Lewin's NCF Experiment and Lecture

[Magluvin]

The 2 resistors in a loop with short lengths of wire between the resistors of different value to make the loop is very strange.  Now you could probably do a comparison by switching out the wires for D cells in series circuit.  I might expect the outcome before testing to be total voltage say 3v/1kohm=3ma , then multiply the current and each resistor to come up with the voltage across each resistor. But now Im questioning it a bit. lol

Mags

[wattsup]

Try also to predict what will be measured (if anything) across the wire segments.

Should be the same, Zero since the "impartation" will still be equal all around originating from the vertical coil each portion gets the same share. The resistors should not play into this.

In the previous tests by @tinman, is it possible that the resistors require a certain length of time to actually resist to their rated value and maybe the discharge is faster hence shorter then the actual drop in the resistors?

wattsup

[Magluvin]

Should be the same, Zero since the "impartation" will still be equal all around originating from the vertical coil each portion gets the same share. The resistors should not play into this.

In the previous tests by @tinman, is it possible that the resistors require a certain length of time to actually resist to their rated value and maybe the discharge is faster hence shorter then the actual drop in the resistors?

wattsup

If we think of the 2 wires as current sources, or just very low turn secondaries, then we look at the probe or meter impedance/internal resistance, and the wires current abilities could take that resistance path just like the other resistors take on those currents. So there could be a voltage measurement there I think. Pretty small, but probably not zero.

Mags

[wattsup]

If we think of the 2 wires as current sources, or just very low turn secondaries, then we look at the probe or meter impedance/internal resistance, and the wires current abilities could take that resistance path just like the other resistors take on those currents. So there could be a voltage measurement there I think. Pretty small, but probably not zero.
Mags

@Magsmaman

It's a trick question. The vertical coil is producing a "magnetic" (let's play the game) influence all around. The loop is catching this influence equally all around. The scope probe will see 5 volts and the ground will see 5 volts and the waveform will show the differential being zero volts. The zero volts does not mean there is zero in the loop, only that the scope sees the same energy level at both the probe and the ground clip. Zero does not mean zero. Ouch. We like to "think" that this is producing a "current flow" in "one direction" but that in my view is never the case. Each micron of that loop is its own generator because the vertical coil is only receiving one impulse so only one chance to create some change in the vicinity of the loop so regardless of where you are probing all points are equal so you will always see zero on the scope.

This is what I was trying to explain to @tinman how any times. Measuring across a cvr only means so much and it definitely does not prove "current flow". But again this only depends on how you handle your logic.

wattsup

[tinman]

Good stuff Brad.

Yes, you have replicated all the results and shown the effects of Lewin's experiment. As such, it would appear that Lewin's emphatic push to denounce Kirchhoff as not holding in such a case seems to be true, correct? (One can easily come to this conclusion because when summing the voltages measured around the loop, we come to the same voltage as the calibrated voltage, and it is not zero as Kirchhoff would prescribe).

Well, Lewin's experiment only demonstrated half the picture, and attempted to mislead the audience into thinking he was measuring potential differences in the circuit. The fact is that this is not entirely true, and the experiment as demonstrated only captures one perspective of reality. He is in fact measuring the E field, and it so happens that the E field in both perspectives overlaps when it comes to the values measured across the resistors. It does not however overlap when it comes to measuring the wire segments.

How then do we examine the experiment from the second perspective (the "real" one Lewin conveniently left out of his demonstration)? Simply make a twin lead cable long enough that it can run from your scope probe, to the ceiling (screw-hook), then drop down right above the solenoid so it can clip onto the various measurement points. This effectively decouples the measurement device from the experiment and permits us to measure the "real" potential differences across all of the components, i.e. the resistors and wire segments in between.

Redo the measurements using this "perspective", and compare your results. Be sure to measure across points D and A again too (i.e. across the middle of the wire segments as per Lewin's diagram), but first try to make a prediction of what voltage (if any) you will measure there.

Ok
Well i would expect that the result's across each resistor would be the same as in the last test,as the two wires leading out from each resistor in the last test, would both see the same electric and magnetic field,and so would cancel out any effect or induced voltage from the E field.
At points A and D (middle of each wire connecting the resistor's) i would expect to see a voltage value that is between(in the middle of) the voltage across the 100R and 1KR resistor-so some where near 400mV.

I have my daughters and grandkids over for a movie/sleep over night,so may not get to testing this tonight--but will see what time they all fall asleep lol.


Brad