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More Ohm's Law debate
- Thread starter blazedaces
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blazedaces,
Ratch
http://forum.allaboutcircuits.com/showthread.php?t=11674&highlight=ohm's+law
Your curiosity should be satisified by the link below.
Ratch
http://forum.allaboutcircuits.com/showthread.php?t=11674&highlight=ohm's+law
Mine wasn't. Good luck in your quest to downgrade Ohm's law. It might succeed, after all, Pluto was downgraded from a planet by international decree.
Fourier's view of homogeneity and "electrical resistance"
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I am suprised at such a juvenile discussion.
Many workers who carried out important work have been honoured by having an equation, a law, a unit of measurement, a proposition, conjecture or theorem named after them.
In some cases the worker was directly responsible for the subject eg Godel's theorem and no other worker was involved.
In other cases the worker was not actually responsible for the theorem but carried out work recognised to be important in the same field.
Ohms law and the Ohm as a unit of measurement falls into this category, as does the Siemen as a unit of conductance and the Pascal as a unit of normal stress.
Sometimes the wrong guy has been fingered eg Laplace was not responsible for his famous transform, Heaviside was.
Sometimes lesser men, as in Joe's link wish to catergorise rather than add new work, and they get things wrong.
We would all be better employed striving to put forward one new molecule of knowledge, rahter than reorganising the pack already provided by forerunners and trying to rename the cards in it.
Many workers who carried out important work have been honoured by having an equation, a law, a unit of measurement, a proposition, conjecture or theorem named after them.
In some cases the worker was directly responsible for the subject eg Godel's theorem and no other worker was involved.
In other cases the worker was not actually responsible for the theorem but carried out work recognised to be important in the same field.
Ohms law and the Ohm as a unit of measurement falls into this category, as does the Siemen as a unit of conductance and the Pascal as a unit of normal stress.
Sometimes the wrong guy has been fingered eg Laplace was not responsible for his famous transform, Heaviside was.
Sometimes lesser men, as in Joe's link wish to catergorise rather than add new work, and they get things wrong.
Fourier appreciated (as did Hooke) that it is perfectly normal to have strain without stress.
We would all be better employed striving to put forward one new molecule of knowledge, rahter than reorganising the pack already provided by forerunners and trying to rename the cards in it.
JoeJester,
I am not downgrading Ohm's law. I am just pointing out what it really is, the electrical linearity of a material. Pluto was just properly categorized lately, not downgraded.
Your link to the article about Fourier still repeats the same mistake in equating the resistance/impedance formula to Ohm's law, instead of recognizing it for what it really is.
Ratch
Yes, it was. You might not agree with my averment, but your curiosity was satisfied. Curiosity is not agreement.
I am not downgrading Ohm's law. I am just pointing out what it really is, the electrical linearity of a material. Pluto was just properly categorized lately, not downgraded.
Your link to the article about Fourier still repeats the same mistake in equating the resistance/impedance formula to Ohm's law, instead of recognizing it for what it really is.
Ratch
thingmaker3
- Joined May 16, 2005
- 5,083
And such is true regardless of whether the formula is known as "George Washington's Birthday" or "Whistler's Mother" or "Ohm's Law." You've read Shakespeare's Romeo and Juliet have you not? (Do so if you haven't. You might like it. And yes, I've heard of Bacon.)
I am quite curious what name you, Ratch, would give to the formula in question. Would you call it "The Artist Formerly Known As Prince?"
thingmaker3,
Ratch
The name should be somewhat descriptive of what the formula does. Most of the names you suggested above are not. Ohm's law should not be used to name a method of voltage/current/resistance calculation because its name is already reserved for the material property of electrical linearity.
No, I have already called it the "resistance formula" or "impedance formula" many times.
Ratch
thingmaker3
- Joined May 16, 2005
- 5,083
Really? Golly! What formula would that be? Are you willing to share it here?
thingmaker3
- Joined May 16, 2005
- 5,083
Ohm Mani Padme Ohm!! 
thingmaker3,
You appear to be asking questions and making comments as though you are unfamiliar with what this thread is discussing. Have you read the link including the links from the two websites and the quotes from two highly regarded physics textbooks?
Ratch
Now you are being indefinite. Were we not talking about V=IR? Or did you mean the electrical linearity of a material? If so, that is a property described by Ohm's law that has no formal formula. It is a direct proportion.
You appear to be asking questions and making comments as though you are unfamiliar with what this thread is discussing. Have you read the link including the links from the two websites and the quotes from two highly regarded physics textbooks?
Ratch
V=IR applies where we have resistive conducting material with a current through it. R can be a function of a variable current or voltage, i.e.
\(R = \frac{\Delta V}{\Delta I}\)
This is the case for a diode where the resistance of the diode is a function of the voltage across it.
Dave
\(R = \frac{\Delta V}{\Delta I}\)
This is the case for a diode where the resistance of the diode is a function of the voltage across it.
Dave
Dave,
Ratch
Indeed, that is true. It is known and mentioned in the physics textbook which I quoted that devices like semiconductors and gas discharge tubes are not ohmic because they do not follow Ohm's law. In other words, their V vs I is not directly proportional, thereby making their resistance variable depending the V or I value. The resistance/impedance formula V = IR or IZ is always correct, but that formula should not be called Ohm's law.
Ratch
thingmaker3
- Joined May 16, 2005
- 5,083
Let's try this again. (And I do apologize for being vague.)
What is "Ohms Law" already the name of? The "material property of electrical linearity" you say? In my admittedly limited experience, I've never run across a material property being called a "law." I usually find them being called "material properties." You seem to have a different experience. Please share it.
Who knows? Your po-tah-toes might well taste as good as my po-tay-toes.
thingmaker3,
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."
-----------------------------------------------------------------------
Ohm's law is the name of the way the electrical resistance/conductance properties of certain materials behave. I will share it by quoting again from the two physics textbooks referenced in the link I gave earlier. The first reference says it just about as clearly as the King's English allows. Be sure to catch the sentence about Ohm's law being a property of a material.
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."
-----------------------------------------------------------------------
To quote the man himself (ok, so he said it in German):
\(I = \frac{V}{R}\)
Now it is correct to say not all devices obey this law for their V-I characteristic, diode being an obvious example where the I-V characteristic (note the switch from V-I to I-V) is defined by the non-linear Ebers Moll relationship.
There is an important point to note - even non-linear devices display Ohmic behaviour - consider the diode which has an exponential I-V characteristic, that is as the voltage across the diode changes (ΔV) so the current varies non-linearly in an exponential manner. If we take the condition that ΔV tends to 0, i.e. we are look at the I-V characteristic locally we see that device behaves Ohmically - that is the resistance at that point, R = V/I. This is the same as saying the resistance is the reciprocal of the derivative of Ebers Moll defined at a voltage V where ΔV is zero. Therefore locally, diodes like all electronic devices, obey Ohms Law.
Dave
This characteristic is embodied in the equation:
\(I = \frac{V}{R}\)
Now it is correct to say not all devices obey this law for their V-I characteristic, diode being an obvious example where the I-V characteristic (note the switch from V-I to I-V) is defined by the non-linear Ebers Moll relationship.
There is an important point to note - even non-linear devices display Ohmic behaviour - consider the diode which has an exponential I-V characteristic, that is as the voltage across the diode changes (ΔV) so the current varies non-linearly in an exponential manner. If we take the condition that ΔV tends to 0, i.e. we are look at the I-V characteristic locally we see that device behaves Ohmically - that is the resistance at that point, R = V/I. This is the same as saying the resistance is the reciprocal of the derivative of Ebers Moll defined at a voltage V where ΔV is zero. Therefore locally, diodes like all electronic devices, obey Ohms Law.
Dave
From this source - http://science.jrank.org/pages/2324/Electrical-Conductivity-Non-ohmic-conductors.html - we see that:
Non-ohmic conduction is marked by nonlinear graphs of current vs. voltage. It occurs in semiconductor junctions, electrolytic solutions, some ionic solids not in solution, ionized gases, and vacuum tubes. Respective examples include semiconductor p-n diodes, battery acid or alkaline solutions, alkali halide crystals, the ionized mercury vapor in a fluorescent lamp, and cathode ray tubes.
Given these examples, how does this affect our understanding of Ohm's law? It really does not seem to have been invalidated by the above, nor must we pay careful attention to the exceptions when calculating voltage drops in a circuit.
What is the point of scrupulously pointing out that Ohm's law is not absolutely universal? As I have said before, this is not a physics - I think I said 'condensed matter' previously - seminar.
Non-ohmic conduction is marked by nonlinear graphs of current vs. voltage. It occurs in semiconductor junctions, electrolytic solutions, some ionic solids not in solution, ionized gases, and vacuum tubes. Respective examples include semiconductor p-n diodes, battery acid or alkaline solutions, alkali halide crystals, the ionized mercury vapor in a fluorescent lamp, and cathode ray tubes.
Given these examples, how does this affect our understanding of Ohm's law? It really does not seem to have been invalidated by the above, nor must we pay careful attention to the exceptions when calculating voltage drops in a circuit.
What is the point of scrupulously pointing out that Ohm's law is not absolutely universal? As I have said before, this is not a physics - I think I said 'condensed matter' previously - seminar.
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