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Author Topic: Lewin's NCF Experiment and Lecture  (Read 45493 times)

minnie

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Re: Lewin's NCF Experiment and Lecture
« Reply #15 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.

poynt99

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Re: Lewin's NCF Experiment and Lecture
« Reply #16 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?

tinman

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Re: Lewin's NCF Experiment and Lecture
« Reply #17 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

tinman

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Re: Lewin's NCF Experiment and Lecture
« Reply #18 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

TinselKoala

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Re: Lewin's NCF Experiment and Lecture
« Reply #19 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".

tinman

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Re: Lewin's NCF Experiment and Lecture
« Reply #20 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

tinman

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Re: Lewin's NCF Experiment and Lecture
« Reply #21 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

tinman

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Re: Lewin's NCF Experiment and Lecture
« Reply #22 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

poynt99

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Re: Lewin's NCF Experiment and Lecture
« Reply #23 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.

poynt99

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Re: Lewin's NCF Experiment and Lecture
« Reply #24 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?

tinman

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Re: Lewin's NCF Experiment and Lecture
« Reply #25 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

poynt99

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Re: Lewin's NCF Experiment and Lecture
« Reply #26 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?

wattsup

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Re: Lewin's NCF Experiment and Lecture
« Reply #27 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

minnie

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Re: Lewin's NCF Experiment and Lecture
« Reply #28 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.

Johan_1955

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Re: Lewin's NCF Experiment and Lecture
« Reply #29 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