Lewin's NCF Experiment and Lecture

[tinman]

 author=poynt99 link=topic=16557.msg482146#msg482146 date=1461760819]

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.

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.

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.

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?

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]

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.

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.

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

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

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]

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.

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.

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.

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]

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]

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.

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%.