# The Naked Scientists Forum

### Author Topic: charged rings  (Read 7953 times)

#### thebrain13

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##### charged rings
« on: 03/03/2007 00:03:23 »
if there was a rotating negatively charged ring, would magnetism arrise?

#### another_someone

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##### charged rings
« Reply #1 on: 03/03/2007 00:11:13 »
That is exactly what you have when you have a coil of wire with electrical current flowing through it that creates an electromagnet - so the answer is yes.

#### daveshorts

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##### charged rings
« Reply #2 on: 04/03/2007 00:13:54 »
however the force would be tiny compared with any conventional electric forces unless the ring was rotating at relativistic speeds, as magnetism is actually a relitivistic correction to the electric force. The fact that this tiny relativistic correction is large enough to be noticed when electrons are moving at only a few m/s, says something about how strong the electric force is.

#### syhprum

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##### charged rings
« Reply #3 on: 04/03/2007 12:27:29 »
If the ring was made of normal material surely the excess electrons that make up the negative charge would be tied to atoms and rotate with the ring, would the rotation of the ring not be equivalent to a current flow that would generate a magnetic field

#### another_someone

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##### charged rings
« Reply #4 on: 04/03/2007 12:48:07 »
If the ring was made of normal material surely the excess electrons that make up the negative charge would be tied to atoms and rotate with the ring, would the rotation of the ring not be equivalent to a current flow that would generate a magnetic field

Most of the electrons, yes - but you can create an imbalance of electrons within a material by applying a voltage across it - that is what a capacitor does - you apply a voltage between two conducting plates separated by a dielectric, and all the electrons go rushing into one plate, and out of the other (well, not all, unless it was a hugely massive charge - but you do get an imbalance in the electrons).

#### lightarrow

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##### charged rings
« Reply #5 on: 04/03/2007 12:51:29 »
If the ring was made of normal material surely the excess electrons that make up the negative charge would be tied to atoms and rotate with the ring, would the rotation of the ring not be equivalent to a current flow that would generate a magnetic field
It would be equivalent, because a rotating (net) charge generate a magnetic field. With net charge I mean a non zero charge in the object's frame of reference.

#### lyner

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##### charged rings
« Reply #6 on: 06/03/2007 23:30:27 »
I have a feeling that the field would be different if the ring were made of an insulator of a conductor. In the case of a charged insulator, the 'current' would be just due to the net effect of  movement of all the captive charges on the surface. With a spinning metal ring the effect of an induced emf  (Lenz's law?) might well cause the charges to stay still whilst the material of the ring revolved. Can anyone get their head round that and help me?

#### another_someone

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##### charged rings
« Reply #7 on: 06/03/2007 23:52:52 »
I have a feeling that the field would be different if the ring were made of an insulator of a conductor. In the case of a charged insulator, the 'current' would be just due to the net effect of  movement of all the captive charges on the surface. With a spinning metal ring the effect of an induced emf  (Lenz's law?) might well cause the charges to stay still whilst the material of the ring revolved. Can anyone get their head round that and help me?

There is only one thing that matters - is there a nett movement of charge.

If the insulator has sufficient charge, and it is moving, it will cause the same effect as if a similar charge was moving within a conductor (the only issue would be whether you can have a sufficient number of charge partices comparable to the number of charge particles that would normally be moving through a conductor under the influence of a voltage).

The relativistic calculations make no distinction regarding the medium in which the charge is moving, or even what particles make up the charge, it is only the amount of charge and its speed that matters.

Don't know if it helps, but here is a site that tries to explain about relativity and magnetism:

http://physics.weber.edu/schroeder/mrr/MRRtalk.html

#### lyner

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##### charged rings
« Reply #8 on: 07/03/2007 11:19:09 »
Cheers; that link helps a lot.  It's amazing what can happen when you multiply a very small number by a very big number!
Extending the idea to account for em radiation is good, too but I think he's a bit cruel about J C Maxwell in his final comment!
I agree about the charged insulator idea; there would be a very small number of  (surplus) electrons available for moving compared with  those in a conductor.

#### thebrain13

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##### charged rings
« Reply #9 on: 07/03/2007 17:46:58 »
I was assuming that the charge remains stationary relative to the rotation of the ring. Also my question was pretty ambiguous, I know there would be magnetism relative to a stationary object outside of the rotating ring, but my question was more along the lines of how would part of the ring view the magnetic effect of other parts of the ring.

#### daveshorts

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##### charged rings
« Reply #10 on: 07/03/2007 17:58:51 »
This is the same question as, what forces will one part of a current loop feel due to the other parts. As long as I haven't got my hand rules the wrong way round they will feel an extra repulsion due to the magnetism. Although as has been said before this will be tiny relative to the electrostatic repulsion.

#### lightarrow

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##### charged rings
« Reply #11 on: 07/03/2007 18:18:30 »
I was assuming that the charge remains stationary relative to the rotation of the ring. Also my question was pretty ambiguous, I know there would be magnetism relative to a stationary object outside of the rotating ring, but my question was more along the lines of how would part of the ring view the magnetic effect of other parts of the ring.
In the ring reference system, there is no magnetic field, just electrostatic field, so only electrostatic repulsion among the charges. The same situation, seen from a stationary object outside the ring, is described with an electric and a magnetic field, so you have the electrostatic repulsion AND a magnetic attraction! Believe me, computations give the same final results in both cases, so there is no paradox.

This is one of the special relativity paradoxes they taught us at the electrodynamic course at univ.; specifically, it was this: 2 electrons move straight at constant speed one next the other. In our ref. frame we see the coulombian repulsion and the (magnetic) Lorenz force. In their ref. frame, only coulombian repulsion. How it's possible?

Ah, physics, how is fascinating!
« Last Edit: 07/03/2007 18:21:10 by lightarrow »

#### thebrain13

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##### charged rings
« Reply #12 on: 08/03/2007 05:57:44 »
Im not sure, how you how you would classify relative motion in a rotating ring. For example, for simplicity consider a gravitron  Instead of thinking of a ring, a gravitron is a ride that spins around very fast, so that everyone inside sticks to the walls.

Inside the gravitron, you would consider everyone at rest, you could make perfect eye contact with anyone in the ride, so you may say there is no relative motion. However, if you think about it the person opposite from you in the gravitron is always traveling in the opposite direction from you at twice your angular velocity. So the question is which frame of reference rains supreme?

#### syhprum

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##### charged rings
« Reply #13 on: 08/03/2007 10:26:02 »
I believe it was Mach who said that we must consider rotation to be relative to the remote stars, but I do not know whether this is the modern view or if we should consider it to be relative to the whole universe or our local galaxy

#### lightarrow

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##### charged rings
« Reply #14 on: 08/03/2007 12:31:05 »
Im not sure, how you how you would classify relative motion in a rotating ring. For example, for simplicity consider a gravitron  Instead of thinking of a ring, a gravitron is a ride that spins around very fast, so that everyone inside sticks to the walls.
Inside the gravitron, you would consider everyone at rest, you could make perfect eye contact with anyone in the ride, so you may say there is no relative motion. However, if you think about it the person opposite from you in the gravitron is always traveling in the opposite direction from you at twice your angular velocity. So the question is which frame of reference rains supreme?
Sorry, I don't understand your question. Can you explain it better?

#### daveshorts

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##### charged rings
« Reply #15 on: 08/03/2007 13:20:13 »
I believe it was Mach who said that we must consider rotation to be relative to the remote stars, but I do not know whether this is the modern view or if we should consider it to be relative to the whole universe or our local galaxy
I think there is an experiment running at the moment to try and find out, it is called Gravity Probe B ( http://einstein.stanford.edu/ ) An absolutely crazy experiment using all the most extreme bits of physics to test gneral relativity. They use billard balls that if they were the size of the earth the variations in their radius would only be a few cm, these are covered with a superconducting layer and cooled in superfliud liquid helium and spun up to a huge speed. They then put this in orbit and measured very carefully the procession of the gyroscopes compared with a fixed star... They are going to report some of the results in April.

#### thebrain13

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##### charged rings
« Reply #16 on: 08/03/2007 18:24:48 »
okay, if you were on one side of a rotating ring, and you were viewing an object on the opposite side of the rotating ring, would you consider the other object at rest relative to you, or would you consider it as always traveling in the opposite direction relative to you?

#### lightarrow

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##### charged rings
« Reply #17 on: 08/03/2007 19:10:42 »
okay, if you were on one side of a rotating ring, and you were viewing an object on the opposite side of the rotating ring, would you consider the other object at rest relative to you, or would you consider it as always traveling in the opposite direction relative to you?
The first you said.

#### The Naked Scientists Forum

##### charged rings
« Reply #17 on: 08/03/2007 19:10:42 »