Intuitive physics

[snousle]

Here's a thought experiment that touches on a lot of different intuitions and assumptions. I'm curious to see how people answer:

You have two hockey-puck shaped permanent magnets, and you place one on each side of a thin sheet of plastic such that their mutual attraction clamps them strongly together. All surfaces are smooth and nearly frictionless (oil them if you like). If you hold the sheet stationary while rotating one of the magnets, does the other one rotate as well? Why or why not?

Comments

[dhpbear.livejournal.com]

That would depend if the North and South poles of each disc were located on their flat surfaces, in which case it wouldn't rotate. However if the poles are located on the perimeter of each disc, then it definitely would rotate!

[bikerbearmark.livejournal.com]

Agreed w/dhpbear.

[kevynjacobs.livejournal.com]

I am going to guess yes, the other one rotates as well.

The reason why I think this is that magnetic force is not simply an amorphous field, but consists of actual lines of magnetic force that will rotate along with the magnet. These lines of magnetic force will be interacting with, and crossing, the lines of magnetic force from the other magnet, pushing on them, thus turning the second magnet.

[come-to-think.livejournal.com]

This argument is an abuse of the notion of lines of force. The two magnets do not have independent fields that reach out & tangle with each other. There is only one magnetic field, and its lines go from the S pole of one magnet to the N pole of the other (thru the sheet), and then from the S pole of that magnet back to the N pole of the first one (thru the surrounding space). These lines are imaginary constructs of arbitrary density (like the lines of latitude & longitude on the earth). They show the direction of the field at any point on them, and if you pick out a bunch of neighboring ones, their density will represent the strength of the field; but they are not threads.

[p0lecat.livejournal.com]

Those magnets have a large numbers of magnetic north/south locations on each side. its not just one location. So its like having a bunch of little magnets side by side. If there was no resistance on the plastic, then the magnets would turn however, that can be quickly lost if turned too fast.

[growler-south.livejournal.com]

It would totally depend on the axis of the poles- and I can imagine two different configurations which would give different results.

If the poles were on the flat faces of the disk magnets they would effectively be very short cylindrical bar magnets connected in series, in which case no, one shouldnt impart the rotation to the other.

If however one half of the circumference was North and the other South, then they would behave like round-ended short bar magnets stuck in parallel, in which case they should be able to transmit a torque due to the two points of attraction.

[bikerbearmark.livejournal.com]

Agreed also with the lovely-smelling Grant!

[come-to-think.livejournal.com]

I agree with the others on this thread that the problem is incompletely specified. However, I believe that it is usual with hockey-puck-shaped magnets for the N-S axis to be perpendicular to the flat faces; at any rate, that is true of the pair that I just plucked off my fridge (whether they stick together or repel each other depends only on which face goes with which, not on the angular position). In that case, the assembly is cylindrically symmetric, its potential energy will not be changed by a mutual rotation, and thus there can be no torque on the second magnet. (I have just tried it, using a piece of paper, and that is indeed the result.)

That's a magnetostatic argument, tho. One might ask whether, if the magnets are conducting and the first one is rotated rapidly, there might be induced currents that would spoil the result. I believe that, once again, the cylindrical symmetry prevents that from happening; but I would be prepared to be surprised, because there are subtleties in applying such arguments to pseudovector fields. (If in addition there were an electric field in the plane of the paper, I would be fully prepared for something weird to happen; but that would break the rotational symmetry too.)

[hudson-phoenix9.livejournal.com]

The other magnet should rotate as well. The reason being, and I know I'm not articulating this well, is that the magnetic field would also move with the magnets and it is the magentic field that is holding the magnets together.

[hudson-phoenix9.livejournal.com]

By the by, I usually think of myself as an intelligent person until I read your posts and the responses to them. You always get me thinking and wanting to learn more about everything.