Is MIT Prof. Lewin wrong about Kirchhoff's law?

[yungman]

I am offering to take this through from first principles to a fruitful discussion about Lewin's experiment.

If you do not want to do this let us abandon it now.

I would like that. As I said, I am an engineer and I consider myseft very strong in the practical experiment and setup. But I am no expert in theory and I am still learning. Actually I think I did something really wrong on the drawing with Cabraham.

 

[yungman]

In an e/m wave, E & H (B) are normal. I'll double check tonight, but I'm perplexed by your inference that the external B & E fileds are both along the z axis. For a transverse e/m wave, E & H/B are perpendicular to each other, not coincident. I'll get back later.

Claude

Your comment here really bordered me for days, I can't help but keep thinking why. I think I was wrong on how I draw the TEM wave coming up on the z axis all together. I am going to start another thread on figuring out the field pattern and the propagation pattern of the solenoid or magnetic dipole in a new thread instead of putting more stuff in here.

I hope you and other people can help me on establishing how the radiation pattern and how it affect the loop outside.

The thread is "Help in TEM wave propagation for a solenoid or magnetic dipole "


Thanks

Alan

 

[cabraham]

Your comment here really bordered me for days, I can't help but keep thinking why. I think I was wrong on how I draw the TEM wave coming up on the z axis all together. I am going to start another thread on figuring out the field pattern and the propagation pattern of the solenoid or magnetic dipole in a new thread instead of putting more stuff in here.

I hope you and other people can help me on establishing how the radiation pattern and how it affect the loop outside.

The thread is "Help in TEM wave propagation for a solenoid or magnetic dipole "

Thanks

Alan

This subject is over everybody's head. Seriously, the late great physics genius Richard Feynman struggled with Faraday's law. If RF struggled, I don't feel so bad. Again, those who have difficulty grasping this problem are in good company. One can be very brilliant & still struggle with this topic. I am in the home stretch of my Ph.D. program at a school known for being tough. I passed the qualifer which was one third e/m theory. It was very tough. Still, I know that my ability to visualize fields in 3 dimensions is limited.

Picturing an E & an H propagating along the z axis w/ multiple reflections, etc. can make my head spin. But let's look at the basic quantities, & it becomes workable.

Does everyone fully understand the concept of "curl"? It is synonymous with "rotation" & "circulation". It is not that hard to grasp. An H field (or B) has a direction defined at some region in space. In a plane, a loop is inserted such that the magnetic flux density B is incident upon the plane of the loop. The integral of B over the area of the loop is the flux linkage "phi". If phi changes w/ time, then there is an E field oriented such that it has "curl/rotation/circulation" in the x-y plane of the loop. That is the definition of curl. B is normal to the x-y plane & E is in the plane w/ a rotation. This rotation means that there is a net force acting on the free charges in said loop.

If the E was in a uniform direction, i.e. no curl, there would be no current in the loop since the forces cancel. To fully visualize this, I suggest studying peer reviewed university texts on EE & physics e/m field theory. Study the curl in detail.

When the concept of curl/rotation/circulation is so well understood that it becomes second nature, you will have taken a big step forward. Comments/questions welcome. BR.

Claude

 

[yungman]

This subject is over everybody's head. Seriously, the late great physics genius Richard Feynman struggled with Faraday's law. If RF struggled, I don't feel so bad. Again, those who have difficulty grasping this problem are in good company. One can be very brilliant & still struggle with this topic. I am in the home stretch of my Ph.D. program at a school known for being tough. I passed the qualifer which was one third e/m theory. It was very tough. Still, I know that my ability to visualize fields in 3 dimensions is limited.

Picturing an E & an H propagating along the z axis w/ multiple reflections, etc. can make my head spin. But let's look at the basic quantities, & it becomes workable.

Does everyone fully understand the concept of "curl"? It is synonymous with "rotation" & "circulation". It is not that hard to grasp. An H field (or B) has a direction defined at some region in space. In a plane, a loop is inserted such that the magnetic flux density B is incident upon the plane of the loop. The integral of B over the area of the loop is the flux linkage "phi". If phi changes w/ time, then there is an E field oriented such that it has "curl/rotation/circulation" in the x-y plane of the loop. That is the definition of curl. B is normal to the x-y plane & E is in the plane w/ a rotation. This rotation means that there is a net force acting on the free charges in said loop.
This is exactly what I drawn out in the other thread. Have you have a chance to look at that? Actually I started that thread because of the discussion with you. As you can see the E field of the TEM is circular and is parallel to the loop with resistors. I don't even dare to say that the E field can induce current in the loop because FL said it is the B field through the loop that cause the emf. If my drawing is correct, then the E field definitely have an effect of the flowing of current in the loop with resistors and your assertion would be correct. I hope that make your day...BUT that would be at odd with the FL! I am confused! Go over there and put in your view.
If the E was in a uniform direction, i.e. no curl, there would be no current in the loop since the forces cancel. To fully visualize this, I suggest studying peer reviewed university texts on EE & physics e/m field theory. Study the curl in detail.

When the concept of curl/rotation/circulation is so well understood that it becomes second nature, you will have taken a big step forward. Comments/questions welcome. BR.

Claude

This is very very hard. I studied the engineering EM twice, now I start studying part of Griffiths book, there is a lot of new materials that not covered in the engineering EM. I am just starting to study Gauge and deeper into Retarded potential which are not covered by any of the engineering EM book in detail. THis is tuff stuff for me studying on my own. But as I said, I don't want to go into this in this thread as it is too long already, better to start fresh in the other.

 

[stevenb]

I'd like to post some preliminary experimental results to provide some material for pondering while partaking of your holiday spirits. I want to be absolutely clear that these are preliminary results and I don't expect anyone to accept them until I've provided very detailed documentation and proof of what I've done. I'd also like to be clear that I have not completed all necessary experimental steps. For example, I have not done the exact differential measurement of yungman/sarumonkey. Further, I want to improve my setup to provide controlled current ramps. A final report on all completed experiments is probably about 2 weeks away because of holidays, sick wife and work schedule.

So far, I've carefully designed and built a solenoid to provide the main flux change for the resistor loop. This involved determining calculated and measured values of, resistance, inductance, time constant, magnetic field inside, magnetic field at the coil ends, sampling of external fields and calculation of total flux versus drive current. This then allowed estimation of voltage (20 V) needed to provide a measurable flux change of about 250 mV for the circuit emf. (I will eventually report all details) My setup for providing the voltage step to the main solenoid is still crude in that I use a knife switch. This is unreliable, and as I said I will build a circuit to do this in a controlled fashion. Still, it is possible to use the knife switch and get some useful data. By monitoring the voltage step with a scope and using that to trigger the scope, I can be sure which test cases have a clean voltage application with no transients. Also note that I'm not crazy about using the knife switch because it seems to be stressing my very nice and expensive Agilent power supply which occationally does an overcurrent shutdown when transients are created by the knifeswitch.

I've attached a jpeg file with picture of the coil with a loop attached around it. The loop has a 90 Ohm resistor (made from 3 parallel 270 Ohm resistors) and a 900 Ohm resistor (made from 2 parallel 1800 Ohm resistors) soldered right next to each other, with a wire used to complete the loop. This loop is as close to the solenoid as possible to minimize flux leakage.

I've also attached a pdf showing 4 test cases and the measured values. Anyone who carefully studies the measurement and the routing of the loops will see that these single-scope measurements reveal that scope measurements do not change when the ground clip is attached to either end of the connecting wire, provided that the routing of the scope leads does not allow penetration of the main flux change into the measurement loop (i.e. loop 3 from my earlier post). Yungman does not comprehend the details of what I've done, so please ignore his previous objections and any further ones that are likely to follow this post. In other words, it is clear that two independent and isolated scopes, as in the Lewin experiment, would not care where on the wire the scope ground is, and this wire effectively is a node for measurement purposes despite the fact that there can be an emf on the wire. I've already provided the theoretical explanation for this nonintuitive result using Faraday's law. (Prof. Lewin has as well, and in much better form.)

As I said, I have not done the differential measurement yet, and it is important that I do so because sarumonkee is on record claiming that this type of measurement allows him to determine the emf on the wire. Also, yungman is addament that this is correct. Personally, I am unable to explain this from a theoretical point of view, but the whole idea of experiments is to let you see things you can't visualize yet. Perhaps the differential measurement using one dual channel scope does something strange to the loops. Again, this seems counter-intuitive to me and seems to violate Faraday's Law, but I'll do the measurements soon and we'll see what we see.

Anyway, this is preliminary so I'd recommend doing more pondering than objecting until I do better experiments and carefully document everything in a form that a good skeptical scientist can accept.

 

[yungman]

Stevenb

I don't understand why you do the scope ground lead around the solenoid in case two. You are intentionally introduce a lope using your ground leads. The new loop will be from probe tip to the scope impedence, back to the ground leads to the other side of the 90ohm. That you realling going to circle all the flux giving out by the solenoid. Why do you intentionally do this? You are working, don't you even have a duel channel scope that you can hook up both at the same time?

Or if you only have single channel, try hook up in the middle of the wire so you only have half a loop and see what reading you get, see whether you get 230mV in case two. That would not be too hard to try out, right?

Why don't you try for once instead of spiking me, hook up the right way to avoid any loop creating by scope probe and ground leads, repeat what Saru did measuring the wire with the ground lead in the middle of the wire and measure both end of the wire and see what you get. If what your assertion is correct, then you should not get anything. Just make sure your probe ground leads do not go around the solenoid. You have your setup already, it will only take you less than a minute to do this and take a picture and post it here.

 

[stevenb]

Stevenb

I don't understand why you do the scope ground lead around the solenoid in case two. You are intentionally introduce a lope using your ground leads. The new loop will be from probe tip to the scope impedence, back to the ground leads to the other side of the 90ohm. That you realling going to circle all the flux giving out by the solenoid. Why do you intentionally do this? You are working, don't you even have a duel channel scope that you can hook up both at the same time?

Or if you only have single channel, try hook up in the middle of the wire so you only have half a loop and see what reading you get, see whether you get 230mV in case two. That would not be too hard to try out, right?

Why don't you try for once instead of spiking me, hook up the right way to avoid any loop creating by scope probe and ground leads, repeat what Saru did measuring the wire with the ground lead in the middle of the wire and measure both end of the wire and see what you get. If what your assertion is correct, then you should not get anything. Just make sure your probe ground leads do not go around the solenoid. You have your setup already, it will only take you less than a minute to do this and take a picture and post it here.

Your post is nonresponsive. You deliberately ignore my repeated statement that these are preliminary results and that I will be doing exactly the things you mention.

You also ignore the clear intent of the diagram that shows the ground leads are routed in a way to not form an open loop that might encircle flux.

And, once again you are unable to comprehend the significance of the information given. You have a clear measurement that shows the same voltage reading with the wire and without the wire. In other words the wire emf can't be measured in any loop that does not encircle flux. I already explained that this is due to FL demanding that the scope leads also have counteracting emf on them. Too bad you consider this "bla bla bla".

In any event I recommend that you ponder instead of object and wait patiently for a full report, as I asked.

 

[stevenb]

OK, after a rejuvenating dinner, I feel obligated to not dismiss all of this. I don't know if it can help to answer, but why not try.

I don't understand why you do the scope ground lead around the solenoid in case two. You are intentionally introduce a lope using your ground leads. The new loop will be from probe tip to the scope impedence, back to the ground leads to the other side of the 90ohm. That you realling going to circle all the flux giving out by the solenoid. Why do you intentionally do this?

The new loop you speak of is not new at all. It is always there. One if free to think of two paths for the measurements in cases 1-4. One path goes through the 90 ohm resistor and the other path goes through the 900 ohm resistor. One of these paths does not enclose the flux, while the other one does. Faraday's law works on both paths. In one, there is little enclosed flux, so the scope reads the potential on that resistor. On the other path, the flux is enclosed and so you read the potential of the other resistor plus the enclosed flux emf. Both values must be equal. FL always works!

The point of the diagram is to show that you can gradually slide the ground connection along the wire without changing the measurement even if emf is on the wire. The only way to change the measurement is to change the path of the ground lead, not the point that it attaches too. This shows why the emf on the wire need never be thought of. This is the point of Prof. Lewin's experiment. This is the entire point and you miss it.

You are working, don't you even have a duel channel scope that you can hook up both at the same time?

This is a good example of how it can be frustrating talking to you. So many points get missed, and the task of going back and trying to clarify everything becomes difficult, especially since the clarifications themselves will also be misunderstood. I clearly stated that I used the other channel of the scope to monitor and trigger off of the input step voltage. I needed to do this to make sure that I had a high quality step excitation of the coil. I also, mentioned that I'm worried about damaging my expensive power supply because of my poor switching method, which I will improve. So, I'm trying to do the minimal number of experiments before I build the drive circuit. Once I have a reliable drive circuit, I can use the input voltage on the trigger input, and use the dual channels as you say. This is all planned and will be done.

Or if you only have single channel, try hook up in the middle of the wire so you only have half a loop and see what reading you get, see whether you get 230mV in case two. That would not be too hard to try out, right?

Of course that would not be hard to try, so (risking my power supply a couple of times more) I will do that and post the result tomorrow. Do you really think I won't get 230 mV with the ground lead routed on one side and 23 mV with the ground lead routed on the other side? Wow, you really miss the entire point.

Why don't you try for once instead of spiking me,

Why don't I try for once? I'm trying to do experiments now, which I've outlined clearly and even given a time for expected completion and delivery of a report. I've also tried providing theoretical explanations and analysis. I'm not trying to spike you. I'm just trying to limit the damage your misinformation will do to those trying to learn. I can't do that by beating about the bush, so I call it like I see it.

 

[yungman]

OK, after a rejuvenating dinner, I feel obligated to not dismiss all of this. I don't know if it can help to answer, but why not try.

The new loop you speak of is not new at all. It is always there. One if free to think of two paths for the measurements in cases 1-4. One path goes through the 90 ohm resistor and the other path goes through the 900 ohm resistor. One of these paths does not enclose the flux, while the other one does. Faraday's law works on both paths. In one, there is little enclosed flux, so the scope reads the potential on that resistor. On the other path, the flux is enclosed and so you read the potential of the other resistor plus the enclosed flux emf. Both values must be equal. FL always works!
I see that you intensionally introducing the loop with the ground lead to pick up the induction to get the reading.
The point of the diagram is to show that you can gradually slide the ground connection along the wire without changing the measurement even if emf is on the wire. The only way to change the measurement is to change the path of the ground lead, not the point that it attaches too. This shows why the emf on the wire need never be thought of. This is the point of Prof. Lewin's experiment. This is the entire point and you miss it.
You graduate slide the ground only result in graduately increase the loop of the probe ground and slowly get more induction of voltage.

This is a good example of how it can be frustrating talking to you. So many points get missed, and the task of going back and trying to clarify everything becomes difficult, especially since the clarifications themselves will also be misunderstood. I clearly stated that I used the other channel of the scope to monitor and trigger off of the input step voltage. I needed to do this to make sure that I had a high quality step excitation of the coil. I also, mentioned that I'm worried about damaging my expensive power supply because of my poor switching method, which I will improve. So, I'm trying to do the minimal number of experiments before I build the drive circuit. Once I have a reliable drive circuit, I can use the input voltage on the trigger input, and use the dual channels as you say. This is all planned and will be done.
You take things too personal. Why don't you calm down, cut the put down and do the experiment. This is very un becoming and have no place in this highly educated forum.
Of course that would not be hard to try, so (risking my power supply a couple of times more) I will do that and post the result tomorrow. Do you really think I won't get 230 mV with the ground lead routed on one side and 23 mV with the ground lead routed on the other side? Wow, you really miss the entire point.



Why don't I try for once? I'm trying to do experiments now, which I've outlined clearly and even given a time for expected completion and delivery of a report. I've also tried providing theoretical explanations and analysis. I'm not trying to spike you. I'm just trying to limit the damage your misinformation will do to those trying to learn. I can't do that by beating about the bush, so I call it like I see it.

Why don't you try measuring the wire WITHOUT the ground leads looping around and play this kind of trick, just measuring the voltage on the wire and see what happen. Make it so you minimize the measurement effect. I think you are going out of your way to pickup voltage with your probe. Hopefully your scope have an external input that you can use to trigger and free up the second channel for two probe and put one one each resistor.

BTW, all the scope including my old Tek 465 have an external trigger input, this is a standard way of getting two channel display while still get a good trigger source. AND spend less time typing insult towards me. This is very un-becoming. Are you educated? What kind of engineering are you? And what kind of projects you design?

 

[stevenb]

.

BTW, all the scope including my old Tek 465 have an external trigger input, this is a standard way of getting two channel display while still get a good trigger source.

And again, you miss the message. I said that I need to monitor the input voltage to the coil right now, not just do the trigger. I then explain exactly what you just said. That is, once I build the drive circuit, I can trigger on the separate input and then have two channels to work with. Here it is again from the previous post.

Once I have a reliable drive circuit, I can use the input voltage on the trigger input, and use the dual channels as you say. This is all planned and will be done.

So, is it that I'm taking it personally, or am I just personally having the problem that you don't listen to me? I feel it is the latter.

 

[yungman]

And again, you miss the message. I said that I need to monitor the input voltage to the coil right now, not just do the trigger. I then explain exactly what you just said. That is, once I build the drive circuit, I can trigger on the separate input and then have two channels to work with. Here it is again from the previous post.



So, is it that I'm taking it personally, or am I just personally having the problem that you don't listen to me? I feel it is the latter.

Why do you even have to measure the input voltage? You just put in whatever input step/pulse to get an out put. It is the ratio that is important. You get 230mV signal across the larger resistor, and if you don't change that, it would not vary from pulse to pulse. And beside it is the voltage ratio that you care, not the absolute voltage you worry. You are not looking for 10% difference, what does it matter if the output vary 10%? Free up two probes can give you freedom to measure ratio between two different points.

You think clearer if you have a cool head!

 

[stevenb]

Why do you even have to measure the input voltage? You just put in whatever input step/pulse to get an out put. It is the ratio that is important. You get 230mV signal across the larger resistor, and if you don't change that, it would not vary from pulse to pulse. And beside it is the voltage ratio that you care, not the absolute voltage you worry. You are not looking for 10% difference, what does it matter if the output vary 10%? Free up two probes can give you freedom to measure ratio between two different points.

You think clearer if you have a cool head!

I already explained why I need to monitor the input voltage right now and already explained that this is a temporary limitation. However once again, you do not have the courtesy to try and understand what I wrote.

You crack me up. You state such obvious things as if they are pearls of wisdom. Have a little patience man. I'm trying to proceed step by step systematically. All the things you ask will be done and are already planned, as I said.

If I actually do have any trouble thinking clearly, then it has nothing to do with needing to be cool, but would instead have something to do with the holiday time pressure and with my wife being very sick right now.

My wife goes to the doctor this morning. Assuming he does not put her in the hospital for emergency surgery, I may have time to continue working on this today and do some of the very obvious next steps that are already planned. Something else which will help me today is that I plan to bring my array of good scope probes into the lab today. Since i forgot them yesterday, I was stuck using only two old probes which is another limitation I had yesterday, but left this out of the discussion as needless babble. However since you have no patience to read what I wrote and to wait for what I already said I would do, here we are talking about unimportant nonsense.

 

[stevenb]

Or if you only have single channel, try hook up in the middle of the wire so you only have half a loop and see what reading you get, see whether you get 230mV in case two. That would not be too hard to try out, right?

So I have 10 spare minutes on my coffee break and did this measurement. I've attached the diagram and results. Exactly the same. Not sure what you were expecting, but this is what I was expecting.

 

[yungman]

I have no choice but to dig out my electronics components, scope and all to do this experiment myself. I have to dig through my shed, storage room and all to get the components and setup.

I build my electro-magnet by using a bunch of 5” 3/8” hex bolt and bundle them together to form the core approx. 2.5” diameter. I wound about 50 turns of insulated wires on it. My loop is using 100ohm and 1K 1/4W metal film resistors and a piece of wire to connect them together.

As shown in attachment “SB”, I label point between the two resistors point “A”. It is very short. Going around CCW, the other side of the 1K is point “B”. From B to point D is about half length of the loop. Then from D to “C” is the other half of the loop that connected back to the other side of the 100 resistor at “C”.

I don’t have a power supply nor have a pulse generator. So I just use a 1.5V D size cell as the source. I wind a lot of turns on the magnet to get the high ampere-turns. My theory is when I charge the coil up with about 5A, then disconnecting it will cause a big jump in voltage in the coil and induce a decent emf into the loop of concern. It worked. I have to parallel a 33ohm resistor between the terminals of the coil to increase the time constant of the decay time. My scope is an old Tektronics 465 that has no storage capability. So I have to keep tapping the battery to look at the average size of the pulse height. The voltages posted are only the approximate average. Each pulse do vary a bit. But the information is valid because the readings are very consistence and reproducible.

I can reproduce what Stevenb’s experiment. As shown in attachment “SB”.




The experiments are shown in top row of the drawing SB.

a) Condition 1 show exact what Stevenb did with the probe on “A” and the ground lead go around and connect at “C”. I read about +200mV. See the setup in the picture “Full Loop”

b) Then I start moving the ground lead and when I move the ground lead over the top of the magnet core about half way as shown in Condition 2, the reading drop to about +100mV. See setup picture “Half Loop”.

c) Then I move the ground lead of the probe all the way to the left side as shown in Condition 3 where the ground lead totally stayed away from the magnet core. I read very low –ve pulse. See setup picture “No Loop”.

I gave it some thought and interpret the result. Here is my theory. I draw the equivalent loops on the second roll of the diagram, and the electrical equivalent of the circuit for each of the conditions on the third row of the SB. On the second row of the drawing, there are two loops that can pick up the induced emf from the magnet which I called loop 1 ( orange) and loop 2 ( red). I showed the core in the middle to show where the concentration of the magnetic fields. The magnetic field is pointing out of the page from the core. In each conditions ( 1,2 and 3) the loop 1( orange ) is the same. That is the loop we are interested in. Loop 2 (red) is my interpretation of the loop formed by the scope probe and it’s ground lead.




Interpretation of row 2 of the drawing in SB.

a)In Condition 1: You can see, loop 2 start from scope ground, going CCW to point C, then through the 100 ohm resistor to “A”, then to the probe back to the other side of the Rin inside the scope as shown. Loop 2 enclosed the total flux from the core so the voltage induced into loop 2 is 220mV.

b)In condition 2: loop2 (red) only enclosed half of the flux from the core so the voltage induced into loop 2 is about +110mV.

c)In condition 3: loop2 (red) does not enclosed any flux of the core, so the induce voltage into loop 2 is zero.





The third row is the equivalent circuit.

a)Condition 1: I show the V_{in} as a battery of +220mV. Since the scope ground lead circled the complete flux so it get the full induction of +220mV. The 0V reference is scope ground shown.as “Scope GND”. With scope ground as reference 0V, point “C” is driven to +220mV by V_{SL} . Point “A” would be 20mV below due to drop of the 100ohm resistor. And Point “B” is at 0V simply by voltage drop across the resistors.

b)Condition 2: The probe ground lead is over the top of the core and circled only half the flux by loop 2. This implies V_{SL} is only half the voltage and +110mV. Since the point “C” is really set by the V_{SL}, “C” is at +110mV, “B” is about +90mV. And “C” is actually -110mV reference to scope ground.
c)Condition 3: The scope ground lead is totally on the left side and did not circle any of the flux from the core. V_{SL} is 0V which put “C” at 0V, “B” at -220mV and “A” and about -20mV.



I repeated the experiment quite a few times to verify the observation. Important to node the voltage value is approximation by just observation the average peak but repeating taping the battery. So treat the number only accurate to 20%. But you can see the correlation. The interpretations are up for discussion. I have to set the whole thing up last night, did the observation, think about the reasoning behind the data and type all this by mid day today. My wife was furious when she saw all the stuff in the study room and we are going to have a party tomorrow. I have to tear everything down and clean up by tomorrow morning. So if any of you have other ideas to try out, please post back and I can do it tonight and post back.

There is second part of the experiment where the probe ground hooked up to “D” as what me and Saru talked about. I’ll post it a little later.


The three picture is how I did the experiment:
1) Full loop: the probe ground is going all the way around and attached to “C” as Stevenb.

2) Half loop: notice the ground lead is on top of the core and catch only half of the flux.

3) No loop: The probe ground is totally out of the way from the core and catch no direct flux from the core.

You have to be very careful in doing this experiment on where the scope probe lead is placed. You can really get fool by the observation. In this case the voltage reading is really in reference to the V_{SL} which set the voltage at "C". If you don't think it through, you really think the the measured voltage is INDEPENDENT to where you hook up the ground. In the picture, the ground lead of the probe is just hanging because I have to let it go and stand far away with my small camera in order to stay focus. I played a lot with the probe ground lead and is quite sensitive in some cases. The whole experiment took only a few minutes to go through, but I spent like two hours to just jiggling, tucking the ground leads, squeezing the lead to see the effect of the size of the loop created by the probe ground.

To prove my point. I actually remove the scope probe from the resistor loop, hook up a 330ohm resistor between the scope tip and the scope ground lead to form a loop and place it so it enclose the magnetic flux. I get exactly the same reading as what Stevenb's experiment +200mV. To confirm this, I flip the scope loop upside down and observe -200mV pulses. I concluded this the scope probe setup is the main cause of the reading in Stevenb and in condition 1. As I said, where and how you put the grounding is everything. Please join in the discussion. This is very reproducable. The length of the probe ground it not critical, I really don't see much noticable effect whether I let it hang slopply like in the picture or tuck it very tight around the core. Maybe, like 20% or so. But by moving the probe over the core, you really see a major difference, you can see the amplitude of the pulse just shrink in front of your eyes.

 

[stevenb]

Your pictures reveal that you have not been careful to run the ground wire right next to the main loop as I showed in my diagram. This is absolutely critical because otherwise the loop 3 captures a great deal if leakage flux. My final report will go into great detail on the proper setup, field measurements to show the leakage flux, screen capture of scope traces, pictures etc. etc. Feel free to do the same and we'll let others judge who has shown proper and careful measurements with theoretical backup of all conclusions.

I think anyone with a basic understanding of FL can already see the flaws in your pictures. I certainly can.

 

[yungman]

This is the second part where I hook the ground lead of the scope probe at point “D” and measure “B” and “C”. I saw exactly what Sarumonkee described. I saw -110mV on B and +110mV on C. But I did go further to prove because the experiment is very sensitive to the placement of the probe ground lead. I took the probe that was on “B” and start pulling the ground lead tight and closer to the loop on the magnet as shown in

a)Condition 1: of the attachment “ Center tap”. Notice the ground lead ( green ) is hugging the core. The signal shrink and fall off the scope. That cause me to rethink how sensitive the grounding is.

b) Condition 2: I relax the ground lead of the scope and let some loop area appeared. I think with the extra area, it actually caught part of the flux curl back as shown. It showed the flux are point into the paper. This is like introducing a voltage source I call V_R and you can see in the equivalent circuit.

c) Condition 3: Then I did the same thing like the other experiment and move the probe ground lead over the magnet core as shown in Condition 3. The voltage drop. In the equivalent circuit, it is like the V_{SL} decrease because the loop only catch half the flux.

d) Condition 4: When I finally move the probe ground lead all the way to the left side. The output actually change to -220mV. The reason is because now the probe ground is following the wire from “D” to “C” and have –ve 110mV induced into it. The equivalent circuit shown below.

This is my interpretation of the observation. Feel free to put in your comments. As I said, the pulse height is approximation, treat the reading as 20% uncertainty. Right at the transistion, I saw a lot of very high speed ringing, I only look at the part when it started to become a exponential decay pulse. I think putting a toroid onto the probe coax to introduce common mode rejection might help, but I don’t have one!

 

[yungman]

Your pictures reveal that you have not been careful to run the ground wire right next to the main loop as I showed in my diagram. This is absolutely critical because otherwise the loop 3 captures a great deal if leakage flux. My final report will go into great detail on the proper setup, field measurements to show the leakage flux, screen capture of scope traces, pictures etc. etc. Feel free to do the same and we'll let others judge who has shown proper and careful measurements with theoretical backup of all conclusions.

I think anyone with a basic understanding of FL can already see the flaws in your pictures. I certainly can.

I did tuck it tight, move it around. I can't show every single move. Read my second post about tucking the ground close to the core and move it around. I spent a lot of time moving the ground probe around. I don't mention on things that don't seem to make a big difference. Of cause I don't have the fancy setup as you are at work. But believe me, I know how to set up equipment. I spent years working in EM and CE testing and signal integrity. I have been an engineer and manager of engineering for over 25 years and I publishing two papers in Review of Scientific Instrument in American Institude of Physics. I spent years designing circuits and systems of various of mass spectrometer that have a lot of pulsing circuits that generate pulses like this kind of high speed exponantial decay pulse. Please don't talk to me as if I borned yesterday. We are all educated people here.

You seems to jump into conclusion really fast without giving much thoughts, how do you think I can take the picture and hold the ground lead and place it in the right place because I only use a small camera and have to stand far in order to get it to focus! As I said, I have the setup until tomorrow morning, if you think what can go wrong in my experiment, you are free to point out what I should do and I'll run it again.

Actually I was going to use cut coax to do all the measurement and I did on some. If you look at the picture very carefully, you see in the picture I have two coax point to point on the two resistors. Problem about the looping is there is not easy and short way to run the ground lead over the magnet core. Tucking the lead tight changing a little, but nothing close to changing the conclusion of my observation.

 

[stevenb]

I did tuck it tight, move it around. I can't show every single move. I spent a lot of time moving the ground probe around. Of cause I don't have the fancy setup as you are at work. But believe me, I know how to set up equipment. I spent years working in EM is CE testing and signal integrity. I have been an engineer and manager of engineering for over 25 years and I publishing two papers in Review of Scientific Instrument in American Institude of Physics. Please don't talk to me as if I borned yesterday. We are all educated people here.

You seems to jump into conclusion really fast without giving much thoughts, how do you think I can take the picture and hold the ground lead and place it in the right place because I only use a small camera and have to stand far in order to get it to focus! As I said, I have the setup until tomorrow morning, if you think what can go wrong in my experiment, you are free to point out what I should do and I'll run it again.

Well I only judged what you showed in the picture and what you showed is completely consistent with your measurements. Fair comments though, if you did something different I'll judge that later when you document. I also did quite a bit of checking and the results I showed can only be obtained if you completely close up the measurement loop 3 so that none of the circulating leakage flux goes back through this loop. Prof Lewin was also very careful to do this otherwise there is no way to quantify the total flux captured in loop3. Again this is all trivial stuff with FL.

 

[yungman]

Well I only judged what you showed in the picture and what you showed is completely consistent with your measurements. Fair comments though, if you did something different I'll judge that later when you document. I also did quite a bit of checking and the results I showed can only be obtained if you completely close up the measurement loop 3 so that none of the circulating leakage flux goes back through this loop. Prof Lewin was also very careful to do this otherwise there is no way to quantify the total flux captured in loop3. Again this is all trivial stuff with FL.

Well, now that we boths really get own hands wet, maybe we can come up with something. Take a look at my equivalent circuits and see what you think.

As I said I was going to use all coax to do point to point connection instead of scope probe. Problem is I only have 3 pieces of lemo coax, two are only 15" long, I cut two already! If I strip the shield 4" off to do the loop waving around, I don't have any length left to hook to the scope and there is no return! I have very limited resources...Something people take for granted while they are working, able to access the full lab!

BTW, I have nothing more to add, I describe the whole process in detail. In my two post, if you follow the 3 conditions, look at the drawing and the equivalent circuits, everything are there. Basically the emf induced on the probe ground lead V_{SL} \hbox { and }V_R are the main cause of false reading. I prove by moving the probe ground over the top and to the left side and I observed the pulse amplitude change drastically as explained in the two posts. I follow up with the probe forming a loop with it's own ground lead and loop over the core and observe the same voltage and polarity to show what you see is actually due to scope probe, not from the loop of concern. I double verified by flipping the probe loop over and see equal but opposite polarity pulses to confirm the effect is absolute coming from the scope probe itself, nothing to do with the professor's loop at all. So I stand by my conclusion for now and feel free to challenge and ask question. I still believe the professor measured wrong here.

 

[stevenb]

Well, now that we boths really get own hands wet, maybe we can come up with something. Take a look at my equivalent circuits and see what you think.

OK, I will do that.

I've been completely out of commission since last Thurs. Even my previous message and previous reading of your last couple of posts before that was made in a fog from my bed. The good news is that my wife was OK after a day at the hospital Thursday. However, I somehow caught a bug Thurs. morning and had to leave work and go home. I was then in bed unable to eat for 3 days, and slept right through Christmas. Then, after recovering, we got hit by the blizzard in New England, so I spent yesterday digging out. I'm not trying to complain or make excuses, but I do want those reading here to understand my late response. On a positive note (being a glass-half-full kind of guy), this is the first Christmas I lost weight rather than gained weight.

I still want to keep to my original schedule on providing detailed measurements and report and work through this issue slowly. I believe there may be more here that meets the eye and a measurement I did this morning seems to confirm that. Again, this is all preliminary. However, I did a single scope measurement and then compared the reading when I hooked up additional scope probes and grounds on a quad-scope I have access too. The readings were different, and I can't explain why yet. I believe that additional loops (from the 4 separate ground connections to the quad-scope) are somehow capturing flux, but I can't explain exactly how yet. I mention it now to allow others to think about it.

I will say that I stand by my previously-posted single-scope measurements and reconfirmed them again this morning. I also stand by my statement that the Lewin experiment is OK because he is using two isolated scopes that can be analyzed in the simple way I (and he!) mentioned. I'm not trying to restart that aspect of the debate, but just want to be clear that it still checks out from my point of view. However, i also see strange flux reading on wire when a differential (two probe with dual channel scope) measurement is done. When I use one probe only, there is no emf reading on the wire. I truly believe that a theoretical understanding is needed here. I'll think about it more, and I hope others do to.

I really need more time to think about this and study it experimentally. I won't try debating any of this yet. I just mention it to allow others to think also. I plan to set up two isolated and independent scopes and compare the reading with using one dual channel scope. Then, I'm going to try to make sense of the experimental results from a FL and loop analysis, if I can.

 

[stevenb]

Then, I'm going to try to make sense of the experimental results from a FL and loop analysis, if I can.

I believe that I can resolve this issue by looking at the open ground loops created by the independent ground paths through the scope probes. If you sketch it out it becomes clear. A simple fix for this appears to route wires from both probes (if using a dual channel scope) right next to each other and force both ground wires to follow the same path. Then the wires need to be routed in a way that completely closes all possible loop paths so that no flux can enter any closed path through other ground connections too. Very tricky ! A quick experiment seems to show that the expected results are obtained if this is done. The wire emf does not show on the scope and the Lewin results work out perfectly. I'll double check everything and document, but I feel this will be explainable in the end.

I clearly see why sarumonkey had the measurement he did. Great care and precautions are needed to get this measurement setup correct if a dual channel scope is used. Even with isolated voltmeters, great care is needed, but with a dual channel scope the problems are compounded. Trust me, I'll document and justify all this in the end. If you don't agree, I don't blame you yet. Please wait.

 

[yungman]

Sorry to hear about the misfortune and I hope you feel better. Take your time, the theory can wait. Now that we have two set up, we can double verify everything and put our heads together. I have both my grand kids here today and are going to stay with us over night, so I am kind of out of commision also.

Yes, I think there is more than eye can see. I can tell you, I still have not think of a way to avoid the pesty loop yet. If you have two scope, you are ahead of me. So far, I mainly use one probe to simplify things since the output is quite predictable. Anyway, take care of yourself first, this whole thing can wait.

Happy New Year

Alan

 

[stevenb]

Yes, thank you very much Alan. Happy New Year to you as well, and anyone else reading. Definitely a tricky setup and more time can only help make the final sense of things. Anything I am saying now should just be considered brainstorming until I can double check and verify.

Just to help make some sense of my above desciption which I expect is not easy to follow, I attached a picture here. I used channels 3 and 4 of the quad-scope to monitor the input voltage to drive coil. This allows me to leave those grounds hanging, and I use a differential voltage of CH4 minus CH3 to tell me the drive voltage and be sure it is a good quality experiment. Then channels 1 and 2 are used to monitor the two resistor voltages in the Lewin style. Note that the lead of one probe runs right next to the other leads which closes all paths that might capture leakage flux.

I can redo this experiment with the CH1 and CH2 probes moved to the outside of the respective resistors to monitor the emf on the half wire sections. No emf can be seen on the scope with this test. This type of setup may then provide the basis for doing the correct measurement. I need to clean it up and double check it, but it seems to make sense to me.

 

[yungman]

I have a few minutes looking at this. You need to tell us what experiment you are doing, we have been talking about Saru's experiment and also the experiment of measuring the 900ohm with ground lead moving around. My guess from your picture is Saru's experiment where the ground is in the middle of the wire at the back ( D ) on my picture and measure B and C. But I need to confirm this.

Second, did you route the probe lead around the coil and then hook the ground clip onto point D? Also the ground clip of the left probe also hooked onto point D also. Just want to confirm this. I have to go because my grandson is chasing me to play with the Wii or what ever! Too old to keep track what kind of game anymore, never been a gammer in my life!

 

[stevenb]

I have a few minutes looking at this. You need to tell us what experiment you are doing, we have been talking about Saru's experiment and also the experiment of measuring the 900ohm with ground lead moving around. My guess from your picture is Saru's experiment where the ground is in the middle of the wire at the back ( D ) on my picture and measure B and C. But I need to confirm this.

Yes, I've been keeping the ground point in the middle. I did two experiments this way using the dual scope.

First I did the Lewin Experiment, but remember his is with two isolated scopes while mine is with one scope with dual channels and common ground tied both at the scope and at the loop.

The second experiment was more similar to Saru's where I move the scope probe to the other side of the resistors (I believe these are the B and C points) to see if emf on the wire can be measured.

Second, did you route the probe lead around the coil and then hook the ground clip onto point D? Also the ground clip of the left probe also hooked onto point D also. Just want to confirm this.

Yes, both ground leads tied to the middle D point. The challenge is to do all this without creating any open loop that can capture the leakage flux. Of course, one can't do this perfectly, but the way I showed seems to reduce the loop areas sufficiently to get close to the results I expected.

 

[stevenb]

First I did the Lewin Experiment, but remember his is with two isolated scopes while mine is with one scope with dual channels and common ground tied both at the scope and at the loop.

I can provide the screen capture from the scope for the repeat of the Lewin test.

In the image, the yellow trace (CH1) monitors the 90 ohm resistor, the blue trace (CH2) monitors the 900 ohm resistor and the red trace is the monitor for the applied voltage to the coil. The applied voltage is initially 20V, but then the power supply goes into current limit (set to 10A). All information on time scale and voltage scale for each trace is on the screen.

Based on this, I conclude that the wire routing I use is reasonably successful in closing all possible loops that would capture leakage flux.

 

[stevenb]

The second experiment was more similar to Saru's where I move the scope probe to the other side of the resistors (I believe these are the B and C points) to see if emf on the wire can be measured.

I can provide the screen capture from the scope for the repeat of the Sarumonkey test across the wire itself.

In the image, the yellow trace (CH1) monitors the other side of the 90 ohm resistor, the blue trace (CH2) monitors the other side of the 900 ohm resistor and the red trace is the monitor for the applied voltage to the coil. The applied voltage is initially 20V, but then the power supply goes into current limit (set to 10A). All information on time scale and voltage scale for each trace is on the screen.

From this I conclude that the arrangement and routing successfully closes the loops that would corrupt the measurement. Essentially no wire emf is seen to be detected here. There is a very tiny pickup of emf, but I believe that this is from that little bit of leakage flux which gets into the measurement loop.

 

[stevenb]

So, if I put the above results into my document, I find it makes a passable first draft of what I wanted to do in terms of documentation. Although, I wanted to add more analysis, maybe it's better if I post this version and then I can update as new information becomes available in any discussions. To this purpose, I put a revision (starting at Rev A) marking at the top of the pages.

So, the attached pdf is an effective summary of what I did, but I left out the single scope measurements since those should be relatively simple to understand and verify. My personal view does show up in a few places in the document. I found this necessary to give the document some context. I do feel the measurements support my personal view, but I'm open to see if the measurements are also consistent with other viewpoints. I was objective and careful when doing the actual measurements, so I have a high degree of confidence in the data shown. Hopefully, even those that don't agree with my viewpoints, will at least find some value in the data itself.

 

[Studiot]

Hi Steve.

Is there a misplaced decimal point in entry line 4 of the B column in Table1?

 

[yungman]

The second part where you repeat what Sarumonkee did, agree with my result in post #226. I put the theory and equivalent circuit and what experiment I did also. Take a look at my attachment and explanation why you see very little voltage on the probe. My theory is because the probe ground lead and the part of the wire form a loop that pick up the exact same voltage. I have detail explanation where the loop of the probe in the equivalent circuit.

Also, I did experiment just with the setup and flip the ground lead over the top of the coil and onto the other side and see the voltage come up and max on the other side as in Condition 2 and Condition 3. And in row three of the attachment, I draw out the loop formed by the probe that cause the problem. This is explained in detail in #226.

If you look at my experiment and the drawings, I think we see the same observations. I don't think we have any disagreement on the result at all. It is just the interpretation of the result that is totally different.


Also finally I actually hook up the scope probe independently with the resistor between the probe tip and the scope probe ground lead to form the loop, loop it over the coil in the same direction and see the pulse as if I measure the 900 ohm ( 1K in my case) and when I flip the probe loop around, I read equal and opposite pulse. This show the loop form by the scope probe drawn will give you the same voltage as the loop I drawn in row three. This mean the observation is contaminated by the probe.

I have not come up with a good way to avoid the scope probe forming a loop that pick up the voltage yet. I am building a new coil so I can flip the probe ground lead back and fore a lot easier. This time I am planning to do away with the scope probe and use total coax to ensure there are no extra length of unwanted leads. I am going to repeat the experiment over to make sure I get the same result since we both are using regular scope to do the experiment so far. We are going to get to the end of this. Please read my theory and comment whether you agree and post your theory also...as I said, I think we are in allignment with the experiment so far and there's not much to debate about. Let me do the coax experiment to verify. If you have a chance, try flipping the probe ground lead back and fore and on top of the coil to see whether you observe what I posted.

It might take me a few days because I have to take down the Christmas Tree ( depressing ) and get ready for my grand daughter's birthday party.