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Relmuis
2006-Jan-01, 08:16 PM
Can a magnetic field be generated by a rotating object made of an electrically conducting material?

I am thinking of massive spheres, spherical shells, cylindrical shells or rings rotating around their axis of symmetry. I am thinking of materials like silver rather than iron.

The reason behind this idea is this: a conducting material is a rigid matrix of positive charges (atom rumps) superimposed on a fluid of negative charges (electrons). If the rigid matrix is forced to rotate, the fluid might not (fully or immediately) share this rotation, which would cause a net positive charge to move in a circle. Or, alternatively, if the fluid would share the rotation, it might become rarefied in the center and concentrated on the outskirts by "centrifugal" forces. In other words, it would need a "centripetal" force which could only be supplied by the electric field generated by rarefaction of the fluid in the center and concentration on the outskirts. And this would cause a net negative charge to move in a circle.

phunk
2006-Jan-01, 08:38 PM
Electrons are very very very lightweight, not alot of inertia there. I don't think there would be a measureable current induced simply by acceleration unless it was a superconductor.

papageno
2006-Jan-01, 11:32 PM
Considering the strength of the interaction between electrons and ions, and the fact that the conduction electrons undergo a sort-of brownian motion, I doubt it.

But let's assume that a rotation can generate separate charge densities, one negative from the accumulation of electrons and one positive from the uncompensated ions: a kind of mechanical Hall effect.

If the axis of rotation is the same as the axis of the ring (like a spinning wheel), the two charge densities would not give rise to a net current.
So, there would not be any magnetic field.

If the axis of rotation is in the plane of the ring (like a wheel hanging from a string), in principle you can get a magnetic field only if the negative charge density is at a different distance from the axis of rotation than the positive charge density (for example, electron accumulate on the inside of the ring).
In this case, you have two opposite currents whose effects do not cancel each other out exactly.
But you can observe a magnetic field only at distance from the ring which of the same order as the thickness of the ring (not the overall size).
If your ring has a size of 10 cm and is 1 cm thick, you should be able to observe a magnetic field at 1 cm from the outer boundary of the ring, but at 10 cm it should be negligibly small.

In conclusion, if you want to generate a magnetic field with a metallic ring, get a superconductor and induce a current by electromagnetic means.

Gsquare
2006-Jan-07, 05:46 AM
Can a magnetic field be generated by a rotating object made of an electrically conducting material?

I am thinking of massive spheres, spherical shells, cylindrical shells or rings rotating around their axis of symmetry.....

The answer to your question is...Yes, absolutely, Relmuis! as long as it is a superconductor. A rotating superconductor develops a magnetic dipole field aligned with the spin axis.
How do you think we monitor Gravity Probe B's gyroscope axis tilt!

What you have intuitively foreseen was first predicted by Fritz London around 1948 and is generally referred to as the "London Moment".

A rotating superconductor develops a magnetic field (dipole moment) which is oriented (aligned) with the spin axis of the material. In fact, this phenomena was fortuously used in the design of Gravity Probe B to solve the problem of how to monitor the tiny changes in the direction of the gyroscope spin axis' without affecting the gyro spin.

A SQUID loop is used to detect the tiny change in the London moment as the spin axis changes direction. (The gyroscopes are coated with niobium metal).

There are some other very interesting experimental results that have been verified based on measurement of the London moment....like, for example, one of the most accurate measurements of the value of the electron charge to mass ratio ! ;)

Gsquare

P.S. Very insightful question, the answer of which has some very thought provoking consequences.

papageno
2006-Jan-07, 03:40 PM
Just to show that I have learned something new thanks to Gsquare, I found this (http://www.nap.edu/html/ssb_html/GravityProbeB/gpbch3.shtml):
London showed that electromagnetic coupling between the positive ions in a lattice and the superconducting electrons would produce a magnetic field in the interior of a spinning superconductor. The magnetic moment of a rotating sphere has a number of ideal properties for indicating the motion of a gyroscope. The field is directed along the spin axis and is independent of the specific material properties of the superconductor.

Gsquare
2006-Jan-07, 06:34 PM
Just to show that I have learned something new thanks to Gsquare, I found this (http://www.nap.edu/html/ssb_html/GravityProbeB/gpbch3.shtml):

Thanks for the post (blushing).
Surprisingly, knowledge of the London moment by a spinning superconductor is not so common, even among 'experts'. Your link did provide, however, a very good explanation both of the London moment and the SQUID readout technique as it applies to GPB.

If you are like me, you will find more surprsing the fact that the very characteristic that defines superconductivity (expulsion of the magnetic field from the bulk material) now becomes "reversed' by spinning the material so that magnetization now pervades the interior.

The strength of the external magnetic field is directly proportional to the spin velocity of the material.
The simple London equation probably isn't even available on the web (I've never seen it), and is given by:

B = -2(omega)e/mc

where omega is the spin velocity, and e= electron charge, m=elect. mass

Since London also 'discovered' Flux quantization of the magnetic field of a superconductor, and since the equation contains e/m, it is easy to see how the spinning SC could be designed as a sensitive test of the value of e/m.
:)

Gsquare

papageno
2006-Jan-07, 09:49 PM
Surprisingly, knowledge of the London moment by a spinning superconductor is not so common, even among 'experts'. Your link did provide, however, a very good explanation both of the London moment and the SQUID readout technique as it applies to GPB. I am somewhat familiar with SQUIDs (never actually worked with them or studied them, but I heard a few talks involving SQUIDs), but the explanation of the London moment from the link baffles me.

I tried to do a search on the web, but I only found a few abstracts (at the moment I have no access to proper jornals), and arXiv gave a handful of paper (which I have not had time to read, yet).

If you are like me, you will find more surprsing the fact that the very characteristic that defines superconductivity (expulsion of the magnetic field from the bulk material) now becomes "reversed' by spinning the material so that magnetization now pervades the interior. It is surprising.