Ohm's Law

Status
Not open for further replies.
Coulter,

< i(t) = v(t)/R
< In a reactive circuit, current leads or lags the voltage--not simultaneous

Interesting point.

Ohms law seems to be a point in time.

In Calculus, you relate the equation to /dt
and then see that nothing is simultaneous.
In physics, Everything is in contant flux.
 
Coulter,

I like your comments, emphasizing the Physics of Reality.
I agree basically for Resistive Devices.

But
< diodes, SCRs
are not resistive devices.

Here Diode R(impedance) is related to /dv (change in voltage).
For a linear increase of Voltage, the forward current through a diode
may increase along the lines of an anti-log (exponential) curve.

And, to boot, as I recently was reminded (gently)
by some engineers at Linear Technologiy's LTspice forum,
that the Substrate Impedence is an additional factor
in the apparent (useable) length of the Exponential curve
of Diode forward current. We were considering Log Amps.
To wit, the substrate plays an additional non-linear part in current flow.

SCRs depend on the gate bias, again not a linear function.
And, as I was instructed,
the substrate plays a non-linear part in current flow.

I like your comments, emphasizing the Physics of Reality.
I agree basically for Resistive Devices.
 
Coulter,

I expect you to come back and remind me that
"Diode R(impedance) is related to /di (change in current)

And I may come a little short on a response,
since those were my student days 30 years ago.
 
Ohm's Law

"Ohm's law cannot be applied to alternating-current circuits since it does not consider the reactance which is always present in such circuits."

Not to nit-pick, but must take exception to the statement above.

Consider an audio oscillator with a 600-ohm resistor across its output terminals.

There is no inductance, no capacitiance, and thus no reactance in this circuit.

While I realize this forum is primarily about 'power' circuits, there are AC circuits without reactance.
 
glene77is said:
My understanding is that the Fields of Force on Copper wire
are nearly instantaneous,
from one end of the wire to the other.

My understanding is that electrons move in Copper
at a rate of some millimeters per second,
from one end of the wire to the other.
Click to expand...
If you have a garden hose filled end-to-end with marbles, and force one more into one end of the hose, a marble will pop out of the other end instantly, yet all of the marbles themselves only moved a distance equal to the diameter of a single marble.

The effect of current moves through the conductor at the speed of light, regardless of the speed of an individual electron.
 
I expect you to come back and remind me that
"Diode R(impedance) is related to /di (change in current)...
Click to expand...
glene77is said:
...Linear Technologiy's LTspice forum,
that the Substrate Impedence is an additional factor
in the apparent (useable) length of the Exponential curve
of Diode forward current. ...
....
Click to expand...
No, I really wouldn't have any knowledge about phenomena reacting to the physics of "substrate impedance".

glene77is said:
... SCRs depend on the gate bias, again not a linear function. ....
Click to expand...
Up until today, I likely would have considered SCR impedance after it was turned on - knowing that it was not a linear function.

carl
 
080624-2002 EST

Ohm's law expressed as a mathematical equation is a way to describe a physical process. Thus, it can be used in any fashion you want if what it models gives you useful and meaningful results.

v = i*r works for a fixed resistor, where the resistor exhibts stationary statistical properties, and we can predict what the applied voltage is for a given current.

It is equally valid where r is a known and predictable function of i.

This same equation with different definitions for the variables can define velocity
v = d/t

We can measure the internal resistance of a battery by measuring the change in the terminal voltage for a given current change, and applying Ohm's law. This may not be linear, but it may be sufficient to assume it is linear.

.
 
LarryFine said:
If you have a garden hose filled end-to-end with marbles, and force one more into one end of the hose, a marble will pop out of the other end instantly, yet all of the marbles themselves only moved a distance equal to the diameter of a single marble.

The effect of current moves through the conductor at the speed of light, regardless of the speed of an individual electron.
Click to expand...

Perhaps my favourite post of all time,

Just today I was asked if we keep conductor lengths the same in some circuits (3P filters for example) because of the speed of electricity. I said no it was an inductance thing.

When challenged, I used your hose example but I had to loose the word garden when I was also challenged that garden hoses are flexible.

I believe the ?Hose? theory to be simply put and concise,

Bravo Mr. Fine!

Something else I ponder. C = 3 x 10^8 m/s in a vacuum. Anyone know how copper or aluminum media affect that?
 
crossman said:
I just read the thread title and the OP is indeed referring to ohm's law.:cool:
Click to expand...

And after U posted Thread 11, was that a ball or a strike on Thread 14 ?
 
I am a little mystified by some of the responses in this thread (haven’t been here in a while). It isn’t Ohm’s Theory, or Ohm’s Postulate, or Ohm’s Best-Guess. It is Ohm’s Law, and as such, it is always applicable. It doesn’t take a vacation and it is not selective as to when it gets used. It doesn’t matter if the voltage, current, or resistance is a linear function or nonlinear function. Nor does it matter if the parameters are given in P-P or RMS. It always applies. That’s what a scientific Law requires for it to be a Law.

For a given resistance (or impedance), current and voltage will remain proportional. If the resistance (impedance) is not fixed, then it too is a variable, and all three parameters can change, yet still satisfy the equation. Even a diode still follows Ohm’s Law, but the resistance is a complex, nonlinear function.

Taking the one example that was brought up, when the voltage to a motor decreases, it is not a violation of Ohm’s Law that the current will generally increase. The reason why the current increases is because the impedance is a function of the rotational frequency of the motor, and it is the reduction in the impedance that causes an increase in the current. In other words, it is not the change in voltage that causes an increase in current, but a reduction in the frequency, and therefore, a reduction in the impedance that caused an increase in the current. If the speed of the motor remained unchanged, then a reduction in the voltage would result in a reduction in the current too.

Some people are confusing transformer function with Ohm’s Law. The difference between primary and secondary of a transformer is not a function of Ohm’s Law, but from Conservation of Energy, in that Power-in equals Power-out.
 
Last edited:
Rick Christopherson said:
I am a little mystified by some of the responses in this thread (haven?t been here in a while). It isn?t Ohm?s Theory, or Ohm?s Postulate, or Ohm?s Best-Guess. It is Ohm?s Law, and as such, it is always applicable. It doesn?t take a vacation and it is not selective as to when it gets used. It doesn?t matter if the voltage, current, or resistance is a linear function or nonlinear function. Nor does it matter if the parameters are given in P-P or RMS. It always applies. That?s what a scientific Law requires for it to be a Law.

For a given resistance (or impedance), current and voltage will remain proportional. If the resistance (impedance) is not fixed, then it too is a variable, and all three parameters can change, yet still satisfy the equation. Even a diode still follows Ohm?s Law, but the resistance is a complex, nonlinear function.

Taking the one example that was brought up, when the voltage to a motor decreases, it is not a violation of Ohm?s Law that the current will generally increase. The reason why the current increases is because the impedance is a function of the rotational frequency of the motor, and it is the reduction in the impedance that causes an increase in the current. In other words, it is not the change in voltage that causes an increase in current, but a reduction in the frequency, and therefore, a reduction in the impedance that caused an increase in the current. If the speed of the motor remained unchanged, then a reduction in the voltage would result in a reduction in the current too.

Some people are confusing transformer function with Ohm?s Law. The difference between primary and secondary of a transformer is not a function of Ohm?s Law, but from Conservation of Energy, in that Power-in equals Power-out.
Click to expand...
Good to see you back Rick.
Yes that exactly was one of the first old wives tales I was shattered to break.
I did a small chemical refinery in the late 80s and one of the product pumps had to be put on a generator so that no matter what was going on you could always sell product.
This genset had an adjustable voltage function to it and I wanted to see just how much the amperage raised when i lowered the voltage.
Much to my surprise when I lowered the voltage it lowered the amperage also.
My jaw hit the ground so I went back to the drawing board to find out why.
 
080625-0607 EST

Rick:

For an equation to be a useful model of a physical process it may well require some limitations or additional definitions.

Consider a sine wave voltage source that has a full positive half wave cycle and the negative half is phase shift adjustable from 180 to 360 degrees. The load is a diode and resistor in series. The connection is such that the diode conducts on the positive half of the source voltage.

In this case using RMS measurements and a fixed positive peak voltage the resistance varies as the phase angle changes. Leave the phase angle fixed and adjust the peak voltage and the resistance is approximately constant for a real world diode. But these resistances are different than the resistance of the resistor alone, and are variable as a function of the input. Is it useful to define these different resistance values, or is it better to use some other analysis technique?

If RMS measurements of voltage and current are made on a linear resistive load and the waveform is arbitrary, then power can be calculated from V*I, V^2/R or I^2*R. Not so for non-RMS measurements.

For a non-linear load, such as the series diode and resistor, there are better ways to determine the power dissipated in the load rather than thru some combination of a varying R and I or V.

The usefulness of v = i*r is going to be a function of how it is applied and other peripherial information.

.
 
gar said:
The usefulness of v = i*r is going to be a function of how it is applied and other peripherial information.
Click to expand...

Just because the formula is not useful does not mean it is not in effect. Remember it is a law, at any point in time (snapshot) it is always true.
 
080624-0902 EST

Jim:

On an instantaneous basis v=i*r is true and useful in many cases. Obviously this is valid for anything if you want to sufficiently inter-relate the variables.

What is not very useful is to try to extend it to arbitrary loads and waveforms with RMS and other measurements, and continue from there to power calculation.

A simple example with a non-arbitrary load and RMS vs average measurement:

Measure a white noise source with a Simpson 260 voltmeter and calculate from this voltage measurement the power dissipated in a linear resistive load. The answer will be incorrect without a correction factor. Change the waveform and the correction factor will change.

Instead use a true RMS voltmeter, within the capability of that meter to read true RMS, and the calculated power from the voltage measurement will be correct for any waveform.

How useful a particular interpretation of Ohm's law is will be dependent on the application.

.
 
gar said:
080624-0902 EST
How useful a particular interpretation of Ohm's law is will be dependent on the application..
Click to expand...
The situation in your example is a failing of the meter, not Ohm's Law. You are only putting in a magnitude for a complex voltage function, and because that magnitude was incorrect, you got an incorrect result.

The beauty of Ohm's Law is that you can still use it if you put in a simple magnitude (the correct one) to represent the voltage function, or you can put in the complex Fourier transform. The point is, you have to enter the correct information into the equation to get a correct result.

Ohm's Law will still solve that other complex example you gave with a diode and "whatnot" (I didn't follow the whole thing), but you need to mathematically model the resistance. Ohm's Law is not just for instantaneous results in these situations. It only appears to be limited to instantaneous results because you have not fully defined the impedance function as a time varying mathematical function (or what ever else it may vary by). I am not savvy enough to derive the formula for your example, but it can be done, and then it can be plugged directly into Ohm's Law for a complete solution.
 
080624-1155 EST

Rick:

My point is that you can not simply put values into the equation without adequate supplemental information to define the values and their interpertation in the equation.

.
 
jim dungar said:
Just because the formula is not useful does not mean it is not in effect. Remember it is a law, at any point in time (snapshot) it is always true.
Click to expand...
But like gar keeps saying, only in certain contexts. The original law has been expanded to cover more areas than the original law-giver was covering. So to be clear, you would really need to say "Ohm's law for..."

Always true?

What is Ohm's law for a superconductor? What is Ohm's law for a carbon nanotube? I'm no expert but I hear that these do not obey Ohm's law.
 
gar said:
...My point is that you can not simply put values into the equation without adequate supplemental information to define the values and their interpertation in the equation.
Click to expand...
gar -
This isn't news. It's true for all mathematical models.

carl
 
Status
Not open for further replies.
Top