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Author Topic: Lewin's NCF Experiment and Lecture  (Read 45486 times)

tinman

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Re: Lewin's NCF Experiment and Lecture
« Reply #45 on: April 27, 2016, 03:11:37 PM »
 author=poynt99 link=topic=16557.msg482146#msg482146 date=1461760819]



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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. :)

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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.

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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.

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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?

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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

poynt99

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Re: Lewin's NCF Experiment and Lecture
« Reply #46 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.

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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.

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

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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.

tinman

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Re: Lewin's NCF Experiment and Lecture
« Reply #47 on: April 27, 2016, 04:06:36 PM »





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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.

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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.

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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.

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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

lumen

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Re: Lewin's NCF Experiment and Lecture
« Reply #48 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.


poynt99

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Re: Lewin's NCF Experiment and Lecture
« Reply #49 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.

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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%.

lumen

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Re: Lewin's NCF Experiment and Lecture
« Reply #50 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.


poynt99

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Re: Lewin's NCF Experiment and Lecture
« Reply #51 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?


lumen

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Re: Lewin's NCF Experiment and Lecture
« Reply #52 on: April 28, 2016, 12:46:10 AM »
Yes, thank you I did miss that post.

 :D

tinman

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Re: Lewin's NCF Experiment and Lecture
« Reply #53 on: April 28, 2016, 01:07:03 AM »



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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.

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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

tinman

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Re: Lewin's NCF Experiment and Lecture
« Reply #54 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.


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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 ?.

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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

poynt99

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Re: Lewin's NCF Experiment and Lecture
« Reply #55 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.

tinman

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Re: Lewin's NCF Experiment and Lecture
« Reply #56 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

poynt99

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Re: Lewin's NCF Experiment and Lecture
« Reply #57 on: April 28, 2016, 03:13:51 AM »
I would like to get it to be exact.
Good.

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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.

Artoj

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Re: Lewin's NCF Experiment and Lecture
« Reply #58 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. 

poynt99

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Re: Lewin's NCF Experiment and Lecture
« Reply #59 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).