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Solid States Devices => solid state devices => Topic started by: poynt99 on April 24, 2016, 04:20:07 PM

Title: Lewin's NCF Experiment and Lecture
Post by: poynt99 on April 24, 2016, 04:20:07 PM
This topic is regarding Walter Lewin's hotly debated non-conservative fields experiment and lecture, where he professes Kirchhoff no longer holds whenever Faraday induction is involved.

A solenoid coil producing a uniform B field, around which is placed a simple circuit with a 100 Ohm and 900 Ohm resistor on opposite sides of the solenoid. This circuit essentially forms a secondary for the solenoid. The solenoid is energized by a step from 0 to some DC voltage, large enough to produce an emf of about 1V in the secondary (1mA).

Scope probes are placed across points D and A as shown, and the voltages across them captured on screen. The resulting peak voltages are as Lewin demonstrates, i.e. -100mV (R1) and 900mV (R2).

[edited to fix "Walter's" name.]
Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on April 24, 2016, 04:36:09 PM
So we are talking about a circuit such as below,where the solenoid has a 1 volt potential placed across it,where the magnetic field is coming out from the page,and the 100R and 900R make a completed loop around the outside of the solenoid coil--but not connected to the solenoid coil?.
Yes, but the input voltage is whatever is required to induce about 1V on the secondary (1mA flow).

Quote
If so,i still do not see how he could have 900mV measured on one side,and 100mV measured on the other side,when both sides of each resistor are joined by a common wire-or the resistors wire it self.
The measurement is correct. There is only one current in the circuit, agreed? Therefore the potential difference across each of the resistors must be different.

Quote
The two voltage potentials should still be the same value,and same polarity :o
Again, looking at the current in the circuit is key. Follow the loop arrows around and see what values and polarities you end up with.

Quote
Maybe it is the electric field he is measuring?,but even then,how do you get a difference in potential across a piece of wire that short.
;)

Is it a potential difference across the wires, or an induced emf?
Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on April 24, 2016, 04:51:59 PM
The previous diagram may be perplexing to some, but the following one should make sense.

Do they both make sense? Why or why not?
Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on April 24, 2016, 04:57:18 PM
What voltage will the left meter indicate?
Title: Re: Lewin's NCF Experiment and Lecture
Post by: tinman on April 24, 2016, 05:24:03 PM
The previous diagram may be perplexing to some, but the following one should make sense.

Do they both make sense? Why or why not?

If we look at the moded diagram below,you can see i have placed both a blue line between the bottoms of the resistors,and a red line between the two tops of the resistors.
There should be no voltage drop(or a very extremely small amount)between the points of either the red line,or the blue line. This means that the voltage should be the same for both scope positions marked on the diagram.

I think the problem here lies within the measurement device leads them self-circled in red,where they set up there own loop that the electric field from the solenoid acts upon.
CH1,and CH2 should read the same.
Just my thoughts.

Brad
Title: Re: Lewin's NCF Experiment and Lecture
Post by: tinman on April 24, 2016, 05:41:14 PM
Tomorrow i will set up this experiment.
I will place 1 scope probe directly in the middle of the secondary turn with the resistors in place,and so the scope probe and ground lead go directly over the top and at the center of the coil.
I will dump the energy from a large cap into the coil,and keep increasing that caps stored energy amount until i get an EMF of 1 volt on the scope.

Then we can start changing scope probe positions,and see what happens every pulse.


Brad
Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on April 24, 2016, 05:46:10 PM
I think the problem here lies within the measurement device leads them self-circled in red,where they set up there own loop that the electric field from the solenoid acts upon.
CH1,and CH2 should read the same.
Just my thoughts.

Brad
Yes, the measurement leads are part of the experiment in these configurations, therefore should they read the same voltage?
Title: Re: Lewin's NCF Experiment and Lecture
Post by: tinman on April 24, 2016, 05:48:07 PM
What voltage will the left meter indicate?

Well if there is any sense to this at all,it should read minus 100mV,as there is no voltage drop (or very little) across the conducting wires between the resistors.
But in saying that,it could just as easily read minus 900mV,as it's at the same measuring point as that that read's 900mV.

Will find out tomorrow for sure though.


Brad
Title: Re: Lewin's NCF Experiment and Lecture
Post by: tinman on April 24, 2016, 05:52:45 PM
Yes, the measurement leads are part of the experiment in these configurations, therefore should they read the same voltage?

No.not until current starts to flow through the single loop/resistor combo.



Brad
Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on April 24, 2016, 06:04:12 PM
Tomorrow i will set up this experiment.
I will place 1 scope probe directly in the middle of the secondary turn with the resistors in place,and so the scope probe and ground lead go directly over the top and at the center of the coil.
I will dump the energy from a large cap into the coil,and keep increasing that caps stored energy amount until i get an EMF of 1 volt on the scope.

Then we can start changing scope probe positions,and see what happens every pulse.


Brad
With an induced emf of 1V, would you expect your decoupled measurement (vertical leads) to measure 1V between points D and A as you have shown?

(it won't be)

Here is how you "calibrate" the experiment: if your scope ground lead is long enough, simply loop it around the solenoid and clip it to the probe tip. If it is not long enough, add a piece of wire. Set your scope to 200mV/div and start discharging your source cap at different voltages until you get a peak of 1V on the scope.

You have now measured the E field and set it to induce 1V into the secondary circuit.
Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on April 24, 2016, 06:08:42 PM
No.not until current starts to flow through the single loop/resistor combo.

Brad

Why do you say they would measure the same once current starts to flow in the circuit?
Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on April 24, 2016, 06:14:29 PM
It is important that your solenoid have a length to diameter ratio of about 4:1 (or higher), and that you place the "circuit" in the middle.  See attached.
Title: Re: Lewin's NCF Experiment and Lecture
Post by: minnie on April 24, 2016, 06:32:13 PM



  If the volt meter was "perfect" no current would flow and even if the leads
  made a loop what would Ohms law think?
             John.
Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on April 24, 2016, 06:43:03 PM


  If the volt meter was "perfect" no current would flow and even if the leads
  made a loop what would Ohms law think?
             John.
It's a moot point, and topic for another discussion perhaps.
Title: Re: Lewin's NCF Experiment and Lecture
Post by: Johan_1955 on April 24, 2016, 07:40:52 PM
Hi Point,

But to measure a Double Load and Double Source serie Circle circuit, with only one reference-point, like PW also did explain short ago, will first mislead every, me to! ;-))

Thanks for your way of and time!

Regards, Johan
Title: Re: Lewin's NCF Experiment and Lecture
Post by: minnie on April 24, 2016, 11:29:17 PM



   So even though the volt meter is in effect an open circuit there will be a small
   displacement current and thus an emf.
   Can't wait to see the "real answer"
   I've had a couple of real gruelling days at the office and I enjoy this bit of
   light relief.
   I'm depending on poynt to put me right,here.
Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on April 25, 2016, 01:59:25 AM


   So even though the volt meter is in effect an open circuit there will be a small
   displacement current and thus an emf.
   Can't wait to see the "real answer"
   I've had a couple of real gruelling days at the office and I enjoy this bit of
   light relief.
   I'm depending on poynt to put me right,here.
Not sure if I'm getting to where you're going, but I'll give it a try:

- The 1V induced E field is present regardless if a piece of wire is there or not.
- The 1V induced emf is present in the circuit regardless if a meter is placed across the resistors or not.
- The combined loop voltage across the resistors is 1V.
- The meter leads if wrapped around the solenoid and shorted together will measure an induced emf close to 1V.
- If the meter had an infinite impedance, the leads would still have an induced voltage of 1V across them.
- I don't see any reason displacement current (current across the plates of a capacitor) would be present anywhere. Where do you see it?
Title: Re: Lewin's NCF Experiment and Lecture
Post by: tinman on April 25, 2016, 04:35:21 AM
Ok,well my test results turned out to be the same as i would have expected,where there is very little difference at all between the wave forms shown on the scope.

I had to settle for a 680R instead of the 900R,as i dont have one. But the results should have shown the difference we were looking for-but did not.
A slight difference in peak voltage was all that was seen,but the polarity remains the same,and also the wave forms them self are near identical.

Below is a picture of the setup-along with the circuit as it is,and the scope capture to go along with the test.
As Poynt said,the scope tip and ground were rapped around the coil,and joined together. The voltage to the cap was then increased until i had my 1v potential seen on the scope trace.
The end resulting voltage will of course be lower,as i used a 680R resistor instead of the 900R.

So i do not get the results shown by Lewin,or as predicted by Poynt.
I will parallel a couple of resistors,and get the required 900R,and see if that makes a difference. But i see only the result being the same,but where the peak voltage may rise.


Brad
Title: Re: Lewin's NCF Experiment and Lecture
Post by: tinman on April 25, 2016, 05:44:28 AM
What voltage will the left meter indicate?

Ok,have an answer to this one.
The left meter in this situation reads very close to 0v.
I would expect the slight gap due to the resistor as being the reason for a slight voltage reading across the meter/scope.

Brad
Title: Re: Lewin's NCF Experiment and Lecture
Post by: TinselKoala on April 25, 2016, 06:37:30 AM
Brad, look at your schematic, and consider the direction of conventional current as you have indicated with the arrows. Look at the locations of the probe Tips and Grounds with respect to the current flow, hence the voltage drop across the resistors.   Also consider that the probe ground references are connected together at the scope chassis.

I think it might be interesting to make these two measurements separately. That is, do a shot with only CH1 connected, save the waveform, then do a shot with only CH2 connected. Then compare the waveforms. Also you might try the CH2 measurement (separately) with probe tip connected to the "current in" side of the resistor and ground connected to the "current out" side.The CH1 probe is already connected this way, with tip on "current in" and ground on "current out".
Title: Re: Lewin's NCF Experiment and Lecture
Post by: tinman on April 25, 2016, 08:08:55 AM
Brad, look at your schematic, and consider the direction of conventional current as you have indicated with the arrows. Look at the locations of the probe Tips and Grounds with respect to the current flow, hence the voltage drop across the resistors.   Also consider that the probe ground references are connected together at the scope chassis.

I think it might be interesting to make these two measurements separately. That is, do a shot with only CH1 connected, save the waveform, then do a shot with only CH2 connected. Then compare the waveforms. Also you might try the CH2 measurement (separately) with probe tip connected to the "current in" side of the resistor and ground connected to the "current out" side.The CH1 probe is already connected this way, with tip on "current in" and ground on "current out".

That is not my schematic,it is Poynt's.
I have the scope hooked as per the volt meters on the schematic.

Why would the common ground of the scope matter ?,as the wire between the two resistors is common anyway.

I stated in the other thread that the polarity should be the same ,due to the common wire on each side of each resistor,and looking at all the scope shot's i have taken,that is exactly how it is.
I will try each channel 1 at a time,but i dont see that making any difference.

Below is Poynts schematic of the test setup,where i have a 1k ohm resistor instead of the 900R.
I still see no reverse voltage--both traces show same polarity.


Brad
Title: Re: Lewin's NCF Experiment and Lecture
Post by: tinman on April 25, 2016, 08:36:38 AM
Well i have no idea why,but we get the desired result by using 1 channel of the scope at a time.
I know my scope has a common ground,but the wire between the two resistors is also common--so why the difference?.

The first scope shot below shows the trace across the 1k ohm resistor only,and the second shows the trace across the 100 ohm resistor only. Here we now see that we do have the reversed polarity we were looking for,but in order for me to get that trace across the 100ohm resistor,the loop formed by the scope probe and ground wire,must be a vertical loop as apposed to the horizontal loop of the two resistors. If the scope probe and ground wire also form a horizontal loop,then only a very small voltage is seen across the 100 ohm resistor.


Brad
Title: Re: Lewin's NCF Experiment and Lecture
Post by: tinman on April 25, 2016, 03:09:38 PM
So when we just started to think we had the answer's.

Decided to scope across the wire from one resistor to the other-->and what do you know :D,The very same voltage potential and polarity as the previous scope trace across the 100R in my last post.

And no,the channel is not inverted ???

See scope probe position in schematic.


Brad
Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on April 25, 2016, 03:18:16 PM
Well i have no idea why,but we get the desired result by using 1 channel of the scope at a time.
I know my scope has a common ground,but the wire between the two resistors is also common--so why the difference?.
This is highly germane to the entire problem at hand. ;)

Consider this; taking into account that we are dealing with Faraday induction here, is it correct to presume the wire between the resistors is simply wire, and it is doing nothing more than connecting one resistor to another?

The answer is no. The wire is where the emf is being induced, so if you have shorted it out with your scope's gnd leads, it isn't going to work properly.

Quote
The first scope shot below shows the trace across the 1k ohm resistor only,and the second shows the trace across the 100 ohm resistor only. Here we now see that we do have the reversed polarity we were looking for,but in order for me to get that trace across the 100ohm resistor,the loop formed by the scope probe and ground wire,must be a vertical loop as apposed to the horizontal loop of the two resistors. If the scope probe and ground wire also form a horizontal loop,then only a very small voltage is seen across the 100 ohm resistor.


Brad
You may want to double check that it is 100 Ohms, and not 10 Ohms or something. I am not sure why you had to rotate the loop 90 degrees in order to measure -100mV, but you shouldn't have to. I didn't.
Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on April 25, 2016, 03:24:52 PM
So when we just started to think we had the answer's.

Decided to scope across the wire from one resistor to the other-->and what do you know :D ,The very same voltage potential and polarity as the previous scope trace across the 100R in my last post.

And no,the channel is not inverted ???

See scope probe position in schematic.


Brad
If all is going as expected, you should measure essentially 0V between the wires when the measuring probes are in the horizontal plane. What should you measure between the wires when the measuring probes are in the vertical plane, i.e. decoupled from the experiment?
Title: Re: Lewin's NCF Experiment and Lecture
Post by: tinman on April 25, 2016, 03:47:40 PM
If all is going as expected, you should measure essentially 0V between the wires when the measuring probes are in the horizontal plane. What should you measure between the wires when the measuring probes are in the vertical plane, i.e. decoupled from the experiment?

With the scope prob and ground wire forming a loop that is horizontal to the resistor loop,i get the same voltage reading as i do across the 100R.
With the scope probe and ground wire forming a loop that is vertical to that of the resistor loop,i get a much higher voltage potential.

And why the inverted voltage to that of the scope shot showing the scope probe position across the 100R in my second last post?.

Brad
Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on April 25, 2016, 03:57:24 PM
With the scope prob and ground wire forming a loop that is horizontal to the resistor loop,i get the same voltage reading as i do across the 100R.
With the scope probe and ground wire forming a loop that is vertical to that of the resistor loop,i get a much higher voltage potential.

And why the inverted voltage to that of the scope shot showing the scope probe position across the 100R in my second last post?.

Brad
It would appear something is not sufficient in your setup. You should not be measuring a voltage between the wires, or at least very little.

In regards to your question, I'm not sure what you are asking. Do you want to know why the 100 Ohm resistor shows a negative polarity while the 900 Ohm shows a positive polarity? Is that what you are asking?
Title: Re: Lewin's NCF Experiment and Lecture
Post by: wattsup on April 25, 2016, 04:51:09 PM
If we look at the moded diagram below,you can see i have placed both a blue line between the bottoms of the resistors,and a red line between the two tops of the resistors.
There should be no voltage drop(or a very extremely small amount)between the points of either the red line,or the blue line. This means that the voltage should be the same for both scope positions marked on the diagram.

I think the problem here lies within the measurement device leads them self-circled in red,where they set up there own loop that the electric field from the solenoid acts upon.
CH1,and CH2 should read the same.
Just my thoughts.

Brad

@tinman

I agree. The probe wires are just as prone to the influence of the solenoid since the ground reference that is supposed to shield the probe conductor is not separate from the scope's differential calculation of both ground and probe.

What if you put the scope on A-B math and only use the two probes across each resistor separately. Will you get the same results?

wattsup
Title: Re: Lewin's NCF Experiment and Lecture
Post by: minnie on April 25, 2016, 05:36:55 PM



  With words like moot and germane cropping up I had to reach for my
  dictionary.
    One thing is clear, everybody except for poynt and Koala just haven't
  got a clue.
     It's amazing how two resistors can baffle everyone.
Title: Re: Lewin's NCF Experiment and Lecture
Post by: Johan_1955 on April 25, 2016, 05:48:50 PM
     It's amazing how two resistors can baffle everyone.

With this only 4 component circuit, you're also missing 2 pcs in counting?
From only 2 pages, you're also missing 2 posts?

Tells also a lot!

Regards, Johan
Title: Re: Lewin's NCF Experiment and Lecture
Post by: minnie on April 25, 2016, 10:28:59 PM



 Johan,
         yes,you've got me Sussed out!
   I'm crap really at this sort of stuff, just look at what happened to
  my latest little project. I put it on test and went to work and when
  I came back there was a dreadful smell. Luckily I didn't burn the
  place down.
      Over the years not that many who contribute to this forum have
   a good grasp of measurement techniques (only my opinion). Just
   look at the Rosemary Ainslie saga.
       Someone could do this Lewin thing with a bit of paper and a pencil
   I'm sure. I just wish I could do that sort of thing.
               John
Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on April 26, 2016, 12:26:57 AM
What were you building there, and why did it go up in smoke?
Title: Re: Lewin's NCF Experiment and Lecture
Post by: tinman on April 26, 2016, 01:28:02 AM




Quote
It would appear something is not sufficient in your setup. You should not be measuring a voltage between the wires, or at least very little.

Well as there are only two resistors,and two short pieces of copper wire joining the two resistors together-where that copper wire is the same diameter as the resistors wire,i am not sure what could be insufficient in the setup?.
Did you try and measure a voltage across the wire it self when you were testing your setup?.

Quote
In regards to your question, I'm not sure what you are asking. Do you want to know why the 100 Ohm resistor shows a negative polarity while the 900 Ohm shows a positive polarity? Is that what you are asking?

No.
I mean,when the ground is fixed in position as marked in the schematic below,and the probe is on the other side of the 100R,i get a negative voltage spike across the 100R. We then leave the ground where it is,and measure across the wire it self-along the bottom wire that joins to the 1KR,and we still get a negative spike to the very same value as seen across the 100R,and yet the probe is now on the leading side of the current flow to that of the ground of the scope.

There has to be a voltage drop across the wire's,as if there was not,then at any point along the two wires,the potential difference must be the same,and there for the same on either side of each resistor.
There is also the fact that we are measuring current flow,and not just the electric field,as we can clearly see an overshoot in all the scope shots-caused by the collapse of the magnetic field when the cap has drained.

It would seem to me that the seen voltages are a result of the loop in the measuring equipment and resistors them self,and not an actual measurement of a voltage drop across the resistors/ loop.
If there is not suppose to be a voltage seen across the wire itself,then how is there a difference in potential from one end of the two wires to the other?,which will be across each resistor.

Im going to take a guess here,and say that i believe if i measure across the top wire in the circuit,i will see the same voltage as across the 1k resistor. But i will have to change the position of the coil on my setup,as i cannot fit the scope probe between the backing board and coil when the scope probe and ground need to be on a horizontal plane.

Each scope shot below was taken with only 1 scope channel hooked to the circuit.

Perhaps you should dig out that old setup of yours Poynt,and scope across the wires them self?.



Brad
Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on April 26, 2016, 03:20:13 AM
Well as there are only two resistors,and two short pieces of copper wire joining the two resistors together-where that copper wire is the same diameter as the resistors wire,i am not sure what could be insufficient in the setup?.
You said you had to reposition the probe somehow while measuring the 100R in order to get a voltage reading if I'm not mistaken. You shouldn't have to do that. That is a sign of one potential problem. You also have not taken care to minimize the effect from the source wiring. Take a close look at mine and you will see what I did. The only horizontal wiring  is twisted to cancel the "stray" magnetic field. The top conductor of the solenoid travels down the inside so it can exit at the bottom, twisted with the other end.

Quote
Did you try and measure a voltage across the wire it self when you were testing your setup?.
I'm fairly certain I did, but it was a few years ago and I could be mistaken.

Quote
There has to be a voltage drop across the wire's,as if there was not,then at any point along the two wires,the potential difference must be the same,and there for the same on either side of each resistor.
If you look back at my diagrams, I show three probe positions for the meter on the left. What are the meter voltages at each of the three positions?

Quote
If there is not suppose to be a voltage seen across the wire itself,then how is there a difference in potential from one end of the two wires to the other?,which will be across each resistor.
Same response as above.

Quote
Perhaps you should dig out that old setup of yours Poynt,and scope across the wires them self?.



Brad
Yes perhaps I will. Tell me what you measure at those three positions first.
Title: Re: Lewin's NCF Experiment and Lecture
Post by: Johan_1955 on April 26, 2016, 12:01:21 PM

 Johan,
         yes,you've got me Sussed out!
   I'm crap really at this sort of stuff, just look at what happened to
  my latest little project. I put it on test and went to work and when
  I came back there was a dreadful smell. Luckily I didn't burn the
  place down.
      Over the years not that many who contribute to this forum have
   a good grasp of measurement techniques (only my opinion). Just
   look at the Rosemary Ainslie saga.
       Someone could do this Lewin thing with a bit of paper and a pencil
   I'm sure. I just wish I could do that sort of thing.
               John

Hi John,

Maybe to take it to: http://overunity.com/15158/food-for-thought-our-world

Out of respect for TM and .99.

Regards, Johan
Title: Re: Lewin's NCF Experiment and Lecture
Post by: Magluvin on April 26, 2016, 11:36:57 PM
You said you had to reposition the probe somehow while measuring the 100R in order to get a voltage reading if I'm not mistaken. You shouldn't have to do that. That is a sign of one potential problem. You also have not taken care to minimize the effect from the source wiring. Take a close look at mine and you will see what I did. The only horizontal wiring  is twisted to cancel the "stray" magnetic field. The top conductor of the solenoid travels down the inside so it can exit at the bottom, twisted with the other end.
I'm fairly certain I did, but it was a few years ago and I could be mistaken.
If you look back at my diagrams, I show three probe positions for the meter on the left. What are the meter voltages at each of the three positions?
Same response as above.
Yes perhaps I will. Tell me what you measure at those three positions first.

These are strange results it seems.

If there is a question of whether the meter leads are being induced, set the meter to the left of the coil, connect the test leads to the middle of the wire connecting the resistors, one lead on each side, take the measurement, then without disconnecting the test leads, move the meter to the right side, lifting the test wires over the coil assy and set  the meter down on the table. Then check the meter again. Is it the same reading?

Mags
Title: Re: Lewin's NCF Experiment and Lecture
Post by: tinman on April 27, 2016, 01:21:27 AM
These are strange results it seems.

If there is a question of whether the meter leads are being induced, set the meter to the left of the coil, connect the test leads to the middle of the wire connecting the resistors, one lead on each side, take the measurement, then without disconnecting the test leads, move the meter to the right side, lifting the test wires over the coil assy and set  the meter down on the table. Then check the meter again. Is it the same reading?

Mags

I am only discharging a cap into L1,and capturing one cycle at a time. So i cant use a DMM or meter to do any checking like that.

I have to run my top wire of L1 down through the center of L1,and twist the ends of the coils leads to eliminate stray inductance they may be causing in the scope probe lead loop.


Brad
Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on April 27, 2016, 03:33:17 AM
Mags raises a very good question, and actually it makes no difference whether you use meter leads (and DMM) or scope probes, the result will be the same. The problem of course is that the signal is fleeting, and it therefore must be captured on a scope.

However, that doesn't mean we are limited to using only the probes that come with the scope, i.e. the traditional scope probe. If you want, you can construct a simple unshielded "probe" from simple twin-lead (speaker wire), two alligator clips, and a BNC connector. Depending on how noisy your environment is, you may or may not have interference on your signals. My speaker wire "probes" are about 6' long, and I don't have any problems with noise. You may want to construct a probe like this for part II of the experiment, which is next when you're ready.
Title: Re: Lewin's NCF Experiment and Lecture
Post by: lumen on April 27, 2016, 03:35:41 AM
It might help to think of the wire segments as batteries placed in the direction of the arrows. It's the same thing because you are inducing a current these segments.
Once you look at the problem this way, everything makes sense and the results are correct.
Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on April 27, 2016, 03:47:22 AM
No quite.

Once you understand what is happening from both perspectives, then everything makes sense. Until perspective two is explored,  the results for perspective one (the one we are working with now) appear confusing when looking at the measurements and imagining the wires as sources of emf.

Hold on to those thoughts lumen and let's get through this one step at a time.
Title: Re: Lewin's NCF Experiment and Lecture
Post by: tinman on April 27, 2016, 01:10:03 PM
OK,here we go.

All tests at each point were done 3 time's,and results are all the same in each of the 3 tests.

Below is the modified circuit,so as the top lead out of the coil now go's down the center of the coil,and twisted with the bottom lead in of the coil--as pictured below. I have twisted it as much as i can-right to the switch,as the wire is very thick,and the coil it self is square shaped ali insulated single strand wire-->more like rod.

So for this post,i will show a picture of the setup now,and a scope shot of the scope probe looped around the coil at the center--so as you can see we are close to the required 1v potential--thats as high as my PSU go's in voltage-31volts,so as close to the 1 volt potential as i can get with this coil.

In my next post,i will supply a scope shot taken at each point indicated in the schematic.
Poynt--you better dust of the old setup ;)


Brad
Title: Re: Lewin's NCF Experiment and Lecture
Post by: tinman on April 27, 2016, 01:23:16 PM
below are my test results for each supplied schematic.
Please note the VPD on each scope shot,as i change them as required in relation to voltage potential reached for each test position.

It is clear from my test,that as i stated before,the potential across both resistors should be the same,and that the scope's probe and ground lead is what influences the result,s.
It is also clear that the voltage potential across the wire(which should be 0),is as high(if not higher) than the potential across the 100R resistor.

All test were carried out with the scope probe and ground lead loop in the horizontal plane to that of the resistors loop.

Brad
Title: Re: Lewin's NCF Experiment and Lecture
Post by: tinman on April 27, 2016, 01:35:31 PM
A close look at the results by separating the scope shot's.
By looking at two at a time,we get a clearer idea.
40mV across R1,and 40mV across R2 ;)

Brad
Title: Re: Lewin's NCF Experiment and Lecture
Post by: tinman on April 27, 2016, 01:39:43 PM
And the other two scope shots
A clear indication that it !is! the scope leads loop position that determines the voltage potential seen on the scope,and not the actual potential across the resistors--as i said before,should be the same.
500mV across R1,and 500mV across R2

Brad
Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on April 27, 2016, 02:40:19 PM
It is clear from my test,that as i stated before,the potential across both resistors should be the same,and that the scope's probe and ground lead is what influences the result,s.
Good. It still seems something isn't quite right as even though you have 1V emf, you don't have 1V total across the resistors, correct? Is that wire fairly low resistance? Is it ferromagnetic? You are only getting about half the voltage. Aren't you wondering where the other half is? I'm guessing your wire is highly resistive (which would also explain why you're measuring a voltage across the wire when you shouldn't). I used wire from 14/2 house wiring, so it is good old solid copper.

Does it surprise you that the scope measures the same regardless of the probe position?

What does it mean when you have one probe on the left and one probe on the right measuring across the same resistor, but are getting two completely different results? That brings us to the end of Lewin's demonstration and lecture basically. Congratulations, you have replicated the experiment. Ready for perspective two?

Quote
It is also clear that the voltage potential across the wire(which should be 0),is as high(if not higher) than the potential across the 100R resistor.
I'm going to verify this with my setup. Again, if the wire has a low resistivity relative to the resistor values, you should measure very little across the wires.
Title: Re: Lewin's NCF Experiment and Lecture
Post by: tinman on April 27, 2016, 03:11:37 PM
 author=poynt99 link=topic=16557.msg482146#msg482146 date=1461760819]



Quote
Good. It still seems something isn't quite right as even though you have 1V emf, you don't have 1V total across the resistors, correct? Is that wire fairly low resistance? Is it ferromagnetic? You are only getting about half the voltage. Aren't you wondering where the other half is? I'm guessing your wire is highly resistive (which would also explain why you're measuring a voltage across the wire when you shouldn't). I used wire from 14/2 house wiring, so it is good old solid copper.

The wire used in the main coil/solenoid is 3.5mm x 2mm rectangular ali laminated wire from the primary of an old arc welder. The copper wire that is soldered to the resistors wire is .8mm copper wire,so the resistance there is almost non existent-very low. There is the fact that we are only talking less than 1mA of current here as well.

I have 550mV across R1,and 550mV across R2. I have 1100 ohms of resistance total.
1.1volts across 1100 ohms = a current of--you guessed it,1mA as predicted. :)

Quote
Does it surprise you that the scope measures the same regardless of the probe position?

No,it dose not,as that is what i stated from the very start-the voltage across the two resistors should be the same,and my test show that it is.

Quote
What does it mean when you have one probe on the left and one probe on the right measuring across the same resistor, but are getting two completely different results?

It means that depending on turn direction of the scope probe and ground lead,it either adds too or subtracts from the EMF in the resistor loop.

Quote
That brings us to the end of Lewin's demonstration and lecture basically. Congratulations, you have replicated the experiment. Ready for perspective two?

Yes,ready for perspective two--what ever that is?

Quote
I'm going to verify this with my setup. Again, if the wire has a low resistivity relative to the resistor values, you should measure very little across the wires.

It is good that you are going to get your setup up and running again.
I have a feeling that you may get a surprise when you measure across the wire it self ;),and i believe that this is due to the scope it self.


Brad
Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on April 27, 2016, 03:36:32 PM
I have 550mV across R1,and 550mV across R2. I have 1100 ohms of resistance total.
1.1volts across 1100 ohms = a current of--you guessed it,1mA as predicted. :)
Not correct, you have 500mV across R2, and 40mV across R1 for a total of 540mV. That does not equal 1000mV. Also you have about 100mV across each wire for a total of about 740mV. Still not 1V total so something is not right.

Quote
It means that depending on turn direction of the scope probe and ground lead,it either adds too or subtracts from the EMF in the resistor loop.
Nope. It means that the value measured (polarity aside for the moment) is dependent on the path taken by the measurement probe. It should also be somewhat evident that this measurement does not measure the potential difference of the resistors per se, rather it is the E field.

Quote
Yes,ready for perspective two--what ever that is?
Confirm you understand both statements above, then we're ready to move on.

Quote
It is good that you are going to get your setup up and running again.
I have a feeling that you may get a surprise when you measure across the wire it self ;) ,and i believe that this is due to the scope it self.
I have a feeling I won't, but we'll see soon enough.
Title: Re: Lewin's NCF Experiment and Lecture
Post by: tinman on April 27, 2016, 04:06:36 PM





Quote
Not correct, you have 500mV across R2, and 40mV across R1 for a total of 540mV. That does not equal 1000mV. Also you have about 100mV across each wire for a total of about 740mV. Still not 1V total so something is not right.

Sorry Poynt,but i cant agree with this. The voltage seen by the scope is clearly defined by the probe and ground loop position. We are not measuring the actual voltage across the resistor's,that should be the same. We dont just have a resistor loop circuit once we add the scope probe and ground wire loop. When we do that,we now have a second loop,and there for a second circuit.

Quote
Nope. It means that the value measured (polarity aside for the moment) is dependent on the path taken by the measurement probe. It should also be somewhat evident that this measurement does not measure the potential difference of the resistors per se, rather it is the E field.

Where there is an E field that changes with time,there is also a magnetic field. It is the magnetic fields change in time that gives rise to current flow through the resistors loop. If i raise the value of the 100R to 200R,and the value of the 1K to 2K,then i would expect the EMF value seen on the scope to rise as well,as we reduce the current,but increase the voltage.
The produced EMF seen across the resistors loop would also change depending on switching speed-as it dose with any transformer,and also how clean that switching is. I only have a home made copper plate tap switch,and you can see the arcing ripple before the spike-if you look closely at my scope shots.

Quote
Confirm you understand both statements above, then we're ready to move on.

I understand what you are saying,i just do not agree with it at this time-well not fully anyway.
You could say we are in half agreement ATM.

Quote
I have a feeling I won't, but we'll see soon enough.

It is my belief that you will see around the same voltage as that seen across the 100R ,but the value may change depending on your resistors loop around your main coil.


Brad
Title: Re: Lewin's NCF Experiment and Lecture
Post by: lumen on April 27, 2016, 05:52:14 PM
Those last scope shots make perfect sense and everything is accounted for.
When the probe wires wrap around the inducing coil, the total voltage is induced into the probe minus the voltage of the DUT (device under test)
So the output is the difference from the area tested from the total which is then the same as the other.

Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on April 27, 2016, 06:00:16 PM
Sorry Poynt,but i cant agree with this. The voltage seen by the scope is clearly defined by the probe and ground loop position. We are not measuring the actual voltage across the resistor's,that should be the same. We dont just have a resistor loop circuit once we add the scope probe and ground wire loop. When we do that,we now have a second loop,and there for a second circuit.
You are disagreeing with your own numbers, they speak for themselves. It is your interpretation of the numbers etc. that is not correct. You have a sum total of 540mV across the resistors, that is a fact.

Quote
If i raise the value of the 100R to 200R,and the value of the 1K to 2K,then i would expect the EMF value seen on the scope to rise as well,as we reduce the current,but increase the voltage.
In theory, it should not, and you will not see what you expect. 1V emf is 1V emf, so the voltages should stay close to the same, but the circuit current will be reduced somewhere near 50%.
Title: Re: Lewin's NCF Experiment and Lecture
Post by: lumen on April 27, 2016, 10:05:27 PM
The scope shot your missing is the one that is not connected to anything!
Just a closed loop around the inductor using the scope probe and ground wire.

What you get when checking R1 around the inductor is then the full circle voltage minus R1 which equals the voltage at R2.
Checking R2 around the inductor is the full circle voltage minus R2 voltage which equals the voltage at R1.
I see no mystery.

Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on April 27, 2016, 10:35:37 PM
The scope shot your missing is the one that is not connected to anything!
Just a closed loop around the inductor using the scope probe and ground wire.
I guess you missed the posts where I advised him to do that, and where he showed the result?

Title: Re: Lewin's NCF Experiment and Lecture
Post by: lumen on April 28, 2016, 12:46:10 AM
Yes, thank you I did miss that post.

 :D
Title: Re: Lewin's NCF Experiment and Lecture
Post by: tinman on April 28, 2016, 01:07:03 AM



Quote
You are disagreeing with your own numbers, they speak for themselves. It is your interpretation of the numbers etc. that is not correct. You have a sum total of 540mV across the resistors, that is a fact.

Well ATM it is not a fact,as i have 100 odd mV across the wire between the resistors them self.
So from the results of my test so far,there is not just 540mV total across the two resistors.
First we have to find out why i have a larger voltage across the resistor joining wire than i do across the 100R resistor. Until such time as that is worked out,i cannot totally agree with what you say.

Quote
In theory, it should not, and you will not see what you expect. 1V emf is 1V emf, so the voltages should stay close to the same, but the circuit current will be reduced somewhere near 50%.

I am going to spend the time tonight,and wind another solenoid out of the building wire such as you have used. I will also make a neater resistor loop-one i can change out with various resistor value loops.

At this point in time,i think it best if you get your setup up and running,and see if you too get that voltage across the resistor joining wires. If not,then we have to work out why i am seeing a voltage across those joining wires. Until that is sorted one way or the other,i cannot make any kind of accurate accounting of what im seeing.

Brad
Title: Re: Lewin's NCF Experiment and Lecture
Post by: tinman on April 28, 2016, 01:15:57 AM


What you get when checking R1 around the inductor is then the full circle voltage minus R1 which equals the voltage at R2.
Checking R2 around the inductor is the full circle voltage minus R2 voltage which equals the voltage at R1.


Quote
The scope shot your missing is the one that is not connected to anything!
Just a closed loop around the inductor using the scope probe and ground wire.

Post 41 ?.

Quote
I see no mystery.

So the voltage across the wire between the two resistors has a larger voltage value than across the 100 ohm resistor why ?-not to mention that it also is the same polarity,even though the probe is now on the other side of the ground,and as such,the loop formed by the probe and ground wire is also now inverted.

Brad
Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on April 28, 2016, 01:17:27 AM
For the resistor circuit I used the solid 14/2 copper wire, stripped.

For the solenoid, I used jacketed 14 GA stranded AC wire, but solid may work fine as well.

Surely you agree the sum of 500mV and 40mV is 540mV? That is the voltage across the resistors. You should have 1V across the resistors, agreed?

So yes, as I have been saying, there is a problem with your setup. But in the conceptual sense, you are still getting the experiment correct.
Title: Re: Lewin's NCF Experiment and Lecture
Post by: tinman on April 28, 2016, 01:43:07 AM
For the resistor circuit I used the solid 14/2 copper wire, stripped.

For the solenoid, I used jacketed 14 GA stranded AC wire, but solid may work fine as well.

Surely you agree the sum of 500mV and 40mV is 540mV? That is the voltage across the resistors. You should have 1V across the resistors, agreed?

So yes, as I have been saying, there is a problem with your setup. But in the conceptual sense, you are still getting the experiment correct.

I would like to get it to be exact.
So first i/we need to find out why i am seeing a higher voltage across the resistor joining wire than i am across the 100 ohm resistor.
I am keen to see if you measure a voltage between your two resistors,across the wire.


Brad
Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on April 28, 2016, 03:13:51 AM
I would like to get it to be exact.
Good.

Quote
So first i/we need to find out why i am seeing a higher voltage across the resistor joining wire than i am across the 100 ohm resistor.
I am keen to see if you measure a voltage between your two resistors,across the wire.


Brad
I do and I don't.

I do if I measure with a "flying" gnd lead.

I remembered however (its been a few years) that since we're dealing with stray magnetic fields and at least a thousand to one resistor to wire resistance ratio, in order to get an accurate reading for the wire segments, I had to solder two pieces of wire onto the wire so the two could closely follow the circuit wire and be brought out to the middle and probed there. See the illustration. Measure it like that and you should see very little voltage across the wires.

With 32VDC on the cap, I get 400mV peak emf (calibration step with scope probe only), then about 360mV across R2, and -40mV across R1. With 0V across the wire segments, all measurements are as expected and the tally adds up perfectly.
Title: Re: Lewin's NCF Experiment and Lecture
Post by: Artoj on April 28, 2016, 03:18:34 AM
Just to be sure we don not mix, schematics(abstract) with actual wiring.  I hope this helps, the values are guesses only. Regards Arto. 
Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on April 28, 2016, 03:22:39 AM
We're not quite there yet, but thanks for the diagram.

I already have a diagram prepared, which is coming next with the discussion of perspective no. 2 (once we get over this wire segment hump).
Title: Re: Lewin's NCF Experiment and Lecture
Post by: tinman on April 28, 2016, 01:03:05 PM
Ok,so following your instructions Poynt, i ran some lead wires along the resistor loop wire,and away from the solenoid--as seen in pic below.
Happy to report no voltage at all detected across the wire between the resistors after 10 go's. ;)

So i applied the same to the resistors,and also happy to report that i have a total voltage across the resistors that is very close to that of the 980mV measured with the scope probe and ground loop around the solenoid.

So all things are as they should be now ;)
Now-one problem ::). Dose this not confirm Lewins experiment?.


Brad
Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on April 28, 2016, 03:25:30 PM
Ok,so following your instructions Poynt, i ran some lead wires along the resistor loop wire,and away from the solenoid--as seen in pic below.
Happy to report no voltage at all detected across the wire between the resistors after 10 go's. ;)

So i applied the same to the resistors,and also happy to report that i have a total voltage across the resistors that is very close to that of the 980mV measured with the scope probe and ground loop around the solenoid.

So all things are as they should be now ;)
Now-one problem ::) . Dose this not confirm Lewins experiment?.


Brad
Good stuff Brad.

Yes, you have replicated all the results and shown the effects of Lewin's experiment. As such, it would appear that Lewin's emphatic push to denounce Kirchhoff as not holding in such a case seems to be true, correct? (One can easily come to this conclusion because when summing the voltages measured around the loop, we come to the same voltage as the calibrated voltage, and it is not zero as Kirchhoff would prescribe).

Well, Lewin's experiment only demonstrated half the picture, and attempted to mislead the audience into thinking he was measuring potential differences in the circuit. The fact is that this is not entirely true, and the experiment as demonstrated only captures one perspective of reality. He is in fact measuring the E field, and it so happens that the E field in both perspectives overlaps when it comes to the values measured across the resistors. It does not however overlap when it comes to measuring the wire segments.

How then do we examine the experiment from the second perspective (the "real" one Lewin conveniently left out of his demonstration)? Simply make a twin lead cable long enough that it can run from your scope probe, to the ceiling (screw-hook), then drop down right above the solenoid so it can clip onto the various measurement points. This effectively decouples the measurement device from the experiment and permits us to measure the "real" potential differences across all of the components, i.e. the resistors and wire segments in between.

Redo the measurements using this "perspective", and compare your results. Be sure to measure across points D and A again too (i.e. across the middle of the wire segments as per Lewin's diagram), but first try to make a prediction of what voltage (if any) you will measure there.

Title: Re: Lewin's NCF Experiment and Lecture
Post by: wattsup on April 28, 2016, 06:25:48 PM
Redo the measurements using this "perspective", and compare your results. Be sure to measure across points D and A again too (i.e. across the middle of the wire segments as per Lewin's diagram), but first try to make a prediction of what voltage (if any) you will measure there.

Zero, because the potential will be equal from both sides.

wattsup
Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on April 28, 2016, 08:50:29 PM
Try also to predict what will be measured (if anything) across the wire segments.
Title: Re: Lewin's NCF Experiment and Lecture
Post by: Magluvin on April 28, 2016, 11:10:09 PM
Zero, because the potential will be equal from both sides.

wattsup

Should be in normal cases.  But this is sort of a current source event considering the 2 different value resistors are in a current loop and the wires are the current induced parts.

We once did an audio system in a mercedes. We had an engine/alternator wine in the tweeters we just couldnt figure out. Checked all grounds, rca cables. Repl amps, head unit, etc.   Come to find that the passive crossover network box was located near a car wire harness. It was inducing the coils in the crossover. Not that it was the same thing you guys were figuring out, but.....

Nice work on working out the scope lead problems. ;)

Mags

Title: Re: Lewin's NCF Experiment and Lecture
Post by: Magluvin on April 29, 2016, 12:03:34 AM
The 2 resistors in a loop with short lengths of wire between the resistors of different value to make the loop is very strange.  Now you could probably do a comparison by switching out the wires for D cells in series circuit.  I might expect the outcome before testing to be total voltage say 3v/1kohm=3ma , then multiply the current and each resistor to come up with the voltage across each resistor. But now Im questioning it a bit. lol

Mags
Title: Re: Lewin's NCF Experiment and Lecture
Post by: wattsup on April 29, 2016, 01:28:36 AM
Try also to predict what will be measured (if anything) across the wire segments.

Should be the same, Zero since the "impartation" will still be equal all around originating from the vertical coil each portion gets the same share. The resistors should not play into this.

In the previous tests by @tinman, is it possible that the resistors require a certain length of time to actually resist to their rated value and maybe the discharge is faster hence shorter then the actual drop in the resistors?

wattsup

 
Title: Re: Lewin's NCF Experiment and Lecture
Post by: Magluvin on April 29, 2016, 02:31:15 AM
Should be the same, Zero since the "impartation" will still be equal all around originating from the vertical coil each portion gets the same share. The resistors should not play into this.

In the previous tests by @tinman, is it possible that the resistors require a certain length of time to actually resist to their rated value and maybe the discharge is faster hence shorter then the actual drop in the resistors?

wattsup

If we think of the 2 wires as current sources, or just very low turn secondaries, then we look at the probe or meter impedance/internal resistance, and the wires current abilities could take that resistance path just like the other resistors take on those currents. So there could be a voltage measurement there I think. Pretty small, but probably not zero.

Mags
Title: Re: Lewin's NCF Experiment and Lecture
Post by: wattsup on April 29, 2016, 03:29:24 AM
If we think of the 2 wires as current sources, or just very low turn secondaries, then we look at the probe or meter impedance/internal resistance, and the wires current abilities could take that resistance path just like the other resistors take on those currents. So there could be a voltage measurement there I think. Pretty small, but probably not zero.
Mags

@Magsmaman

It's a trick question. The vertical coil is producing a "magnetic" (let's play the game) influence all around. The loop is catching this influence equally all around. The scope probe will see 5 volts and the ground will see 5 volts and the waveform will show the differential being zero volts. The zero volts does not mean there is zero in the loop, only that the scope sees the same energy level at both the probe and the ground clip. Zero does not mean zero. Ouch. We like to "think" that this is producing a "current flow" in "one direction" but that in my view is never the case. Each micron of that loop is its own generator because the vertical coil is only receiving one impulse so only one chance to create some change in the vicinity of the loop so regardless of where you are probing all points are equal so you will always see zero on the scope.

This is what I was trying to explain to @tinman how any times. Measuring across a cvr only means so much and it definitely does not prove "current flow". But again this only depends on how you handle your logic.

wattsup

Title: Re: Lewin's NCF Experiment and Lecture
Post by: tinman on April 29, 2016, 02:24:01 PM
Good stuff Brad.

Yes, you have replicated all the results and shown the effects of Lewin's experiment. As such, it would appear that Lewin's emphatic push to denounce Kirchhoff as not holding in such a case seems to be true, correct? (One can easily come to this conclusion because when summing the voltages measured around the loop, we come to the same voltage as the calibrated voltage, and it is not zero as Kirchhoff would prescribe).

Well, Lewin's experiment only demonstrated half the picture, and attempted to mislead the audience into thinking he was measuring potential differences in the circuit. The fact is that this is not entirely true, and the experiment as demonstrated only captures one perspective of reality. He is in fact measuring the E field, and it so happens that the E field in both perspectives overlaps when it comes to the values measured across the resistors. It does not however overlap when it comes to measuring the wire segments.

How then do we examine the experiment from the second perspective (the "real" one Lewin conveniently left out of his demonstration)? Simply make a twin lead cable long enough that it can run from your scope probe, to the ceiling (screw-hook), then drop down right above the solenoid so it can clip onto the various measurement points. This effectively decouples the measurement device from the experiment and permits us to measure the "real" potential differences across all of the components, i.e. the resistors and wire segments in between.

Redo the measurements using this "perspective", and compare your results. Be sure to measure across points D and A again too (i.e. across the middle of the wire segments as per Lewin's diagram), but first try to make a prediction of what voltage (if any) you will measure there.

Ok
Well i would expect that the result's across each resistor would be the same as in the last test,as the two wires leading out from each resistor in the last test, would both see the same electric and magnetic field,and so would cancel out any effect or induced voltage from the E field.
At points A and D (middle of each wire connecting the resistor's) i would expect to see a voltage value that is between(in the middle of) the voltage across the 100R and 1KR resistor-so some where near 400mV.

I have my daughters and grandkids over for a movie/sleep over night,so may not get to testing this tonight--but will see what time they all fall asleep lol.


Brad
Title: Re: Lewin's NCF Experiment and Lecture
Post by: ramset on April 29, 2016, 03:18:34 PM
Grandkids and a sleepover !!
Hide the Magnums or it'll be an all nighter !!

[especially those Chocolate Carmel ones ]

 8)
Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on April 29, 2016, 04:52:23 PM
How about the measurement across the wire segments. Any guesses?
Title: Re: Lewin's NCF Experiment and Lecture
Post by: wattsup on April 29, 2016, 06:44:55 PM
How about the measurement across the wire segments. Any guesses?
Reply 67 & 69
Title: Re: Lewin's NCF Experiment and Lecture
Post by: lumen on April 30, 2016, 07:41:06 AM
How about the measurement across the wire segments. Any guesses?

If you can run the probe leads vertical in a manner where current will not be induced in them, then measuring the wire segments will produce about half the signal as when the leads circled the coil. Each segment is a winding on the transformer core and would each provide about half of a full loop current.

Title: Re: Lewin's NCF Experiment and Lecture
Post by: Pirate88179 on April 30, 2016, 03:23:05 PM
Would slipping a small Faraday cage (Like a can shaped one) over the coil stop it from influencing the probe wires?  It would seem to me this would be the easiest way if it works.

Bill
Title: Re: Lewin's NCF Experiment and Lecture
Post by: wattsup on April 30, 2016, 05:26:46 PM
Would slipping a small Faraday cage (Like a can shaped one) over the coil stop it from influencing the probe wires?  It would seem to me this would be the easiest way if it works.
Bill

Actually what would be interesting to see is the following as shown below.

Keep the vertical coil (VC) (orange) and the center positioned loop (blue) but position two metal cones (grey) as shown to see if the impress from the VC to the loop is mainly perpendicular. Then remove the top cone and try again. Then remove the bottom cone and replace the top cone and try again. So what would this show you?

Then relate this to which is the open side of the VC that is then being connected to the discharge. Any relation there?

Then remove the cones and slide the loop 1/4 way up, try it, then near the top, try it, then 1/4 way down from center, try it, then all the way down and try it again. So what would this tell you? Simple little things can say a lot.

You can then compare the same as above but replace the cap discharge with a steady pulse. Now what will be the difference?

wattsup

Title: Re: Lewin's NCF Experiment and Lecture
Post by: tinman on May 01, 2016, 02:06:07 AM
Ok,below are the test results from the vertical wire test,where the two wires from the measuring points to probe are vertical to that of the resistor loop,and around 1.5 meters high-away from the DUT.
The scope probe and ground are then hooked to the end of these vertical wires-speaker like cable.

Results seem to be much as i expected.


Brad
Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on May 01, 2016, 02:35:14 PM
Brad.  ;)

When you have a chance, all that is left are the wire segment measurements.

Yes, your prediction of the voltage from D to A was correct.
Title: Re: Lewin's NCF Experiment and Lecture
Post by: tinman on May 01, 2016, 02:54:56 PM
Brad.  ;)

When you have a chance, all that is left are the wire segment measurements.

Yes, your prediction of the voltage from D to A was correct.

Ah yes-i missed that one Poynt.
Had a busy weekend with kids,grandkids and some work on my 4x4.
Will get it done ASAP-probably tomorrow night.

Brad
Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on May 01, 2016, 03:01:46 PM
OK Brad, no problem.

What is your prediction for the wire segments?

And sorry, your prediction AND measurement for the voltage across points D and A was/is correct.
Title: Re: Lewin's NCF Experiment and Lecture
Post by: tinman on May 01, 2016, 03:32:35 PM
OK Brad, no problem.

What is your prediction for the wire segments?

And sorry, your prediction AND measurement for the voltage across points D and A was/is correct.

I believe that it should be 0 as seen it the test with the wires running along side the resistor connector wire,but something is telling me that that will not be the case.

In all this(so far) dose this not make Lewins claim correct?,as we do not have 0 volts across the resistors as Kirchhoff would suggest.

Brad
Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on May 01, 2016, 03:37:17 PM
I'm not going to give you the answer (I know the answer), just curious what your thoughts were.

Perform the measurement and let's see what it is.
Title: Re: Lewin's NCF Experiment and Lecture
Post by: tinman on May 02, 2016, 03:01:07 PM
Well oddly enough,i get around 400mV across the bottom wire,and around 400mV across the top wire.

This was with the duel cable going to the scope probe vertical to the resistor loop,as in the other tests.

Why the two are different?-i have no idea,but i carried out the test many times on each wire,and the results were always the same.

Brad
Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on May 02, 2016, 04:59:39 PM
Well oddly enough,i get around 400mV across the bottom wire,and around 400mV across the top wire.
;) It is not 0V as most would seem to expect/guess.

Quote
Why the two are different?-i have no idea,but i carried out the test many times on each wire,and the results were always the same.

Brad
Do you mean why in the first case the measurement was 0V on each wire segment, and in this last case the sum is close to the calibration emf?

I was hoping you would have put it all together, as you've done quite well so far. Isn't this just a step-down transformer? We have 50 to 100 turns on the primary, and one turn on the secondary, loaded with a resistance of R1+R2.

You are measuring the actual emf induced in the secondary loop because the measurement device is decoupled from the primary (and secondary). That is why the measurements are different.

Now as a final step, use your vertical probe to go around measuring the 4 components (R1-->wire_seg-->R2-->wire_seg) in serial fashion following the current path and write the voltage down for each. If performed correctly, two measurements will be positive and two negative in polarity (the two resistors will be one polarity, and the two wire segments will be the other polarity). Add the 4 values together and you should be left with something close to 0V.
Title: Re: Lewin's NCF Experiment and Lecture
Post by: tinman on May 03, 2016, 01:30:02 AM
;) It is not 0V as most would seem to expect/guess.
Do you mean why in the first case the measurement was 0V on each wire segment, and in this last case the sum is close to the calibration emf?

I was hoping you would have put it all together, as you've done quite well so far. Isn't this just a step-down transformer? We have 50 to 100 turns on the primary, and one turn on the secondary, loaded with a resistance of R1+R2.

You are measuring the actual emf induced in the secondary loop because the measurement device is decoupled from the primary (and secondary). That is why the measurements are different.

Now as a final step, use your vertical probe to go around measuring the 4 components (R1-->wire_seg-->R2-->wire_seg) in serial fashion following the current path and write the voltage down for each. If performed correctly, two measurements will be positive and two negative in polarity (the two resistors will be one polarity, and the two wire segments will be the other polarity). Add the 4 values together and you should be left with something close to 0V.

Yes,the sum equals 0,but what has that to do with what Lewin claims?.
We can sum up voltages around any circuit to equal 0 if we measure from certain points using certain polarities.

It would seem to me that Lewins claim is correct,when measuring across the two resistors,as the sum is not 0,it is the induced 1volt EMF in this case,being the total across each resistor.
If we have a current flow,then we are not measuring just the E filed,but the EM field.
If it were just the E field,then Kirchhoff law may hold,in that the measuring was carried out incorrectly. But as we also have current flow,and a magnetic field,then it would seem to me that Kirchhoff's law dose not hold in this situation.


Brad
Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on May 03, 2016, 01:53:27 AM
Yes,the sum equals 0,but what has that to do with what Lewin claims?.
It has everything to do wtih Lewin's claim that Kirchhoff does not hold in this case. In his lecture he is clearly not only comparing apples and oranges, but he is muddling the whole affair entirely.

Quote
We can sum up voltages around any circuit to equal 0 if we measure from certain points using certain polarities.
In other words, if you always ensure your measurement apparatus is decoupled from the DUT, you will not only measure the correct voltages across each component, but you will prove that Kirchhoff holds each and every time. Lewin claims that it does not, and you just proved that it indeed does hold every time.

Quote
It would seem to me that Lewins claim is correct,when measuring across the two resistors,as the sum is not 0,it is the induced 1volt EMF in this case,being the total across each resistor.
If we have a current flow,then we are not measuring just the E filed,but the EM field.
If it were just the E field,then Kirchhoff law may hold,in that the measuring was carried out incorrectly. But as we also have current flow,and a magnetic field,then it would seem to me that Kirchhoff's law dose not hold in this situation.
I can't make much sense out of this last bit. Each statement on its own is either wrong or nonsensical and seems to indicate that you don't understand what you should have just learned.
Title: Re: Lewin's NCF Experiment and Lecture
Post by: tinman on May 03, 2016, 01:43:47 PM
It has everything to do wtih Lewin's claim that Kirchhoff does not hold in this case. In his lecture he is clearly not only comparing apples and oranges, but he is muddling the whole affair entirely.
In other words, if you always ensure your measurement apparatus is decoupled from the DUT, you will not only measure the correct voltages across each component, but you will prove that Kirchhoff holds each and every time. Lewin claims that it does not, and you just proved that it indeed does hold every time.
I can't make much sense out of this last bit. Each statement on its own is either wrong or nonsensical and seems to indicate that you don't understand what you should have just learned.

Perhaps a review of my posted result's?,as you seemed to have missed something.

Diagram 1 and associated scope shot has to be inverted in order for us to do our loop measurement points in order.So invert the scope probe position,and invert the scope shot,so as to gain the correct value,which will be negative 400mV.

Diagram 2(which is the correct polarity) and associated scope shot,shows us the potential across the 1k resistor. We can safely say that it is close to 800mV.

Diagram 3(which is the correct polarity) and associated scope shot,again shows us a value that is close to negative 400mV.

800mV-400mV-400mV gives us a value of 0 volt's.
But we still have the 100 ohm resistor to measure,and so our total value is not 0 volts.

Kirchhoff's voltage law is based on the assumption that there is no fluctuating/changing magnetic field linking the closed loop. In our DUT,there is clearly a changing/fluctuating magnetic field linking the loop,and this can be seen in all my scope shot's by the reversing current flow across each measuring point during the decoupled tests-and all the other tests carried out in this thread that i have done.

Quote-KVL is based on the assumption that there is no fluctuating magnetic field linking the closed loop.  In the presence of a changing magnetic field the electric field is not a conservative vector field. Therefore the electric field cannot be the gradient of any potential. That is to say, the line integral of the electric field around the loop is not zero, directly contradicting KVL.

The results of my test,the results of Lewins test,and the definition of KVL indicating that a changing magnetic field linking the closed loop make KVL invalid in this case,seems to all fit together,and Lewins statement is correct.

What have i missed here Poynt?.


Brad
Title: Re: Lewin's NCF Experiment and Lecture
Post by: minnie on May 03, 2016, 02:14:33 PM



  I live on the side of a hill. I set out to shepherd my sheep, go down to the river,
  up to the windmill and then go home. When I get home I'm at the same elevation
  as when I started.
      I can't walk anymore and the kids got me this-
Title: Re: Lewin's NCF Experiment and Lecture
Post by: tinman on May 03, 2016, 02:31:25 PM


  I live on the side of a hill. I set out to shepherd my sheep, go down to the river,
  up to the windmill and then go home. When I get home I'm at the same elevation
  as when I started.
      I can't walk anymore and the kids got me this-

It's a good thing that there was not a land slide under your house while you were away from home,as then you would have gained more energy going down hill to the river than it took you to get back home :D

The energy form point A to point B and back to point A can change if there is another force acting upon one of those points,or the area between the points.

I drove my boat from my house jetty, down river to my favorite fishing spot,and used 2 liters of fuel.
I drove my boat back to my house jetty from my favorite fishing spot,following the same path,but used 3 liters of fuel on the return trip. ;)

Brad
Title: Re: Lewin's NCF Experiment and Lecture
Post by: minnie on May 03, 2016, 02:31:50 PM



  I'm a bit baffled today, I came across the term "convection current". Then I
  encountered "self capacitance".
    I was just wondering why after seven decades I've all of a sudden taken an
  interest in these things, and I've worked that one out, it's Google!
             John.
Title: Re: Lewin's NCF Experiment and Lecture
Post by: tinman on May 03, 2016, 02:50:18 PM


  I'm a bit baffled today, I came across the term "convection current". Then I
  encountered "self capacitance".
    I was just wondering why after seven decades I've all of a sudden taken an
  interest in these things, and I've worked that one out, it's Google!
             John.

Quote: The inter-winding capacitance of a coil is sometimes called self-capacitance,[9] but this is a different phenomenon. It is actually mutual capacitance between the individual turns of the coil and is a form of stray, or parasitic capacitance. This self-capacitance is an important consideration at high frequencies. It changes the impedance of the coil and gives rise to parallel resonance. :D

Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on May 03, 2016, 03:05:07 PM
Perhaps a review of my posted result's?,as you seemed to have missed something.
I've missed nothing. I am fully aware that your numbers don't quite add up. We've seen that from the very beginning.

Quote
Diagram 1 and associated scope shot has to be inverted in order for us to do our loop measurement points in order.So invert the scope probe position,and invert the scope shot,so as to gain the correct value,which will be negative 400mV.
I think I speak for all here when I say STOP USING THE INVERT SWITCH ON YOUR SCOPE! It is not required 99% of the time and it only confuses the matter.

Quote
800mV-400mV-400mV gives us a value of 0 volt's.
But we still have the 100 ohm resistor to measure,and so our total value is not 0 volts.
You apparently don't realize it, but your setup or measurements aren't perfect (which is ok, I doubt mine is either), otherwise all 4 voltages would sum to 0V. I sincerely hope you're not implying that a non-zero sum means Kirchhoff doesn't hold?

Quote
Kirchhoff's voltage law is based on the assumption that there is no fluctuating/changing magnetic field linking the closed loop. In our DUT,there is clearly a changing/fluctuating magnetic field linking the loop,and this can be seen in all my scope shot's by the reversing current flow across each measuring point during the decoupled tests-and all the other tests carried out in this thread that i have done.

Quote-KVL is based on the assumption that there is no fluctuating magnetic field linking the closed loop.  In the presence of a changing magnetic field the electric field is not a conservative vector field. Therefore the electric field cannot be the gradient of any potential. That is to say, the line integral of the electric field around the loop is not zero, directly contradicting KVL.

The results of my test,the results of Lewins test,and the definition of KVL indicating that a changing magnetic field linking the closed loop make KVL invalid in this case,seems to all fit together,and Lewins statement is correct.

What have i missed here Poynt?.


Brad
You've gone and googled and drank the coolaid instead of paying close attention and analyzing what I've shown you here.

I am fully aware that the E field is nonconservative, and that Kirchhoff does not apply when generating and measuring the E field. However, Lewin was equating the E field with potential drops across resistors in a circuit and then espousing that Kirchhoff does not hold. It does hold, as you've mostly proved with your measurements. Lewin was clearly mixing the two concepts (potential difference and E field measurements) in an effort to discredit Kirchhoff, but the bottom line is that Kirchhoff holds regardless if the circuit is static (DC), or dynamic (Faraday induction). When the case is dynamic, the DC battery is replaced with an inductance, through which the emf is produced in the circuit, and it is this emf (conveniently not illustrated by Lewin) that is in exact opposition to the potential drops across the resistors, resulting in a zero sum.
Title: Re: Lewin's NCF Experiment and Lecture
Post by: ramset on May 03, 2016, 03:05:51 PM
Minnie
Don't stop walking ... those little battery scooters that "help" people get around .

they _help_ people Lose all mobility thru atrophy .

Re Google and folks your age , They say you'll become addicted because you weren't raised with it .

Chet

Title: Re: Lewin's NCF Experiment and Lecture
Post by: tinman on May 03, 2016, 03:33:20 PM
I've missed nothing. I am fully aware that your numbers don't quite add up. We've seen that from the very beginning.






Quote
I think I speak for all here when I say STOP USING THE INVERT SWITCH ON YOUR SCOPE! It is not required 99% of the time and it only confuses the matter.

I have not used the invert switch on the scope for any of these measurements--you misunderstood what i was saying. The first scope point/polarity has to be inverted to do our loop test point measurements,as the polarity show is opposite to that needed for a series loop measurement.

Quote
I am fully aware that the E field is nonconservative, and that Kirchhoff does not apply when generating and measuring the E field. However, Lewin was equating the E field with potential drops across resistors in a circuit and then espousing that Kirchhoff does not hold. It does hold, as you've mostly proved with your measurements. Lewin was clearly mixing the two concepts (potential difference and E field measurements) in an effort to discredit Kirchhoff, but the bottom line is that Kirchhoff holds regardless if the circuit is static (DC), or dynamic (Faraday induction). When the case is dynamic, the DC battery is replaced with an inductance, through which the emf is produced in the circuit, and it is this emf (conveniently not illustrated by Lewin) that is in exact opposition to the potential drops across the resistors, resulting in a zero sum.

Ah ok,now i see where i was looking in the wrong direction.
Also,would using a longer solenoid coil,and higher value resistor's,enable me to gain more accurate measurements?,as i would like to get this to all work out much closer to correct values to those i have-just to clean up my experiment a bit.

Quote
You've gone and googled and drank the coolaid instead of paying close attention and analyzing what I've shown you here.

Well first up,after hearing this saying many times here on this forum,and being Australian--what the hell is coolaid?--im guessing some sort of wonder drink?.
I always search google for information when i dont see what i should be seeing,or to understand something a little better than i do-->is this not what we all do?.

Quote
You apparently don't realize it, but your setup or measurements aren't perfect (which is ok, I doubt mine is either), otherwise all 4 voltages would sum to 0V. I sincerely hope you're not implying that a non-zero sum means Kirchhoff doesn't hold?

It would appear that my non-zero sum is due to slight measurement error,and/or the circuit not being quite up to scratch,which left me around 100mV under the total 0 volt result i should have seen-hence my trip down google lane to find the answers :-\


Brad
Title: Re: Lewin's NCF Experiment and Lecture
Post by: Pirate88179 on May 03, 2016, 04:50:13 PM
Brad:

Koolaid is a powdered fruit drink mix that has been a favorite of kids over here since the 60's.

There phrase "Drink the Koolaid" comes from a tragic event where a guy named Jim Jones led a group of folks that he had brainwashed to a place in Guyana and ran the place like a drug crazed dictator.  When a US Senator went there to see if the folks were OK, he was shot and killed.  Jones then instructed his followers that God want them to drink the Koolaid with him.  The Koolaid was loaded with poison and all of them were found dead a few days later.  Everyone in the group followed this nut and did whatever he told them to do including killing themselves.

So, when someone is said to have drunk the Koolaid, it simply means they have blindly accepted and followed something someone else has told them without any reasoning on their own.

Sorry, if the above is not totally clear but, it will give you some idea of the history of this often used phrase.

Bill

PS  Next, we should talk about the phrase "Jump the shark" which comes from a Happy Days tv program episode.
Title: Re: Lewin's NCF Experiment and Lecture
Post by: tinman on May 03, 2016, 05:05:06 PM
Brad:

Koolaid is a powdered fruit drink mix that has been a favorite of kids over here since the 60's.

There phrase "Drink the Koolaid" comes from a tragic event where a guy named Jim Jones led a group of folks that he had brainwashed to a place in Guyana and ran the place like a drug crazed dictator.  When a US Senator went there to see if the folks were OK, he was shot and killed.  Jones then instructed his followers that God want them to drink the Koolaid with him.  The Koolaid was loaded with poison and all of them were found dead a few days later.  Everyone in the group followed this nut and did whatever he told them to do including killing themselves.



Sorry, if the above is not totally clear but, it will give you some idea of the history of this often used phrase.

Bill

PS  Next, we should talk about the phrase "Jump the shark" which comes from a Happy Days tv program episode.

Quote
So, when someone is said to have drunk the Koolaid, it simply means they have blindly accepted and followed something someone else has told them without any reasoning on their own.

Well if that is the meaning behind it,then Poynt needs to retract his comment,as i blindly followed no one. I did some research in the hope of finding answers that arose from my own test result's-as i always do. If i did not,then it could be just as easy to say that i drank the Poynt coolaid,where i blindly followed him,without researching the answers for my self.


Brad
Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on May 03, 2016, 09:31:55 PM
I have not used the invert switch on the scope for any of these measurements--you misunderstood what i was saying. The first scope point/polarity has to be inverted to do our loop test point measurements,as the polarity show is opposite to that needed for a series loop measurement.
I assumed these were fresh measurements, as I had suggested that you do all 4 in a row. But apparently you have not done that. You may wish to try doing them, as there is the possibility they may sum to 0V.
Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on May 03, 2016, 09:43:15 PM
Well if that is the meaning behind it,then Poynt needs to retract his comment,as i blindly followed no one. I did some research in the hope of finding answers that arose from my own test result's-as i always do. If i did not,then it could be just as easy to say that i drank the Poynt coolaid,where i blindly followed him,without researching the answers for my self.


Brad
You posted/quoted something that 99% of academics espouse, and I assume you take it for granted that it is correct. That is ok, as most of the time they are.

You have just proven on your bench however that they are wrong (and right, depending on the perspective).

It really comes down to defining  precisely what one is referring to when making statements about dynamic fields and KVL. Most get it all mixed up or are simply unclear or too general (like your quote). If the method of measurement isn't made, then the statement is ambiguous. It must be qualified, otherwise it is just noise.
Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on May 03, 2016, 09:54:53 PM
Quote-KVL is based on the assumption that there is no fluctuating magnetic field linking the closed loop.
False. You have proven this yourself. If the measurements are performed to measure the potential differences, KVL does hold.

Quote
In the presence of a changing magnetic field the electric field is not a conservative vector field.
True.

Quote
Therefore the electric field cannot be the gradient of any potential. That is to say, the line integral of the electric field around the loop is not zero, directly contradicting KVL.
True, but no one is trying to apply KVL on the E field itself; that is their incorrect assumption! So the point is moot.

When dynamic fields are the source of emf in a circuit, KVL still applies to the potential differences in the circuit.
Title: Re: Lewin's NCF Experiment and Lecture
Post by: minnie on May 03, 2016, 10:41:59 PM
Ok
Title: Re: Lewin's NCF Experiment and Lecture
Post by: tinman on May 04, 2016, 01:16:41 AM
False. You have proven this yourself. If the measurements are performed to measure the potential differences, KVL does hold.
True.
True, but no one is trying to apply KVL on the E field itself; that is their incorrect assumption! So the point is moot.

When dynamic fields are the source of emf in a circuit, KVL still applies to the potential differences in the circuit.

Well i am happy that i carried on through with this experiment,and i thank you Poynt for teaching me something new :)

Time well spent,and can only lead to more accurate measuring techniques,and understandings in the future.


Cheers
Brad
Title: Re: Lewin's NCF Experiment and Lecture
Post by: poynt99 on May 04, 2016, 04:45:18 AM
My pleasure Brad.

Cheers.