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Author Topic: Lewin's NCF Experiment and Lecture  (Read 45495 times)

poynt99

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Lewin's NCF Experiment and Lecture
« 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.]
« Last Edit: April 25, 2016, 02:04:47 AM by poynt99 »

poynt99

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Re: Lewin's NCF Experiment and Lecture
« Reply #1 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?

poynt99

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Re: Lewin's NCF Experiment and Lecture
« Reply #2 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?

poynt99

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Re: Lewin's NCF Experiment and Lecture
« Reply #3 on: April 24, 2016, 04:57:18 PM »
What voltage will the left meter indicate?

tinman

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Re: Lewin's NCF Experiment and Lecture
« Reply #4 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

tinman

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Re: Lewin's NCF Experiment and Lecture
« Reply #5 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

poynt99

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Re: Lewin's NCF Experiment and Lecture
« Reply #6 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?

tinman

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Re: Lewin's NCF Experiment and Lecture
« Reply #7 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

tinman

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Re: Lewin's NCF Experiment and Lecture
« Reply #8 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

poynt99

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Re: Lewin's NCF Experiment and Lecture
« Reply #9 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.

poynt99

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Re: Lewin's NCF Experiment and Lecture
« Reply #10 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?

poynt99

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Re: Lewin's NCF Experiment and Lecture
« Reply #11 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.

minnie

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Re: Lewin's NCF Experiment and Lecture
« Reply #12 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.

poynt99

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Re: Lewin's NCF Experiment and Lecture
« Reply #13 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.

Johan_1955

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Re: Lewin's NCF Experiment and Lecture
« Reply #14 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