8, 2009
3. ### MrAlWell-Known MemberMost Helpful Member

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Just a little curious here...did you even read my previous post at all?

4. ### DaveNew Member

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So then what is the limit of ohms law stability as a function of the slope of the VI curve of the conductor? The law itself does not exist at ALL unless that boundary is described, so you can't prove the 'law' itself even exists if you can't tell me the exact deviation from a slope of 1 in the VR curve where it becomes non-ohmic. Otherwise EVERYTHING you have said from this point and back is open to interpretation.

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6. ### unclejed613Well-Known Member

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Ohm was the person who first discovered and defined on paper this basic physical principal. the scientific community does not use the term "Law" loosely. if a principle is a very basic one they call it a law. if there are laws underlying the principle, it's termed a "principal" or a "theorem". when i was young, Bernoulli's principle was called Bernoulli's Law. since then there have been laws of fluid dynamics discovered that are more basic, and Bernoulli's principle is no longer a law of physics, but an interaction of more basic laws of physics (hence it's a "principle" not a "law").

also, using simple math one can take Ohm's law and define it 3 ways E/I=R, I*R=E, or E/R=I. it's the same mathematical relationship. the only thing that changes is the quantity you are solving for. to say that it's not Ohm's law because i'm not solving for R, is like saying that if i calculate the mass of matter turned into energy in a nuclear reaction using E/c²=m then i'm not using Einstein's equation, because it's not written in the classic form.

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Thank you so much unclejed that was very succinct. Very much at the core of what I feel is the point. Better so far by a lot than I have said at least.

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8. ### unclejed613Well-Known Member

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Ohm's LAW is a basic law of physics. any of the 3 quantities are solveable using the other two. that one or more of them is nonlinear is irrelevant. the relationship between the QUANTITIES exists whether a device is linear or nonlinear. that's what makes Ohm's law basic enough to be a Law. it defines the relationship between the quantities without regard to the properties of the materials in a device or circuit.

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But as far as Ohm's law's experiments went, it didn't prove those laws, it only SUGGESTED them.

10. ### Tesla23Member

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So if I modify my summary to be:
The true facts are:
1. in most homogeneous materials (i.e. no junctions), for all practical purposes V = IR where R is a function of temperature only
2. For any two terminal device with a voltage V across it and a current I flowing through it, you can calculate a quantity R = V/I that has the dimensions of resistance. Once you have determined R in this way, then V=IR and I=V/R hold as long as you don't change V or I.

Would anyone disagree with this?

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11. ### unclejed613Well-Known Member

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as we used to say in high school "close enough for rock n roll".....

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Tesla 1. is again wrong. There are no materials that are perfectly linear in relation to Ohm's law.

2 works for me.

13. ### Tesla23Member

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Sceadwian, 1 doesn't say 'perfectly linear' it says 'for all practical purposes'. I refer you to the reference below and the quote included. In particular the phrase There are very few (for example, some biological) materials to which the law is not usefully applied. Isn't this similar to 'most materials' and 'for all practical purposes'? Do you have examples in your work where deviations from V=IR are significant?

"Georg Simon Ohm and Ohm's Law" by Gupta, Madhu Sudan
IEEE Transactions on Education, vol. 23, issue 3, pp. 156-162

from which I quote:

VI. OHM'S LAW IN RETROSPECT
The term "law" has been used in the sciences for a variety of
types of results: from exact laws (like Coulomb's inverse
square law in electrostatics) that are at present considered
fundamental laws of nature, to approximate empirical relationships
(like Boyle's law for gases) that apply under idealized
conditions or over a limited range of parameter values. Ohm's
law is also an empirical relationship, but it is applicable in a
remarkably wide range of situations. If the qualifier "under
isothermal condition" is added to the law, it is experimentally
verifiable for metals from pA/cm2 to gA/cm2 [23], although
some conflicting evidence is also available [24]. It is also possible
to generalize Ohm's law to include the effect of temperature
rise caused by current flow; the resulting current-voltage
relationship then depends on the postulated mechanisms for
heat loss and is nonlinear [251. There are very few (for example,
some biological) materials to which the law is not usefully
applied.

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14. ### MrAlWell-Known MemberMost Helpful Member

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Hi,

Tesla, thanks for the quote.

Hi,

Sorry, that is not the same argument, but perhaps a good secondary 'law' to compare with what

E=mc^2, m=E/c^2, c^2=E/m,

are all usable whereas for a diode:

R=E/I

is the ONLY form that works, not the other two. Something that obeys Ohm's Law
can use all three equations.

You just disagreed with many universities and even disagreed with Prof. Saslow who indirectly added
to the proof that for Ohm's Law 'R' has to be constant.

You two guys still should tell us what your definition of 'ohmic resistance' is, and what other types

Ok sure...
If a resistor quacks like a diode it is no longer ohmic.

Ohmic is a relative term, relative to the significance of the circuit, but we never claim that
a diode is ohmic because it deviates wayyyy too far from the resistor.

BTW, a slope of 1 indicates the normalized slope. That is not the only slope that works unless you
first normalize the VI curve. In other words, every resistors VI curve normalizes to a slope of 1.

I am still waiting for YOUR definition of what an 'ohmic resistance' is and what it is not.

15. ### suhasmNew Member

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I can find the resistance of a ohmic resistor when you give me V and I.
I can do the same for a diode also.

I can find the drop across a resistor if you give me I and R.
I can do the same for a diode also.

I can find the current in a resistor if you give me V and R.
I can do the same for a diode also.

If the equation R=V/I works for a diode , then naturally all the other two derivations must work too.

16. ### MrAlWell-Known MemberMost Helpful Member

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Hi suhasm,

The problem is, i wont always be there to 'give' you the time of day,
much less what the resistance or any other quantity is...and that
is what Ohm's Law is for. Problem is, it doesnt work for diodes.
If you dont believe that then you should read back some posts to
the experiment i gave.

The experiment is where we have a voltmeter and ammeter and
we measure R of a resisitor and R of a diode at the same current.

The problem starts when we drop the voltmeter and it breaks
and we are no longer able to measure voltage, just current,
and we want data points for other currents for both devices.
The funny thing is, we can STILL calculate the voltage for
the resistor, using guess what, Ohm's Law, but we are then
unable to calculate the diodes voltage because we can no longer
read voltage. Thus, Ohm's Law helps us with the resistor but
it does not help us with the diode. This illustrates the 'ohmic'
vs the 'non ohmic' device or material.

Interesting screen name BTW

Last edited: May 9, 2009
17. ### MikebitsWell-Known Member

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The first time I hear a diode quack, I promise to go back to church. Or maybe Betty Fords clinic.

18. ### suhasmNew Member

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interesting 'screen' name ?
Thats my real name by the way - Suhas Mahesh

I'm from India.

19. ### MikebitsWell-Known Member

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Who's screen name are you referring to? By the way, nice blog site you have going.

20. ### suhasmNew Member

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I'm referring to my own screen name...It was intended to be a reply to post #175 by Mr.AI....

21. ### MrAlWell-Known MemberMost Helpful Member

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Hi again,

Oh ok, good. Nice site there too.

Back on subject, i found another indisputable 'experiment' that illustrates
Ohm's Law and what is not Ohm's Law with respect to diodes and resistors.

Again we start with a resistor and diode, where the resistor is in series
with the diode and the cathode of the diode is grounded and we have
a current generator connected to the top of the resistor. Thus, we have a
resistor biasing a diode into forward conduction.
The resistor is marked R1 and the diode is marked D1 (unspecified diode
but we know it is silicon) . We also have a current meter in series with
the resistor (which also puts it in series with the diode).
Now this time we want to find the voltage drop across the resistor and
the voltage drop across the diode and come up with a formula for each
that always works. Simple right?

Now since we know the current through the resistor we can calculate the
voltage across that device (VR) using Ohm's Law:
VR=I*R1
no problem, but we would also like to calculate the voltage across the diode
(VD).
Question is, if diodes obey Ohm's Law then how do we use Ohm's Law to solve
for the voltage across the diode???
An attempt like this:
VD=I*RD
fails because we dont know what RD is.

Now one could say that although we dont know what RD is we also dont know
what R1 is, so nothing works for either. The interesting thing however is that
VR=I*R
is a complete fomula, while
VD=I*RD
is not because we never know what RD is unless we already measured VD
and that would defeat the whole purpose of having a formula to compute
VD.

Last edited: May 9, 2009