## Moving Charges and Magnetic Moments

A point charge moving at a constant velocity will have the same magnetic field shape as a bar magnet.

 Quote by quantumfoam Nooooo! I don't think we are on the same page here is all. (: I don't think imagining our charge to be a wire is going to help because a wire and a point charge have different magnetic field shapes. What I meant by the donut shape of our point charge is that it is not completely in the shape of a donut, but it has some characteristics. In a real donut, there is a hole, which I totally understand why you got confused (that was my mistake by not being clear enough), but in the magnetic field of our point charge there is no hole. It has the same curving features of a donut but it doesn't have a whole. It has the same magnetic field shape as a solenoid. Our moving point charge is moving at a constant velocity and through a vacuum (classical vacuum). If it is not accelerating or near any force, how will the direction between north and south change?
Oh!!! Now I get what you want to say.... Well then, if that is the case, then surely the magnetic field has a constant direction and hence a fixed north and south. Now we can safely ask whether there's a moment or not. But there's still one problem... the distance between the two poles! How do you get that?
Probably not possible, but there's a detour.
This donut shaped motion of the charge will be equivalent to current flowing through a coiled wire. And we all know the magnetic moment in such a situation.
 Right! That is what I was thinking! Thank you very much deep838!! (:

Mentor
 Quote by quantumfoam A point charge moving at a constant velocity will have the same magnetic field shape as a bar magnet.
No. They are very different.

Do a Google search for "magnetic field of a moving point charge" and "magnetic field of a dipole".
 but then again, coming back to your original question, i don't think that any type of motion will have a magnetic field! the charge in circular motion is a very special case!
 yes moving charges produce a magnetic field. you can explicitly see this when you have a static charged particle in one frame moving relative to another frame. in relativity, neither electric fields nor magnetic fields are invariant in different frames, but the electromagnetic tensor is. You take the Lorentz transformation of the electromagnetic tensor that describes a static charged particle in the rest frame in order to transform it into the moving frame and write out the terms explicitly. You will find that the B'$_{i}$ terms in the moving frame will have nonzero components related to the E field in the rest frame.

 Quote by chill_factor yes moving charges produce a magnetic field. you can explicitly see this when you have a static charged particle in one frame moving relative to another frame. in relativity, neither electric fields nor magnetic fields are invariant in different frames, but the electromagnetic tensor is. You take the Lorentz transformation of the electromagnetic tensor that describes a static charged particle in the rest frame in order to transform it into the moving frame and write out the terms explicitly. You will find that the B'$_{i}$ terms in the moving frame will have nonzero components related to the E field in the rest frame.
oops! sorry i wrongly wrote "magnetic field"!!!!

i meant "magnetic moment".

really sorry for that.

So, does any type of motion have a magnetic moment?
 Actually jtbell, I did the search and they look exactly the same. The only difference is that a point charge has an infinitetismally small distance between it's two magnetic poles.
 everyone, u r all diverting to a different problem here! we started with magnetic moments and lets stick to that in this thread! start a new one for others!!