A bunch of questions regarding radially magetized ring magnet.

[PolaczekCebulaczek]

These magnets or speaker magnets does not have a field gradient along the circumference. So there will be no interaction between the rotating magnet and the compass needle.
This statement is only true if the magnet is uniformly magnetized. Most magnets are not that precicely manufactured.
So in a practical experiment you will probably se some movement in the needle.

Vidar

very interesting... so lets assume that I have a perfectly uniformly magnetized ring (just like one on picture)  then I put compass in the center of the ring and set its needle to point east west earth direction and now needle will STAY STILL at this E-W position forever regardless of magnet rotation? is this correct?

[Low-Q]

very interesting... so lets assume that I have a perfectly uniformly magnetized ring (just like one on picture)  then I put compass in the center of the ring and set its needle to point east west earth direction and now needle will STAY STILL at this E-W position forever regardless of magnet rotation? is this correct?

Yes. However, the earth magnetic field dominates the compass needle. So it will point north/south.
Remove the earth, and the needle stays E-W (except there is no E-W when the earth is gone ).



Vidar

[PolaczekCebulaczek]

Yes. However, the earth magnetic field dominates the compass needle. So it will point north/south.
Remove the earth, and the needle stays E-W (except there is no E-W when the earth is gone ).



Vidar

dominate you say? I think that depends from strength of ring magnet or its inner diameter... so earth will only slightly pull the needle or even not at all.(earth field vs. magnet pull)
by putting compass  inside the ring i was thinking about a way of determining if magnetic field rotate with ring or not, Faraday paradox stuff, yet even if field is rotating needle should not move since field is all around equal and uniform, only induction and open circuit approach can tell if any tiny current is present.

[Low-Q]

The thing is:
At center, the magnet does not pull anything. The magnet could have a thousand gauss. It does not matter. Because at the center, all fields cancel each other out. The extent of the needle is equal in both directions. Any magnetic influence on one side is equal on the other.


So what magnetic field do we have left? The Earths magnetic field - which will be the only field left that directs the needle.


Vidar.

[PolaczekCebulaczek]

The thing is:
At center, the magnet does not pull anything. The magnet could have a thousand gauss. It does not matter. Because at the center, all fields cancel each other out. The extent of the needle is equal in both directions. Any magnetic influence on one side is equal on the other.


So what magnetic field do we have left? The Earths magnetic field - which will be the only field left that directs the needle.


Vidar.

yeah i think you are right, thats correct for case nr 1 and probably for case 2 as well because inner circle of the ring will be completely filled by mag field... so the second picture is wrong.