Explain how this intresting and simple DIY train works 3

[marajendra]

Link
My take is this.
The magnets are conductive.
And any given instant, the magnet completes an electric circuit composed of the battery and a section of the coil (toroid) thereby forming a solenoid around the battery-magnet assembly. The assembly is in the center of the solenoid. We know that the magnetic field of a solenoid is almost constant inside the solenoid (except for fringing effect near the ends)decreases when we move away from the ends. So, this create a non-uniform magnetic field around the assembly.
The north and south poles of the magnet is oriented so that the magnetic field of solenoid and of the magnets are oppositely oriented, and the solenoid field will tend to push the magnets away from the center and towards the weaker field. And hence the assembly moves away, a bit. And in the next instant, it completes another section of the coil, forming another solenoid and the process repeats.

If the assembly is flipped over, it will still work, because the battery polarity will also be reversed and the solenoid will produce magnetic field in the other direction. However, if only the magnets are flipped over the battery, then I think the assembly will tend to be stuck at the center, because the magnetic filed of the solenoid and that of the magnet will tend to pull each other. Also, instead of flipping the magnet, if a different toroid, where the wire is wound the other way is used, it will fail.

Also, if the battery was dead, and if someone was to power the toroid from external supply, it won't work, because, then the magnetic field of the toroid would be uniform and hence no force would apply to the magnet assembly.

Am I right?

 

[OperaHouse]

I can only imagine you are talking about this

https://www.youtube.com/watch?v=J9b0J29OzAU

REALLY, watch it. Especially if you have grandkids who can't figure out what you do for a living.

 

[logbook]

I can only imagine you are talking about this

https://www.youtube.com/watch?v=J9b0J29OzAU

Of course you could have cliked on the link he gave as the first line in his post, cunningly disguised by being called "Link"

 

[logbook]

I think you are right in pretty much everything you have said, except perhaps for this bit …

“Also, if the battery was dead, and if someone was to power the toroid from external supply, it won't work,”

I think it might work. The field in the rest of the solenoid would still be present but in the region around the magnets the coil is shorted by the outside of the magnets. The field is therefore non-uniform at that point.

(I don’t claim any particular expertise here. My “intuition” on this subject is poor. I also distinctly remember getting a “Grade F. See me” on my lab report of the jumping ring experiment some 30+ years ago )

I just wound a solenoid about 3cm diameter in 1.2mm copper wire. 12 turns spread over 5cm. Using tinned copper wire wasn’t such a good idea as the turns short to each other very easily! With 5A through the coil you get a strong force on the neo magnet at the start of the coil, but as soon as you get inside the force drops away.

 

[waross]

Try reversing the battery and then inserting the assembly into the other end of the coil.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[logbook]

Try reversing the battery and then inserting the assembly into the other end of the coil.

I'm not sure to whom that suggestion was made. Assuming it was to me (since the OP is just looking at a video) I am not sure what your point is.

The push (force) on the solenoid coil is going to be symmetrical at both ends of the coil, but due to the longer lead wire on the “front” end, in practice the front of my solenoid moves more than the back when approached by the magnet.

 

[waross]

Concurrent posts logbook. I was not aware of your post when I posted.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[logbook]

It was all going so well

I did a first pass video but when I saved it in YouTube format the text got clipped off. So I went back into the video editor to fix the text and it then said my trial period had ended so it stopped working. The raw files are 240MB which compressed down to 18MB. If I upload the raw files to YouTube it will use up the last of this month’s bandwidth allocation. Besides, I forgot that YouTube is now Google – and that kind of put me off as well.

For those who can’t play flv format videos I show a neo magnet being sucked into the middle of a close wound solenoid, or repelled everywhere (dependant on the polarity). On the loose wound solenoid the magnet can get trapped inside, even when the polarity is set to repulsion. This modifies my earlier test report which said the repulsion force drops as the magnet enters the solenoid. It is all to do with the thin magnet and the loose wound coil.

(The upload was slooooow so you may need to be patient on the download)

 

[IRstuff]

?? The download was barely 2 seconds ;-), but we have a pretty huge pipe

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

Of course I can. I can do anything. I can do absolutely anything. I'm an expert!

 

[MagBen]

This video amused/puzzled/faked lots of people. the only trick for me is that the Cu wire must be NAKED (no insulated!), otherwise there won't be a close electric circuit! Note that the "train" starts to move only when both ends of magnets touch the solenoid to form a close circuit!

 

[marajendra]

It was all going so well pc2

I did a first pass video but when I saved it in YouTube format the text got clipped off. So I went back into the video editor to fix the text and it then said my trial period had ended so it stopped working.cry The raw files are 240MB which compressed down to 18MB. If I upload the raw files to YouTube it will use up the last of this month’s bandwidth allocation. Besides, I forgot that YouTube is now Google – and that kind of put me off as well.

For those who can’t play flv format videos I show a neo magnet being sucked into the middle of a close wound solenoid, or repelled everywhere (dependant on the polarity). On the loose wound solenoid the magnet can get trapped inside, even when the polarity is set to repulsion. This modifies my earlier test report which said the repulsion force drops as the magnet enters the solenoid. It is all to do with the thin magnet and the loose wound coil.

(The upload was slooooow so you may need to be patient on the download)

http://files.engineering.com/getfile.aspx?folder=1b1bcfef-526f-40e2-bd36-2d

Nice video. At 0:50, I can see that the solenoid is pushing the magnet away to right, but at 1:03 (when the magnets passed the midpoint of solenoid), it is pushing the magnet to the left. Even thought the magnetic field is pointing the same direction, we have two different directions of force, something I didn't thought.
When I think about it, I realize that, the magnetic field most strong (and most uniform) in the center and decreases both ways. So, at around 0:50, assuming your north pole is facing left (the problem is symmetric, so that shouldn't matter), the north pole being nearer the solenoid experiences more repulsion (force to right) than the attraction (force to left) experienced by the rear south pole. But past the center, the force experienced by south pole (again to left) is more than the force experienced by the north pole (to the right). The net force is the difference of the two, and flips sign at the center.

After you reverse your polarity: Near the right end, the magnetic field should pull the north to left, more than push south pole to right, so it should suck it in. Past the center, and it should push the north to left less than pull the south to right, and hence again suck it in to center again.

In case of the loose wound wire, which I can see has much less number of turns but same current, is producing much weaker magnetic field. So, when the magnet partially enters the loose wound coil, the difference in magnetic force on the poles should have been too small to move it, and hence it is stable (not stuck I think). Could you crank up the current in the loose wound wire to make its magnetic filed much stronger, and repeat the experiment.
The magnet will be stable at the center even in the case of the other solenoid (not loose wound) with repulsive polarity, if you carefully place it at the center.

So, far the theory sounds fine. One more experiment, and I might have to refine it again.
Its so easy to adjust and correct your theory when you see some experiments happening, but its also easy to miss some aspects at the beginning before you do the experiment.

 

[MagBen]

I admitted the Vedio is interesting, but people are overthinking it! I guess that is why even lot of experts can be puzzled by the Vedio. This is nothing more than reaction between two DC magnetic fields, one is from the local solenoid powered by the battery, the other is from the pair of Neo magnets.

Think about this: the battery is useless if +,- polarities are not connected.

Also note the toroid doesnot connect to any external power supplier in the original vedio. loglook's vedio supplied a 5A current.

 

[tinfoil]

Its subtle, but there seems to be an 'optimum' density of turns in the coil for speed.

Too sparse, and the field is likely lower, so the 'train' slows down.
Too dense, and the 'train' slows down. I suspect it is because adjacent turns of the coil are shorted, cutting down on the mag field.

You can see this quite clearly around 0.38 of OP video.

 

[MagBen]

speed should monotonically increase with density of turns, simply because increase in turns would increase the field strength of the local toroid.

 

[MagBen]

tinfoil, sharp eyes! i reviewed the video again. This is a different story, but shorting off the coils does decrease the field, and slow down the speed.

 

[logbook]

The magnet will be stable at the center even in the case of the other solenoid (not loose wound) with repulsive polarity, if you carefully place it at the center.

No, I don’t think so. The frictional force is remarkably low (by design). If the frictional force was stronger you might get away with it. Assuming no friction, you balance on a knife edge of the force curve. If the air molecules happen to group ever so slightly one side (which they will every so often by random chance) the slight motion will be increased by the field and the magnet will be pushed out of its (unstable) equilibrium position.

In case of the loose wound wire, which I can see has much less number of turns but same current, is producing much weaker magnetic field. So, when the magnet partially enters the loose wound coil, the difference in magnetic force on the poles should have been too small to move it, and hence it is stable (not stuck I think).

Although the field in the loosely wound coil is weaker, it still has a fair amount of strength compared to the frictional force. It is a very difficult experiment in this case because my loose wind coil is not perfectly uniform and the magnetic reaction on the wires is just about visible, especially nearer the end of the coil. Increasing the current (which my power supply can’t do anyway) would increase the distortion effect as well. The magnet is large compared to the coil and I think the edges of the magnet are fitting nicely into the gaps between the turns. It is probable that a smaller round magnet would not do this grippy thing, especially if the magnet were thicker than say 2 coil turn widths. My tight wound coil has turns held in place by varnish and doesn’t suffer from these problems, although it does get quite hot at 5A!

All in all I think my loose wound coil is just a very special case, of limited interest, due to using thin rectangular magnets.

 

[marajendra]

No, I don’t think so. The frictional force is remarkably low (by design). If the frictional force was stronger you might get away with it. Assuming no friction, you balance on a knife edge of the force curve. If the air molecules happen to group ever so slightly one side (which they will every so often by random chance) the slight motion will be increased by the field and the magnet will be pushed out of its (unstable) equilibrium position.

Yep, I see it. My bad. I like the knife's edge analogy. Thanks for the correction.

 

[waross]

The battery probably has a steel case. The magnets on the negative end of the battery magnetize the battery case, hence the magnetic field surrounding the battery is not uniform and is definitely not centered inside the solenoid coil.
If you take the time to do some simple calculations, you will see that the force of a DC solenoid is independent of the number of turns. When you double the number of turns, you double the resistance and halve the current so the Amp turns remain constant. However, there will be an optimum current for best energy transfer from the battery to the energized section of the coil.
The current will depend on the effective voltage which may be reduced by load induced voltage drop of the battery, the resistance of the contact between the magnets and the coil, and junctions effects of the contact between the magnets and the coil.
Increasing the number of turns does increase the induction by a square factor. The increased induction of an increased number of turns with a tighter wound coil may or may not be a factor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[btrueblood]

"The battery probably has a steel case. "

Hmm, yes, thus the magnets "stick" to the battery. But then, wouldn't the solenoid effect work even without the magnets, i.e. if some way of making contact from the battery terminals to the coil can be made?

 

[MagBen]

Without the magnets, the batteray will probably stay in the solenoid. The plarity of the battery, the polarity of magnets, as well as the turning direction of the solenoid all seemed matter.

Since the internal resistance of an AA battery is relatively big about 0.1 Ohms (let alone the contacting resistance etc.), much bigger than the reistance of the local solenoid (about 0.01 ohms/meter for 16 AWG Cu wire at 20C), doubling the turns can almost double the current, so double B field! Although it does double the resistance of the solenoid, the total resitance remains alomst the same.