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Author Topic: Professor Walter Lewin's Non-conservative Fields Experiment  (Read 253051 times)

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It's not as complicated as it may seem...
Agreed.
That being; you assert that the voltage measured between points A and D in the diagram will be "zero" at any point in time, regardless if "B" is changing or not.

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It will not be 'a' voltage measured. IF you are able to measure a voltage the result will vary from -.1 to +.9 on a scope. An analog meter will show zero volts. The effective measurement will be zero or, more correctly identified as 'undefined'. Some measuring devices may show the average between -.1 and +.9 .
Is this not a contradiction of the above? Please clarify your assertion; will the voltage measured with an oscilloscope be "zero", OR some value between "-.1 and +.9"? (I am assuming you are not saying that it can be BOTH).

.99
« Last Edit: 2012-02-22, 13:25:33 by poynt99 »
   

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I am afraid you have the same understanding I had before performing the physical experiment. It made no sense that KVL may not apply in all instances. At that point, I would also find my previous statements to be a contradiction.

"At any point in time" is a valid statement for the application of KVL and can't be applied to non-conservative fields in motion. In such fields you must consider the point in time and other factors as the measurement results may be different for each point in time, relative position and path current has taken.

The point in time and current path may determine if the measuring device is shorted, in parallel, shorting or functionally unconnected to the circuit or portions of the circuit.

The same discussion arises when I tell someone the digital world is only a very small subset of the analog world and they believe the digital world can define all in the analog world. At best, digital can only provide a good approximation of the analog.

KVL vs. Faraday is the same thing. Mangled KVL can provide good approximations of such circuits but simply does not apply on its own. When we apply KVL, in what we think is the pure form, we get the wrong answers to questions like these.

If we apply it in the pure form, i.e. only to path independent situations with summations of measured charge, there is no problem.


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"As far as the laws of mathematics refer to reality, they are not certain; as far as they are certain, they do not refer to reality." - Einstein

"What we observe is not nature itself, but nature exposed to our method of questioning." - Werner Heisenberg
   

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It's not as complicated as it may seem...
WW,

In the interest of brevity and focus:

With reference to the attached diagram, and assuming that the scope leads are largely normal to the loop, will the voltage measured with the oscilloscope be "zero", OR some value between "-.1 and +.9"?

.99
   

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You will measure zero EMF.

Since EMF and PD (Potential Difference) are not the same but both are measured as Volts I can't guess at what you will measure as PD.

If I remember correctly you saw no difference between EMF and PD. If that is true then I have no way to make my answer more clear since the difference between PD and EMF is one thing that defines this experiment.

The only thing else I can say is there is no point to continue until you perform the actual experiment.

Sorry, I should have let the sleepy dog lie.


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"As far as the laws of mathematics refer to reality, they are not certain; as far as they are certain, they do not refer to reality." - Einstein

"What we observe is not nature itself, but nature exposed to our method of questioning." - Werner Heisenberg
   

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It's not as complicated as it may seem...
I'll keep asking essentially the same question until you answer it as it is worded. I have a fair bit of patience. ;)

With reference to the diagram, and assuming that the scope leads are largely normal to the loop, will the oscilloscope measure and display:

a) zero (i.e. nothing, no change), OR

b) some appreciable non-zero value?

a) or b)?

.99
   

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It's not as complicated as it may seem...
Unfortunately, it looks as though WW has decided to withdraw from this discussion.  ???

Is anyone else interested in continuing where we left off with this important debate?

.99
   

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Not withdrawn - swamped.

Above and below a significant non-zero VOLTAGE will be measured but it will not be from induction into the scope probe wires.

I haven't figured the details, yet.

The 'above' and the 'below' measurements will be opposite sine and offset from mid-point (.5V)


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"As far as the laws of mathematics refer to reality, they are not certain; as far as they are certain, they do not refer to reality." - Einstein

"What we observe is not nature itself, but nature exposed to our method of questioning." - Werner Heisenberg
   
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« Last Edit: 2012-02-24, 07:28:39 by Harvey »
   

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Harvey,

What is your opinion of the possible answer to .99's question?

I've never seen a paper covering this detail so I doubt you'll find another link. The link you last provided does cover this question but folks tend to think on one plane at a time.



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"As far as the laws of mathematics refer to reality, they are not certain; as far as they are certain, they do not refer to reality." - Einstein

"What we observe is not nature itself, but nature exposed to our method of questioning." - Werner Heisenberg
   

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It's not as complicated as it may seem...
When the meter and the loop of wire connecting it to the two points mid to the resistors is directly above or below the coil zero voltage is measured because there induction is cancelled by the opposing two meter branches.

Above and below a significant non-zero VOLTAGE will be measured but it will not be from induction into the scope probe wires.

OK.

Since the two above quotes contradict each other, are you now retracting your prior statement shown in the first quote?
   

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No.

The problem is the question doesn't fit the experiment - in the most strict sense.

My last answer is what I expect you will see should you perform the experiment. I would see the same results but translate them differently than I am sure you would.

Not trying to be an ass. Your perspective is still clearly KVL when I know KVL doesn't apply unless your meter is at a very precise position above or below. 'Generally normal' means nothing in this case or the world I work in.




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"As far as the laws of mathematics refer to reality, they are not certain; as far as they are certain, they do not refer to reality." - Einstein

"What we observe is not nature itself, but nature exposed to our method of questioning." - Werner Heisenberg
   

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It's not as complicated as it may seem...
It is a simple question, but it is being unnecessarily complicated by your responses. (you're not being an ass; more like "difficult" or "unreasonable").

Regarding the question at hand, there can only be one of those two answers which are correct, a) or b).

As such, you must retract one or the other of those two quotes. If you do not, then you are obviously undecided on the question.
   

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It's not as complicated as it may seem...
The question IS INDEED PERTINENT to the experiment, Lewin's lecture, and all the papers and books written about this.

I can assure you that this is not a fanciful indulgence on my part; I ask every question with a specific purpose, and although it may be interpreted by some as a "side track", it is actually a means to an end. I ask for some patience and cooperation, that's all.
   
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Harvey,

What is your opinion of the possible answer to .99's question?

I've never seen a paper covering this detail so I doubt you'll find another link. The link you last provided does cover this question but folks tend to think on one plane at a time.



His question implies a time-changing magnetic field, but the necessary information regarding that aspect is missing - therefore we are left to assume he is referring to time-changing magnetic field associated with the title of this thread. In that case, the closed loop integral for the circuit is 1V. Since Ohms law holds, the current is the same throughout the circuit with 1V across the series resistance of 900 + 100 Ohms we get 1 mA. Therefore, depending on which path you take you will read 0.9V from A to D across the 900 Ohm resistor (E=IR) or 0.1V from D to A across the 100 Ohm resistor.

Of course Lewin's actual experiment was reduced by a factor of 10, but the video clearly shows both voltages being measured by two separate probes simultaneously.

The actual physics involved shows that the E-field is non-uniform throughout the loop and that charges are naturally created at the resistive junctions in order to balance the net result. This is counter intuitive because standard textbook theory does not provide a mechanism for introducing these electric charges where they need to be.

In order to get the simulators to respond properly, KVL is modified for inductance and sources must be inserted in the loop to provide the necessary drops to bring the net to 0V. But reality shows the net to be 1V.

When we put inductors onto the substrates in chips, you can be sure that Faraday's law is used. I'm sure you can see how critical that would be in very low voltage applications.

 8)
   

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Undecided?

Yes. I haven't performed the experiment using 100 and 900 ohm resistors.

I don't consider your question a side track.

In my experiment the orientations shown on your diagram resulted in zero volts measured above and below. I used galvanometers and continuous pulses with difference between the resistors  but I don't recall the values. Since a scope can show far more precise values I can only think it will show some fraction of 1V at that position, probably positive voltage and probably slightly more than .5V. The measurement from below will be the opposite with the sum of the two readings being zero.

To provide you with a concise and correct answer I will need more than the usual five minutes I can apply to the problem or I'll need to perform the experiment.
I don't have personal time allowed on MatLab & Femm.

So I can't recant either. I measured zero but you and your scope will probably measure + above and - below with a sum of zero.
 


Harvey,

No need to repeat what you and I see as the foundation of the thread.



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"As far as the laws of mathematics refer to reality, they are not certain; as far as they are certain, they do not refer to reality." - Einstein

"What we observe is not nature itself, but nature exposed to our method of questioning." - Werner Heisenberg
   

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It's not as complicated as it may seem...
WW,

Logic dictates that both your answers can not be true.

Either there will be no change on the oscilloscope display, OR the oscilloscope will measure and display some appreciable non-zero value.

Until you are willing to firm up your assertion and commit to ONE of these answers, you ARE undecided.

Respectfully,
.99
   

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It's not as complicated as it may seem...
His question implies a time-changing magnetic field, but the necessary information regarding that aspect is missing [snip]
The diagram clearly indicates a magnetic field and its direction. When I first posted the diagram I stated that the field was increasing.

I believe WW would agree that we are assuming an induced loop emf of 1V (peak), but the value does not matter when it comes to my question, as long as some appreciable emf is induced.
   

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Then let it be.

I am undecided until I can perform the experiment with a scope.

Considering the much higher impedance of the scope sensing circuit, compared to a galvanometer, my best guess is the graphic position of the scope shown will yield a measurement of positive .4 volts.

It is not my nature to make such guesses. If I don't know for sure I push the questions aside until I find a definite answer.

If I'm wrong, fine. I'll run the test when I have a chance.


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"As far as the laws of mathematics refer to reality, they are not certain; as far as they are certain, they do not refer to reality." - Einstein

"What we observe is not nature itself, but nature exposed to our method of questioning." - Werner Heisenberg
   

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It's not as complicated as it may seem...
Thank you for the answer WW. :)

Harvey, would you care to provide an answer to the same question?
   
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The diagram clearly indicates a magnetic field and its direction. When I first posted the diagram I stated that the field was increasing.

I believe WW would agree that we are assuming an induced loop emf of 1V (peak), but the value does not matter when it comes to my question, as long as some appreciable emf is induced.

Fair enough. While we are assuming, let us also assume that the drawn ellipse is intended as an isometric view of a true circle. Let us also assume that the 100 Ohm resistor is farther back from the viewer than the 900 ohm resistor such that the diagram resembles Lewin's setup. And lastly, and mostly pedantic, let us assume the scope connection is a standard BNC wherein the ring is the reference and the tip is the signal input.

With these assumptions in place, we can properly ascertain the polarity and magnitude of the voltage and current induced in the loop.  O0
   

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It's not as complicated as it may seem...
Harvey,

YES to all the assumptions. I might add one other; the wire connecting the two resistors has a uniform resistance along its length totaling no more than 0.1 Ohms each leg.


Are you interested in offering an answer to the question I posed?
   

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It's not as complicated as it may seem...
Here is a slightly more accurate isometric view (I am not much of an artist) of the diagram (smaller far resistor).

.99
   
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Harvey,

YES to all the assumptions. I might add one other; the wire connecting the two resistors has a uniform resistance along its length totaling no more than 0.1 Ohms each leg.


Are you interested in offering an answer to the question I posed?

With the assumptions in place we know from Lenz's law the direction current will flow in the loop do the increasing flux denoted by Vector B which in most equations is called Phi ( Φ ). Therefore, the conventional current will flow clockwise through the loop such that if we start at A it will first reach the 100 Ohm resistor, then D then the 900 Ohm resistor and back to A.

Consequently, we can expect a voltage (EMF) to be present directly at the leads of the 100 ohm resistor proportional to that current and polarized such that the A side is positive. That voltage would be 0.1V. Similarly, we can expect a voltage to be present directly at the leads of the 900 Ohm resistor such that the D side is positive. That voltage will be 0.9V.

If both resistors are the same in size, and the leads between them are identical for the A side and the D side, then we are forced to realize that at that exact instant in time whereby the induced loop voltage is 1V that a differential exists across those leads 'totaling no more than 0.1 Ohms each leg' such that if we if we move our probe and its reference from the 900Ω resistor where it reads zero point nine volts (0.9V), over to the 100Ω resistor where it reads negative zero point one volt (-0.1V) we know that for the pair of leads (both the reference and the signal) we have 1V differential. And this is exactly that 1V that is induced.

Therefore, when we position the reference at A and the signal probe at D we expect to see the midpoint voltage of zero point four volts (0.4V).

We can illustrate this by dividing the length of the leg into 10 sections using 11 points where point 1 is the -0.1V position and point 11 is the 0.9V position. If the EMF is equally distributed, we can expect 0.1V differential at each point. Therefore, point 6 is exactly midpoint thus representing 0.4V.

Why do we have such a high differential on a wire that measures only 0.1 Ohm? Because of inductive reactance. In order for the experiment to work as stated, the time-changing field must be done with such a time frame so as to allow the wire to act as a resistor to the 1 mA induced for the loop. Remember, this wire is functioning as a generator and thus appears as a supply to the circuit replacing the 1V battery previously referenced in Professor Lewin's video. Thus the reactance is part of the internal source impedance.

 8)

Edit to correct reference polarity  :P
   

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Yea, but what would the measured voltage be if the scope and leads were directly below the loop?  :D

I'll give you one guess  ;)

(without disconnecting the scope probe from its current points)


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"As far as the laws of mathematics refer to reality, they are not certain; as far as they are certain, they do not refer to reality." - Einstein

"What we observe is not nature itself, but nature exposed to our method of questioning." - Werner Heisenberg
   
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Gibbs just did some superposition of the equivalent circuit.  Looks like it is in line with some of the thinking in here.  The voltage/battery used is 1 V ignored impedance.  The two mid points are measured with scope.  1 and 2 Ohms are used to simplify the situation.

The one on the left would reads 0 V.
The one in the middle would reads 1 V.
The one on the right would reads .33V.

One can play around with the voltage induced and resistance to calculate any similar situation. 
   
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