# Confused about charged rotating disk-Magnetism, reference frame

1. Nov 6, 2009

### fluidistic

For some reason I've thought about the following case : A uniformly non conductor charged disk is rotating with an angular velocity $$\vec \omega$$.
If I see it in front of me I'd observe a magnetic field since moving charges produce a current which produce a magnetic field according to Ampere's Law. So if I hang a magnet over the rotating disk it would suffer some force because of the magnetic field created by the disk.
However if I chose, as a reference frame, the center of the disk, I'd see no current since no charge is moving. Hence no magnetic field and the magnet wouldn't be deflected at all while being closer and closer to the disk.

It doesn't seem possible so I'm guessing I'm misunderstanding something.
Can you explain what's happening?

2. Nov 6, 2009

### tiny-tim

Hi fluidistic!
No, a reference frame can't be a point, it's the whole of space at a particular velocity.

3. Nov 6, 2009

### fluidistic

It seems I'm already confused about this, even before having started to learn about Relativity!
If I'm over the disk (say a 2 meters diameter), I would rotate with it, with respect to the Earth's ground. However I would not see/feel any magnetic field while immobile on it. All changes if I leave the disk and look at it from the ground.

4. Nov 6, 2009

### tiny-tim

oh, do you mean standing on the disc, and therefore rotating with it?

But that's a non-inertial frame, and so Maxwell's equations wouldn't apply (well, not without adding "fictional forces").

5. Nov 6, 2009

### fluidistic

Thanks a lot. And a bit late : Hey tiny-tim (my hero!)
You just reminded me of Coriolis force on Earth although I never studied it.
I have some difficulties with inertial frames of reference and I guess I'm not the only one. For example, what if the Universe is made of the disk, a person in front of it, and me over the disk? Would I still be considered as a non inertial frame of reference?

6. Nov 6, 2009

### tiny-tim

You're confusing me

I'm going to bed! :zzz:​

nighty-night!

7. Nov 6, 2009

### fluidistic

Good night! Sorry about that.
Anyone's welcome to clear my doubt.

8. Nov 6, 2009

### Staff: Mentor

Whether or not you are inertial is determined by if you are experiencing any net forces, which you can measure with an accelerometer. What the rest of the universe is doing is not relevant.

9. Nov 7, 2009

### tiny-tim

Hi fluidistic!
(yawn :zzz: … just got up … must eat … :tongue2:)

hmm … you're talking about the http://en.wikipedia.org/wiki/Mach_Principle" [Broken], and so on …

how do we decide whether the disc (and you) are rotating, or (only! ) the whole of the rest of the universe is rotating?

Would it make you happier if I say that in all moment of inertia exam questions, you can always assume Newtonian space-time and mechanics, and that an inertial observer is always non-accelerating and non-rotating?

Last edited by a moderator: May 4, 2017
10. Nov 7, 2009

### Per Oni

Perhaps I’ve not giving this problem enough thought, but when the disk is not rotating then the magnet must be moving. A moving magnetic field produces an electric field which in turn reacts with the charges on the disk.

11. Nov 7, 2009

### fluidistic

Good to know. If I was on the center of the rotating disk with an accelerometer in my hands, would it feel some kind of acceleration? (The only acceleration I could guess is the centripetal one)

Good day. I wasn't aware of this information, nice.
About exams questions, I'd solve them as professors want, but this wouldn't limit my questions so that it doesn't really make me happy not to know well physics.

The disk is non conductor, so would the charges move under the electric field created by the moving magnet?
Anyway at time t=0 I could set the Universe like this : I'm over the disk which rotates with respect to an observer which isn't on the disk. I have a magnet in one hand, not moving with respect to me, hence also with respect to the disk. So I don't see any magnetic field due to the rotating disk, unlike the observer outside the disk which sees one and should see the magnet influenced by the magnetic field of the rotating disk. While I wouldn't see any interaction between the magnet and the disk.

I'd love to do the experiment to see what happens. Now where do I get a charged big rotating disk? :uhh:

Last edited by a moderator: May 4, 2017
12. Nov 7, 2009

### Staff: Mentor

Yes, but I guess I should have been more clear. I was talking about the 6 degree of freedom kind of accelerometers, the kind that can detect 3 directions of linear acceleration and 3 directions of rotation (often called inertial measurement unit).

Recall that the centripetal acceleration is proportional to the radius, so if you were at the center that would be zero, however if you were co-rotating with the disk then that would be measured on one of the gyroscope channels. If you were attached to the edge of the disk then you would measure both the centripetal acceleration and the rotation.

13. Nov 7, 2009

### fluidistic

Ah ok! If I understood well this would imply that I'm not in an inertial reference frame so the magnet will indeed be deflected by the magnetic field of the rotating disk. To explain it from my point of view I'd have to use a fictitious force, like in the case of when I'm sat in a car when turning (I'd be pushed on a side, and I'd have to use the centrifugal force).
That's what tiny-tim' second post mention.
Thank you all! Very much.

14. Nov 7, 2009

### Staff: Mentor

Yes, I think you got it.

15. Nov 8, 2009

### Per Oni

There’s always some ohmic electrical resistance and some small scale stuff going on but ignoring that there will be an electric force but no movement of charges. This force will be either towards or away from the centre.
The observer outside the disk sees 2 magnetic and 2 electric fields.
He will find that the extra forces between the magnetic fields cancel out against the extra forces of the electric fields.
The maths behind this is somehow too complicated for a Sunday (or Monday).
I think the hardest part will be to detect a magnetic field at all of such a disk. Remember that normally magnetic field are generated by the movement of enormous amounts of electrons. When there’s only a fraction of a fraction of that charge on your disk I would not like to approach it and/or go and sit on it.

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