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  • #16
    Originally posted by deemon View Post
    They shows the same polarity and voltage , I checked it of course . And the scope itself cannot inverse the pulse because the "inverted" button on the scope panel ( near the channel B input ) wasn't pressed ...
    In that case, it must be pickup in the probe leads.
    Please can you move one of the probes (while it is still connected) so that it lies parallel to the axis of the ferrite rod. I would expect the voltage to change or even vanish altogether.

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    • #17
      You are really taking this serious??
      C'mon!
      Each students generation do remember some "crackpoty" profesor!
      "The one", funny, interesting and "especial".
      I do remember one pretty weird too !
      In especially prepared cabinet for Physics he used to abet few students to walk in special room and than to perform demostration of how strong low frequencies affects human behavior!
      There was a large glass window between cabinet and that room, so the rest of students could see their colleagues across.
      Than he used to start strong generator, working under 20Hz for short time.

      You can only imagine what than could happen.
      There was a rumor saying he became weird like that because he was the first to use that room.... oftenly!

      So Prof. Lewin is just another "crackpoty" profesor, trying to be funny and interesting.
      But seems this time he trapped into unintentional prank believing he discovered something revolutionary that will reverse some of the main laws in Physics.
      Already seen so many times in the past.
      Seems he became victim of his own unfulfilled ambitions in life!

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      • #18
        I think that the diagram with the 1V battery, two resistors and two stipulated points of reference is fine for the first example he gave. However it breaks down when he changed the circuit by removing the battery. The diagram was no longer complete, it does not account for the induced voltages. There should have been the addition of inductors shown between the resistor as well as additional points of reference. I think the experiment would be hard to duplicate without playing around a lot with lead lengths, etc.

        Food for thought at any rate. Thanks for putting it up Carl.

        Jerry

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        • #19
          I might go out on a limb here and say I understand what he is saying, if you don't I suggest watching it again.

          Listen, Kirchhoff's law is based on the conservation of energy, which is flawed, because he does not follow conservation of energy despite building his theorem on that very foundation.
          It is assuming potential energy is the same as an induced magnetic field, it is not.

          I don't think Kirchoff's law is to be tossed out with the bathwater, as the Prof. said it could be considered a special case of Faraday, that is all.

          What you (and Kirchoff) are not considering is the energy conversion factor of inductors.

          I am simplifying somewhat, as I know I am about to be FLAMED by the usual suspects. Although they say I am "ignored" so maybe they won't read this and we can have a fruitful discussion after all.

          Thanks Carl.

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          • #20
            Remember even a small resistor converts voltage into heat, possibly magnetic fields.

            For this reason, 1V can never equal 1V after going through a 100ohm and 900ohm resistor.

            Practically speaking, it works for our "general" projects though.

            Consider it rounding up the pennies. (Kirchoff's law)

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            • #21
              Originally posted by Qiaozhi View Post
              If you have two scope probes connected to the same point in the circuit, then you must display the same voltage.
              I agree, this is an absolute requirement... otherwise, I could take one single probe, connect it a first time and read 900mV, then remove it and connect it a second time to the exact same point, and it suddenly reads -100mV. No, sorry, physics doesn't work this way.

              Better look closer.

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              • #22
                We can view a PI transmit waveform by simply connecting the cro probe tip to the cro ground lead and presenting this single turn coil to the detector's coil and this is what we see in deemon's post. In other words, one single turn coil formed by the cro leads is on one side of the ferrite coil and the second single turn coil formed by the cro leads is on the other side of the ferrite coil. This would explain the difference in amplitude and polarity and perhaps you should also consider this when using modern day high impedance multimeters.

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                • #23
                  Originally posted by Carl-NC View Post
                  I agree, this is an absolute requirement... otherwise, I could take one single probe, connect it a first time and read 900mV, then remove it and connect it a second time to the exact same point, and it suddenly reads -100mV. No, sorry, physics doesn't work this way.

                  Better look closer.
                  OK - so I haven't gone completely mad ..... yet.

                  Originally posted by crane View Post
                  We can view a PI transmit waveform by simply connecting the cro probe tip to the cro ground lead and presenting this single turn coil to the detector's coil and this is what we see in deemon's post. In other words, one single turn coil formed by the cro leads is on one side of the ferrite coil and the second single turn coil formed by the cro leads is on the other side of the ferrite coil. This would explain the difference in amplitude and polarity and perhaps you should also consider this when using modern day high impedance multimeters.
                  This is why I asked Deemon to reposition the leads so that they lie parallel to the axis of the ferrite core. This should eliminate any pickup. Hopefully he can run this simple test, otherwise I'm going to be forced to repeat the experiment myself.

                  I'm still sticking with my original conclusion:
                  That the probes of the "measuring instrument" across R1 in circuit 2 (no battery) are reversed when compared to circuit #1 (with battery). Hence the polarity reversal. It's a basic measurement error, and there is no conflict between Kirchoff and Faraday.

                  Deemon's replication of the experiment only confuses the issue, due to pickup in the scope leads, and therefore "appears" to agree with the Professor's results by introducing a different error.

                  I'm absolutely convinced that Professor Lewin knows exactly what he's doing with this little demonstration. It's a real teaser!

                  Comment


                  • #24
                    I'll admit my ignorance as I don't understand the setup with the solenoid in the first place. Is he basically energizing the solenoid (roughly) in the center of a loop consisting of the two resistors and wire connecting them? Are his resistors truly only resistive or do they have inductance (wirewound)?

                    Comment


                    • #25
                      Originally posted by bklein View Post
                      I'll admit my ignorance as I don't understand the setup with the solenoid in the first place. Is he basically energizing the solenoid (roughly) in the center of a loop consisting of the two resistors and wire connecting them? Are his resistors truly only resistive or do they have inductance (wirewound)?
                      The setup acts like a transformer. The solenoid is in the primary and the resistor loop is the single-turn secondary. A current is induced in the loop from the solenoid, and flows around the loop in a clockwise direction. The loop is not a complete short-circuit, as there is a 100R resistor on the left, and a 900R resistor on the right. The Professor claims that the voltage across R1, when driven by a battery inserted in the loop (circuit #1) measures as +0.1V. But in the solenoid example, it measures as -0.1V. His conclusion is that Kitchoff's Law breaks down in the second case.

                      This is, of course, total BS.
                      I'm sure the Professor knows this. It's an example designed to provoke debate and encourage experimentation, and it's certainly doing that!

                      Comment


                      • #26
                        Originally posted by Qiaozhi View Post
                        OK - so I haven't gone completely mad ..... yet.
                        This is why I asked Deemon to reposition the leads so that they lie parallel to the axis of the ferrite core. This should eliminate any pickup. Hopefully he can run this simple test, otherwise I'm going to be forced to repeat the experiment myself.
                        Of course I can make it , but after finishing some other experiments .... very interesting experiments , by the way . If I succeed , it will be very interesting concept of inductive balance . I will write about it , of course .

                        And what about of Kirchoff's law , as I understand it - it's not completely wrong , of course , but it was declared for the other conditions . In the usual electric chain we can have some discrete batteries , resistors , etc .... and we must calculate the sum of the voltages of those batteries , and divide this sum to the sum of resistors , and find a loop current . It's a usual procedure in such a cases .... but what we have in the Faraday's case ? We haven't a battery ( with the fixed voltage ) , but now every inch of the wire works like a battery - because some external force pushes electrons in the wire and makes them to move . And every resistor - works now not only like a resistor , but like a part of "battery" too - all electrons inside the resistor being pushed forward the same way . So we cannot calculate the sum of discrete voltage values ( like Kirchoff wants ) but need to calculate the integral on the entire loop . But when we do it - we can divide it by the sum of the resistance and find our the current in the loop , it's not a problem of course . So we haven't any "breaking of the laws" , we only need to use more general law - and I think that professor meant it in his lecture .

                        Comment


                        • #27
                          I agree.

                          Comment


                          • #28
                            Originally posted by Qiaozhi View Post
                            The setup acts like a transformer. The solenoid is in the primary and the resistor loop is the single-turn secondary. A current is induced in the loop from the solenoid, and flows around the loop in a clockwise direction. The loop is not a complete short-circuit, as there is a 100R resistor on the left, and a 900R resistor on the right. The Professor claims that the voltage across R1, when driven by a battery inserted in the loop (circuit #1) measures as +0.1V. But in the solenoid example, it measures as -0.1V. His conclusion is that Kitchoff's Law breaks down in the second case.

                            This is, of course, total BS.
                            I'm sure the Professor knows this. It's an example designed to provoke debate and encourage experimentation, and it's certainly doing that!
                            To me so many things in the setup have changed and the measurement method changed that it seems a silly comparison. What if he were to monitor the current when the battery connection is made or disconnected with a scope probe? On a metal table vs a wood one.. What if he were to do this with the solenoid nearby and not? As you say, it is a steady state power source with no transformer vs a transient state with transformer and pulse power source.

                            Brings me to wonder - in a pulse TX signal, does the coil current vary noticably during the TX pulse if there is a target or not? I would think it would (at high probe sensitivity) but never looked at it. Could this variation, if not for detection purposes, be used for ground balance purposes?

                            Comment


                            • #29
                              Originally posted by Aziz View Post
                              *LOL*
                              Does anybody remember the "energy loss in charging a capacitor" - problem? That's funny too.
                              http://www.smpstech.com/charge.htm

                              Half the energy has disappeared. Where did it go?

                              I'll try to find it. Because I have a TEE-degree ("Trial & Error-Engineer"). Particularly when I'm as drunk as a lord.

                              Cheers,
                              Aziz
                              Funny, I used to give that question when I worked at Bell Labs and was interviewing new EE's for apprentice positions. The question was, something like: Two capacitors, each of identical value C, separated by a switch. Before the switch is closed, one capacitor is initially charged to voltage V, while the other one is discharged.

                              Question #1A: What is the total energy of the system prior to closing the switch?

                              Question #1B: What is the total charge contained in the system?

                              Now the switch is closed, effectively connecting the two capacitors in parallel. After the system has settled,

                              Question #2A: What is now the total energy of the system?

                              Question #2B: What is now the total charge of the system?

                              Question #3: Please comment about the results of your findings.

                              In all the years I interviewed new EE's for entry level positions, only 1 ever gave the correct answer and only 5 answered question 3 stating that the previous answers didn't make sense and there was something wrong, but they didn't know why.

                              Depending on which way they worked the problem, either half the energy was lost, or half the charge disappeared between questions 1 and 2.

                              The simple answer is that Ohm's law and Kirchhoff's circuit laws are derivable from Maxwell's equations after making a couple of assumptions which, in circuit theory, result in two no-nos. Ohm's and Kirchhoff's do not work in cases where you have a current impulse flowing through a capacitor, or a voltage impulse across an inductor (impulse is the Dirac delta function ... essentially an infinite amount even if just for an instant).

                              Another favorite question of mine was giving them a cube made out of soldered 1 Ohm resistors on each edge of the cube. The question was for them to calculate the equivalent resistance of the cube when measuring it across diagonally opposite corners and then verify their answer with an Ohm meter.

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                              • #30
                                Originally posted by Rudy View Post
                                a cube made out of soldered 1 Ohm resistors on each edge of the cube. The question was for them to calculate the equivalent resistance of the cube when measuring it across diagonally opposite corners and then verify their answer with an Ohm meter.
                                =5/6R
                                ??? => http://www.metacafe.com/watch/624539...ble_hypercube/
                                (=2/3R)

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