Ohm's Law

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mivey said:
... What is Ohm's law for a superconductor? What is Ohm's law for a carbon nanotube? ...
(SA answer) v = ir

I didn't know it didn't work for these. Although I did know that one has to be careful when r = 0. v/r = i looks real funny when both v = 0 and r = 0.

Anybody have a reference for a paper on these that show the model?

carl
 
coulter said:
(SA answer) v = ir

I didn't know it didn't work for these. Although I did know that one has to be careful when r = 0. v/r = i looks real funny when both v = 0 and r = 0.

Anybody have a reference for a paper on these that show the model?

carl
The point was that the original Ohm's Law fails for these. As it did for semiconductors. When Ferdinand Braun discovered the semiconductor in 1874, he found the crystal material did not follow Ohm's Law. The original law did not always hold true, but had to be expanded to include the new discoveries. The original law was for simple ohmic materials.
 
mivey -
I read the hypephysics links - like most of the posters here, the articles talk about ohms law doesn't work for for some particular substance/device because the current is not porportional to the resistance - huh? I thought resistance was defined as v/i. (yes the function blows up when v = 0 and i >0 - not discusing that yet) I don't recall any body teaching this concept, I think it comes from me not having a meter that measures ohms directly. All the meters I have send a current through the device, and measure the voltage. The resistance is calculated.

mivey said:
...The original law was for simple ohmic materials.

So where am I going? I think that when the term "ohms law doesn't apply" is used, what is meant is , "dv/di is not a constant"

carl
 
coulter said:
mivey -
I read the hypephysics links - like most of the posters here, the articles talk about ohms law doesn't work for for some particular substance/device because the current is not porportional to the resistance - huh? I thought resistance was defined as v/i.
But the resistance formula is not, in itself, Ohm's Law.

Also, it is not really a law in the strictest sense, but a limited observation of the voltage current relationship in certain materials.

Ohm's Law states the ratio of voltage to current through a conductor is constant if influences, like temperature, are held constant (we know this is true only in a limited range for a lot of materials) and that this ratio is independent of the voltage and current.

This formula describes the resistance of such a conductor, and while commonly called Ohm's Law, it is NOT Ohm's Law in the strictest sense: R = V/I. This formula becomes the general definition for resistance and has been expanded to cover non-ohmic material.

These formulas, while based on the concept, do not describe an ohmic material, and these materials do not necessarily follow Ohm's Law:
Z = v/i
Z = v(i)/i
Z = v/i(v)
R = v(t)/i(t)
R(t) = v(t)/i(t)
Z = Ldi/dt / Cdv/dt

coulter said:
So where am I going? I think that when the term "ohms law doesn't apply" is used, what is meant is , "dv/di is not a constant"
carl
The term might be "doesn't follow Ohm's Law" instead of "doesn't apply". So when we say a material "doesn't follow Ohm's Law", the resistivity is not a constant, independent of the voltage and current.

Again, the formulas are not Ohm's Law, but are resistance definitions or voltage/current relationships. They are Ohm's Law equations that can be said to describe Ohm's Law.

My physics book does call the formula E = pJ "Ohms law for a substance", where E is the electric field, J is the current density, and p is the resistivity for ohmic substances. It states that nonlinear substances are materials where E is not proportional to J.

I have several texts that refer to the V=IR formula as Ohm's Law but that does not make the use totally correct.

FWIW, I also have the BJT thermal version of the Ohm's Law equation: Tj-Ta = @ja * PD where Tj is the junction temp, Ta is the ambient temp, @ja is the thermal resistance, and PD is the power dissipation.

How about Ohm's Law from a material's standpoint?: s = q*n*u where s = specific conductivity, q = charge, n = density of charge carriers, and u = charge carrier mobility.
 
mivey said:
Also, it is not really a law in the strictest sense, but a limited observation of the voltage current relationship in certain materials.

Ohm's Law states the ratio of voltage to current through a conductor is constant if influences, like temperature, are held constant (we know this is true only in a limited range for a lot of materials) and that this ratio is independent of the voltage and current.
Who says that resistance has to be a constant? Put your engineering hat back on, it seems to have slipped off to the side a little. As with all scientific equations, all parameters may be represented as either a simple expression, or as a complex variable. If you held that this was not true, then you would be forced to also state that Ohm's Law is valid only for non-varying DC circuits, because you would be forcing the parameters to be fixed. In for a penny, in for a pound, so to speak.

mivey said:
This formula describes the resistance of such a conductor, and while commonly called Ohm's Law, it is NOT Ohm's Law in the strictest sense: R = V/I. This formula becomes the general definition for resistance and has been expanded to cover non-ohmic material.
If given the same argument, then we need to throw away 90% of the applications for Newtonian Physics too. If "F=ma", then Newtonian Physics completely fail us for a shuttle launch because the mass is not a constant as fuel is burned.

You are taking these equations too literal, and I know that your engineering education has taught you otherwise. Just because an equation is represented by a simple alpha character does not preclude it from being more complex. It is no different than the common acceptance of "V=120 volts", when in fact we really mean v(t)=Vsin(wt).

mivey said:
These formulas, while based on the concept, do not describe an ohmic material, and these materials do not necessarily follow Ohm's Law:
Name one material that does not follow Ohm's Law (and that includes superconductors). If R=0, then for any "I", V=0. For any V>0, I becomes infinite. You seem to have gotten hung up on the term "Ohmic material", and drew the conclusion that if it was not an Ohmic Material, then it doesn't follow Ohm's Law. This is not what your own hyperlinks stated, and it is not what Ohm's Law states.

Don't confuse the difference between atomic-level physics and macroscopic physics. Newtonian Physics do not apply at the atomic level, but that doesn't mean we need to throw away Newtonian physics. Likewise, atomic-level physics do not apply at the macroscopic level either. And neither does relativistic physics apply at non-relativistic speeds.
 
bob said:
Thats exactly what happens in a DC circuit or an AC circuit with a resistive load. As stated ohms law rules Amps = Voltage/resistance. If the resistance is constant, the amps will increase with the voltage


I believe that idea comes from the electrical motor characteristics. The motor current tends to go up if the voltage decreases assuming the torque is constant.

I disagree, bob. If you take a particular motor and apply X voltage it will draw X current. Increase the voltage without rewiring or changing the motor and the current draw will increase until the motor overheats. I doubt it would be proportionally linear though.

Bob
 
bthielen said:
If you take a particular motor and apply X voltage it will draw X current. Increase the voltage without rewiring or changing the motor and the current draw will increase until the motor overheats.
Actually, within a range, the current will drop as voltage increases. Check the current ratings on a 208-230v motor.

I lost the will to thoroughly read this thread a long time ago (in a galaxy far, far away), but isn't it still true that, in a given instant in time, the current through a circuit, device, or node will be the voltage divided by the impedance?

If one volt will push one amp through one ohm, again ignoring moment-to-moment changes, meaning not looking for linearity as the voltage slides, but merely a snap-shot moment in time, won't Ohm's Law apply, as a simple mathematic operation?

In other words (like I left out any), aren't any and all factors that affect current flow considered part of the total impedance, and thus the formula does work? If not, that's what I missed.
 
While Ohm's law as described by Georg Ohm in 1827 http://www.sparkmuseum.com/BOOK_OHM.HTM might not apply to these materials, various equations that look a heck of a lot like Ohm's law are still useful and usefully applied. The original concept has been extended as our knowledge of the world has increased.

For things like LEDs, for example, the concept of 'incremental resistance' is very useful. Rather than using R = E/I , you use dR = dE/dI. Looks pretty similar to the original, but not quite.

Consider a circuit where, if you apply 7.5V, 1A of current will flow, but if you apply 8V then 2A of current will flow, and at 8.5V then 3A will flow. It might be reasonable to say that the circuit has an impedance of 4 Ohms (at 8V), but that doesn't really help you predict anything. However the incremental resistance at 8V is 0.5 ohm, and you could predict the current that would flow given an applied voltage that isn't 7.5, 8, or 8.5V.

-Jon
 
080627-1839 EST

Larry:

Impedance is a measure of values under steady-state conditions.

For transient conditions you would analyze the circuit operation with a differential equation.

If we have a battery, a switch, a resistor, and a capacitor. The initial state of the several components will be known or assumed at the instant before the switich is closed. No current is flowing so the voltage drop across the resistor is zero. The capacitor will have some voltage depending upon its stored charge. This might be zero and is often the initial condition assumed.

Instantaneously when the switch is closed the current thru the resistor is Vbat/R and the capacitor voltage has not yet changed. It takes time for charge to flow into the capacitor. After switch closure the capacitor voltage rises exponentially determined by the valuse of R and C. Obviously the resistor voltage drops exponentially. At any instant the sum of all the voltages around the loop are zero.

Under different conditions at some fixed frequency with a steady state sine wave excitation we can define an impedance for the series RC circuit.

For transient conditions with AC excitation we would go back to using the differential equation approach.

.
 
LarryFine said:
Actually, within a range, the current will drop as voltage increases. Check the current ratings on a 208-230v motor.

Larry, I think the name plate provides the FL current at rated load. In that case, the current would be greater at the lower voltage. However, if we merely increase the voltage with a load such as a fan or pump, the power output and the current go up as well.
 
Rick Christopherson said:
Who says that resistance has to be a constant? Put your engineering hat back on, it seems to have slipped off to the side a little.
Our electrical forefathers said it, within the parameters defined in Ohm's law. Nothing wrong with my hat, just something wrong with your history.
Rick Christopherson said:
If given the same argument, then we need to throw away 90% of the applications for Newtonian Physics too. If "F=ma", then Newtonian Physics completely fail us for a shuttle launch because the mass is not a constant as fuel is burned.

You are taking these equations too literal, and I know that your engineering education has taught you otherwise. Just because an equation is represented by a simple alpha character does not preclude it from being more complex. It is no different than the common acceptance of "V=120 volts", when in fact we really mean v(t)=Vsin(wt).
Don't go overboard. My engineering education taught me to pay attention to the details as well. I have stated before that the original law has been expanded as we learned more about electricity. I don't take issue with any of the expanded representations of the V,I,Z relationship and I use them regularly.

The point is that the V=IR equation can represent Ohm's law if you pay attention to the detail that surrounds the formula. In our expanded version, we have more complex representations of "R". I have not said anything about throwing out any formulas.
Rick Christopherson said:
Name one material that does not follow Ohm's Law (and that includes superconductors). If R=0, then for any "I", V=0. For any V>0, I becomes infinite. You seem to have gotten hung up on the term "Ohmic material", and drew the conclusion that if it was not an Ohmic Material, then it doesn't follow Ohm's Law. This is not what your own hyperlinks stated, and it is not what Ohm's Law states.

Don't confuse the difference between atomic-level physics and macroscopic physics. Newtonian Physics do not apply at the atomic level, but that doesn't mean we need to throw away Newtonian physics. Likewise, atomic-level physics do not apply at the macroscopic level either. And neither does relativistic physics apply at non-relativistic speeds.
No confusion here, just grounded in historical fact. Don't be scared of the phrase "Ohmic material", it is a concept that has been around since the days of G.S Ohm. It mearly means a material that follows Ohm's law.

These are not my conclusions, but the conclusion of our founding fathers. And it is what "following Ohm's law" was all about. The materials that followed Ohm's law were those where the current in the material was a linear function of the voltage. You might not like the history or the science, and as for that, I really don't know what to tell you.
 
winnie said:
While Ohm's law as described by Georg Ohm in 1827 http://www.sparkmuseum.com/BOOK_OHM.HTM might not apply to these materials, various equations that look a heck of a lot like Ohm's law are still useful and usefully applied. The original concept has been extended as our knowledge of the world has increased.

For things like LEDs, for example, the concept of 'incremental resistance' is very useful. Rather than using R = E/I , you use dR = dE/dI. Looks pretty similar to the original, but not quite.

Consider a circuit where, if you apply 7.5V, 1A of current will flow, but if you apply 8V then 2A of current will flow, and at 8.5V then 3A will flow. It might be reasonable to say that the circuit has an impedance of 4 Ohms (at 8V), but that doesn't really help you predict anything. However the incremental resistance at 8V is 0.5 ohm, and you could predict the current that would flow given an applied voltage that isn't 7.5, 8, or 8.5V.

-Jon
Agreed. We certainly have expanded on the original concept.

It is interesting to look back and see what some of these guys spent their time on. They wrote entire book sections on the true measure of resistance. We take for common knowledge the concepts and standards that they developed over years of intensive work.

Things like series and parallel resistance seem ho-hum to us but this was cutting edge stuff for some of these guys. G.S. Ohm was not even taken seriously at first. Can you imagine having to prove Ohm's law was true with the equipment they had? It is really some interesting reading (I guess if you are a nerd).

Ohm's Law was really established earlier by Henry Cavendish in 1781, but was independently discovered by Ohm in 1827.
 
Ranch said:
Ya think ......
As the OP referred to Ohm's law: If he meant a resistor, then of course they change simultaneously. If he meant some of the devices and loads we have today that use an expanded version of Ohm's law, the answer may not be as straightforward.
 
winnie said:
Consider a circuit where, if you apply 7.5V, 1A of current will flow, but if you apply 8V then 2A of current will flow, and at 8.5V then 3A will flow.
Doesn't that merely mean that the impedance changes, and it's 7.5 ohms @ 7.5v, 4 ohms @ 8v, and 2.833... @ 8.5v?

It might be reasonable to say that the circuit has an impedance of 4 Ohms (at 8V), but that doesn't really help you predict anything.
Why must the behavior be predictable (i.e., linear) for Ohm's Law to apply at any given instant, or any given voltage?

However the incremental resistance at 8V is 0.5 ohm, and you could predict the current that would flow given an applied voltage that isn't 7.5, 8, or 8.5V.
That I don't understand. Incremental resistance?
 
mivey said:
As the OP referred to Ohm's law: If he meant a resistor, then of course they change simultaneously. If he meant some of the devices and loads we have today that use an expanded version of Ohm's law, the answer may not be as straightforward.

So true. Mine is a world filled with VFD's and such. It's really been quite a thread

That Rick and Mivey stuff was great - imagine those two taking off with Maxwell's Equations
 
gar said:
If we have a battery, a switch, a resistor, and a capacitor. The initial state of the several components will be known or assumed at the instant before the switich is closed. No current is flowing so the voltage drop across the resistor is zero. The capacitor will have some voltage depending upon its stored charge. This might be zero and is often the initial condition assumed.
Okay so far. I understand RC time constants. (I am 'into' electronics.) I'm with you.

Instantaneously when the switch is closed the current thru the resistor is Vbat/R and the capacitor voltage has not yet changed. It takes time for charge to flow into the capacitor. After switch closure the capacitor voltage rises exponentially determined by the valuse of R and C. Obviously the resistor voltage drops exponentially. At any instant the sum of all the voltages around the loop are zero.
That's simple enough. The VD of the resistor drops as the cap charges and approaches steady-state voltage. The circuit is behaving as if the capacitor's impedance rises as it approaches the supply voltage.

When the cap is at full voltage, it behaves as if it is an open in the circuit, or of infinite impedance.
 
Ranch said:
So true. Mine is a world filled with VFD's and such. It's really been quite a thread

That Rick and Mivey stuff was great - imagine those two taking off with Maxwell's Equations
Then I would win because I have referenced what Maxwell has said about Ohm's law. I did open a class on power factor with a review of Maxwell's equations (as a joke, of course).:grin:
 
Not so Larry,

Not so Larry,

LarryFine said:
Okay so far. I understand RC time constants. (I am 'into' electronics.) I'm with you.

That's simple enough. The VD of the resistor drops as the cap charges and approaches steady-state voltage. The circuit is behaving as if the capacitor's impedance rises as it approaches the supply voltage.

When the cap is at full voltage, it behaves as if it is an open in the circuit, or of infinite impedance.

Larry, it is misleading to say the impedance of the cap changes. the reactance of the cap is always 1/(2*pi*f*C) regardless of the charge on the cap.
 
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