It's not about an admition or a lack thereof, it's about a trivial question that doesn't warrent an answer, ie "If it's a constant, is it a constant." Being a constant means it never changes. What's so hard about that? I've answered your question about the linearity of coefficients. Time to move on.

__________________
You don't need a quadraphonic Blaupunkt -- you need a curve
ball.

 

Yeah, all those websited are written by physicits. NOT!
You'll only find Commumity College professors making claims so clearly wrong. You'll never hear a claim like that in advanced physics lectures.

__________________
You don't need a quadraphonic Blaupunkt -- you need a curve
ball.

 

Hello again,


Nope, universities.
This is FAR too simple for that kind of requirement anyway.

Let me ask you this then...

Why exactly don't you believe that in Ohm's Law that R must be constant?

Before you answer, please be aware that if you make R a variable as in:
y=x*r

then you have just allowed x and y to be any point on the xy plane, and that is
no more a statement of what this 'Law' is then stating what the xy plane itself is.
It wont help if we say, "pick any value for R you feel like picking, and that
is the law", because gee i could make y anything i want it to be for any x.
What kind of physical phenomenon could that possibly describe?
I guess we could say F=m*a right?

When we limit the 'Law' to R being a constant, we say something very
unique about all the possibilities of x and y.

So maybe i dont understand why you think R should be a variable to begin with,
so i ask you the question then:
"Why exactly don't you believe that in Ohm's Law that R must be constant?"

If you like, you can also tell me what law this is:
c=a*b
I would be very interested to hear your opinion about this too.

I also have taken the time to graph both v=i*r and v=i*R in the attachment.
Notice how the pic on the right seems to describe something very particular,
while the left side takes up the whole first quadrant.

Attached Thumbnails

 

 

Heh, you're trying to show a 3 dimential quantity on a 2-D graph. That's a very basic error, so it calls into question all your other mathamatical assertions. Look at the graph I provided earlier, that's the only reasonably you can graph that quantity, unless you use 3-d modeling software.

In physics, there are many, many, many phenomona that can't be graphed. That doesn't mean they are invalid.

It's clear you're trying every dodge you can think of. You might want to ask yourself why this takes to much to prove? A proff should be simple.

__________________
You don't need a quadraphonic Blaupunkt -- you need a curve
ball.

 

I can see you dont truely want to discuss this, just assert your (wrong)
opinion. You make assertion after assertion, yet dont answer the questions
you are asked. You wont even state exactly why you dont believe that
R has to be constant, because you know that will show exactly how and why
you are wrong.

We dont need 3d graphing because what we are talking about is much much
simpler than that. You're still just trying to complicate in order to avoid
commitment to any one argument.

You said r was a variable, and i showed you what happened when we allowed
it to be a variable. We end up saying something like F=m*a or c=a*b
rather than stating what Ohm's Law really is.
Yes, there are other relationships we might look at like v=i*r^2 or whatever,
but it again will not be Ohm's Law.

For the last time...

Ohm's Law is a very very SPECIFIC rule, that states the relationship between
current and voltage in a conductor, and that that relationship isnt just any ol'
relationship, it is the relationship of PROPORTIONALITY.
What is is not is: it is not a more general relationship like c=a*b, or F=m*a.

BTW i asked myself why this takes so much to 'prove', and i realized that first
of all i am not trying to prove anything because it has been proved long ago,
and second that it takes longer to get through to people who have closed
minds.


"In physics, there are many, many, many phenomona that can't be graphed.
That doesnt mean they are invalid"

Yes, and Ohm's Law isnt one of them because it can be graphed quite easily
for v and i, that's the whole point!

Answer some questions for a change? How do you think R varies if it is a variable?

 

Ohm's law can be graphed for vaiablble V and R. It does requre a 3_D graph to see this correctly. But I posted a good approximation for this relationship on an earlier post. You want to just ignor that, so you are the one who clearly doesn't want to discuss this issue. I don't need to answer your questions; I already have. Just because you write an upper-case letter in an equation, that doesn't make it a constant. A constant is a quantity that doesn't change, and we all know that R changes, in fact, there are infinite values for R that Ohm's law is valid for. V, I and R are all variables. All of those quantities can vary in an electric system. It's so elementary. It's been proven over and over in this thread, as well, it's been proven over and over that proportion does not requre that only one quantity be a variable. Also, it's been proven that each quantity of Ohm's law affects the other two, thus all are variables. You decide to ignor all of this and just continue to ask trivial or irrelevant questions. Look the thread over, man, all has been answered. You just keep asking after the answers have been provided.

__________________
You don't need a quadraphonic Blaupunkt -- you need a curve
ball.

 

In an earlier thread we managed to reach agreement by not referring to 'Ohm'. we seemed to be able to agree on:

The 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.

It seems that some folk learnt that (1) is Ohm's law and others learnt it as (2).

 

Thank you for the very cogent, clear, explanation of your thoughts. I see what you're getting at, now. Will continue pondering...

In terms of physical relationships... I guess "one of these things is not like the other" -- resistance is a physical *property* of the circuit, thus a coefficient (like coefficient of drag for a car, say), whereas voltage and current represent, well, a sort of energy state of the system ... or something.

So you're essentially saying that the most important focus of Ohm's Law is describing the relationship between electron flow and electric potential in a circuit? I'll buy that.

__________________
Michael Shimniok
http://bot-thoughts.blogspot.com/
Microcontrollers can solve world hunger, too!

 

Hi again,



It's a little harder to respond to three people with the same post, but
it's nice to see that some people are still interested in the real truth
behind Ohm's Law. I've answered the replies one at a time here.


BrownOut:
Well my friend feel free to show us your 3d graph. I'd be happy to see
what you are trying to say. If you are trying to say that R changes
in a conductor then sure, we all know it does, but that is not what
Ohm's Law is all about. Ohm's Law is about an idealized condition and
not only that, for some currents the law holds over several orders of
magnitude too. If we pump 3000 amps through a 10 foot piece of 22 gauge
wire it wont take long before its resistance shoots up to a million megohms
or more (he he) but that is not what Ohm's Law is about.
Still, if you feel that a 3d graph would help then sure post one for us.

BTW just to clarify, we know that R can change if we force it to change
on purpose. If we swap a 10 ohm resistor for a 20 ohm resistor certainly
things are going to change and we can claim that R had indeed changed,
but that is not what Ohm's Law is trying to say either. It is more about
using the SAME resistor during the experiments.

This might provide a clearer picture...
Ohm's conclusion went something like this:

1. Take a length of wire, pump 1 amp through it.
2. Measure the voltage across the wire ends from end to end. Lets say 3v this time.
3. Now if you increase the current to 2 amps which is 2 times the original current,
the voltage across the two ends will change by the same proportion: 2 times. Thus,
if we increase the current to 2 amps the voltage will rise to 6v for that piece of wire.
4. So the conclusion was that if you increase the current by 2 times the voltage
will also increase by 2 times, and if you instead increase the voltage by 2 times the
current will increase by 2 times.
5. The only way this can happen is if R of the wire is constant.
6. This also leads to the term "ohmic" which is used to describe objects that obey
this law of proportionality.

The converse, making r a variable, would mean 'ohmic' would have no meaning because
everything under the sun could be called ohmic.

A really good geometric analogy i think would be a circle. When R is constant
in the equation for a circle we get a really nice, perfect circle. If R was allowed
to vary however we would get everything BUT a circle, with many resulting shapes
totally unrecognizable. If we did experiments with v=i*r, we would find that
we could not recognize what kind of device we were experimenting with in some
cases, but if v ended up being proportional to i we would know right then and there
that we were working with a resistance like a wire (an ohmic conductor).


Tesla23:
What happens is some people learn the law by looking at V=I*R or similar, and never
get the real truth behind the experiment that led to the result of proportionality.
They are given the math first rather than the experiment first.
Please see the above 6 steps of Ohm's Law and see what you think about that.

shimniok:
Well, the most important part is that current and voltage are proportional.
This is why i tried to reach people through the door of proportionality rather
than trying to argue about Ohm's Law at first, which they have learned through
mathematics rather than outright understanding of the nature of the experiment
that led to the law. This is something that requires 'understanding' BEFORE
'mathematics', not mathematics before understanding. Mathematics can get us
very lost in space while understanding can lead to some wonderful mathematics.
I believe that reading over those 6 steps outlined above can help anyone gain
the understanding they need to get past the 'math before fact' paradox.

Objects that do not obey this rule are said to be 'non ohmic'.

Please take a look at those 6 steps outlined above and see if this makes
more sense after that.


Take care all...

 

Haveing a material that's 'ohmic' is not what the law is about. Here is a very simple illustration:

Take a regualted, adjustable power supply and connect a variable resistor.
Set up the power supply for constant volts.
Vary and volts and observe the current meter varying with the voltage.
Vary the resistance and observe the current meter varying inversly with resistance.
Ohm's law holds for both cases, hence Voltage, Current and Resistance are variables.

Now, set up the source for constant current.
Vary the current and observe voltage changes with current.
Vary the resistance and observe voltage changes with resistance (proportionally)
Ohm's law holds for all cases, hence Voltage, Current and Resistance are all variables.

__________________
You don't need a quadraphonic Blaupunkt -- you need a curve
ball.

 

You must have to realize how ridiculous this statement is? Engineers
for ages have been calling devices that obey Ohm's Law 'ohmic'.
Hence, Ohm's Law and 'ohmic' work the same. You need to do some
more reading im telling you! I'd like it if you could see this some day.


The thing is, i could do a ton of experiments to show just what you did:
that when you setup and perform experiments you have to get the
procedure down perfectly right in order to get the same results that
anyone else performing the same experiment would get.
You set up and performed a different experiment than Ohm did, hence
you are getting different conclusions. The conclusion that Ohm
reached is MUCH SIMPLER than that! It's a simple experiment!
Vary the voltage OR the current and see what happens to the other.
That's all there is to it.
It's funny too because i already talked about the case where we swapped
a 10 ohm resistor for a 20 ohm resistor or vice versa. We already know
that it holds for both resistors, but the experiments are independent of
each other. Yes, you can choose any conductor, but once you choose
the conductor you're stuck with it. You cant swap it out in the middle
of the experiment or you are doing something else.

If you dont want to believe this then at least show what you think
the definition of 'ohmic' is please?

 

You're not stuck with it. You're putting unnessary restrictions on the law. The law holds for the general case of variable resistance, as well as for the speicific case you want to badly to ram home. If you look at how the original experiments were actually performed, you would understand that you have it exactly backwards. Ohm didn't have a variable voltage source or a variable current source. All he had were wires of varying resistance. He connected each wire to his constant voltage source and measured the resulting current. Very similar to the first procedure I described.

"ohmic" referes to a material that behaves as in your simple, special case. However, the law holds for non-ohmic materials as well.

__________________
You don't need a quadraphonic Blaupunkt -- you need a curve
ball.

 

You got back here quick, too fast for me to post a picture that helps
to describe what ohmic really is :-)

You have to show me how the law holds for non ohmic devices.
I've drawn several non ohmic device curves (blue) in the pic
below, and in particular i would like you to show me how the
law holds for the curve numbered '2' in the diagram.

Here is the picture that shows an ohmic device curve (red) versus
several other curves (blue) for non ohmic devices. Note how the
red line shows a very specific relationship between current and voltage,
while the other curves can vary just about any way we feel like drawing
them. There has to be something different about the red line. What is
it? It is a straight line, while the others are true curves. It's slope
is constant, the blue curves slopes all vary.
We could draw thousands of blue curves that show possible other devices,
in fact surely an infinite number of them. The funny thing is, we could
use Ohm's Law to draw the red line, while we can not use Ohm's Law
to draw ANY of the blue curves. Thus, the red line shows an 'ohmic'
device, while the blue curves do not.

I agreed already that you can change the resistor to some other value
and see that the law still holds, but that is not proof that R is a variable
because in Ohm's Law when stated mathematically as v=i*R, R has to
be a constant in order to maintain the proportionality between v1,i1,
and v2, i2. In other words, you can not state that
"1 is proportional to 2"
you have to deal with at least four numbers as in:
"1 is to 2 as 3 is to 6",
or stated another way,
"the proportional ratio is 1:2"
which then implies that any two numbers must obey the proportion
"1:2".
You can also say that,
"4 is to 2 by the proportion 2:1"
but if you only state:
"4 is to 2"
you havent said anything.
In other words, the definition isnt complete until we consider more than
one set of numbers, as i have been trying to get across in this thread.

Ohm's Law does not hold for variable resistance because that causes
a loss of proportionality...
For a resistor of 1 ohm, a source of 2v, causes a current of 2 amps,
the proportion here is 1:1.
Changing the resistor to 2 ohms, same source of 2v, we get now
only 1 amp, so the proportion changed to 2:1, hence the 'device'
(that was fictitiously allowed to change from 1 ohm to 2 ohms)
changed from a proportion of 1:1 to a proportion of 2:1 so it is
not ohmic.
If we instead allow the definition which you want to assert is true,
we would have to call every device under the sun 'ohmic'. We
would loose the uniqueness of what an ohmic device is, even
have to through out the notion of an ohmic device altogether.

I would appreciated it if you would show me how the blue curves are
ohmic, and even more, if you would tell me about one device you have
encountered in life that is ohmic, and one that is not ohmic.

Attached Thumbnails

 

 

I never said the variable resisitance is ohmic. I siad ohm's law does not requre R to be a constant. Just because you can call some mateials 'ohmic' and others 'non-ohmic', that doesn't mean the law can be applied to one class and not the other. As evidence, I showed a graph of a real device several posts above for which Ohm's law holds although resistance was variable.

Ohm's law does not requre R to be constant, neither does it requre I V-I graph to be linear. You're imposing that restriction, and trying to force it. But the law holds for the general case of non-constant R.

__________________
You don't need a quadraphonic Blaupunkt -- you need a curve
ball.

 

Hello again,


I've talked to many engineers about this topic in my years and they all
say the same thing: the correct interpretation is with constant R.
The professors all teach that in universities too. All the books i have
say constant R is required. If R is to vary, then who is to say how it
should vary.

Since you wont believe anyone else all i can do at this point is wish you good luck
with your interpretation of Ohm's Law, so good luck to you...and i mean that too,
and hope that you continue (assuming you are now) to get some enjoyment out
of electronics, electricity and magnetism, and related.