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Author Topic: Lewin's NCF Experiment and Lecture  (Read 45480 times)

minnie

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Re: Lewin's NCF Experiment and Lecture
« Reply #30 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

poynt99

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Re: Lewin's NCF Experiment and Lecture
« Reply #31 on: April 26, 2016, 12:26:57 AM »
What were you building there, and why did it go up in smoke?

tinman

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Re: Lewin's NCF Experiment and Lecture
« Reply #32 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

poynt99

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Re: Lewin's NCF Experiment and Lecture
« Reply #33 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.

Johan_1955

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Re: Lewin's NCF Experiment and Lecture
« Reply #34 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

Magluvin

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Re: Lewin's NCF Experiment and Lecture
« Reply #35 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

tinman

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Re: Lewin's NCF Experiment and Lecture
« Reply #36 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

poynt99

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Re: Lewin's NCF Experiment and Lecture
« Reply #37 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.

lumen

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Re: Lewin's NCF Experiment and Lecture
« Reply #38 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.

poynt99

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Re: Lewin's NCF Experiment and Lecture
« Reply #39 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.

tinman

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Re: Lewin's NCF Experiment and Lecture
« Reply #40 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

tinman

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Re: Lewin's NCF Experiment and Lecture
« Reply #41 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

tinman

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Re: Lewin's NCF Experiment and Lecture
« Reply #42 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

tinman

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Re: Lewin's NCF Experiment and Lecture
« Reply #43 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

poynt99

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Re: Lewin's NCF Experiment and Lecture
« Reply #44 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.