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Name: Steven
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Question:
a positive charged particle will spiral in one direction when moving parallel with a uniform magnetic field, and a negative charged particle will spiral in the opposite direction. But,does a magnetic field affect the forward direction of charged particles. e.g would a positive charged particle be "pulled" towards either the north or south poles of the magnetic field and a negative charged particle in the opposite direction? Thank you for your answer to my question, it is very informative. I dont know if i am allowed to respond to your email, but there is still something about charged particles that bothers me. if the poles of a magnetic field neither repel nor attract charged particles, why do physicists believe charged particles are themselves magnetic dipoles? Surely, if that was the case there would be some repulsion or attraction in resonse to an external magnetic field?


Replies:
Ah! Yes, you are correct, of course. It is a bad habit of mine to disregard the spin of charged particles.

However, that still does not mean that a negative particle will be attracted to one pole of a magnet and a positive particle to the other pole. It means that charged particles themselves are dipole magnets. So it's worth reviewing how dipole magnets interact with other dipole magnets, or with magnetic fields in general.

Basically, the north pole of the dipole is pushed one direction and the south pole is pushed the opposite direction. The net force on the dipole magnet is the sum of these two interactions. This depends on the uniformity of the external field (a perfectly uniform field will exert exactly opposing forces on the two poles of the dipole, giving a net force of zero) and on the orientation of the dipole within the external field (the end immersed in the more intense region of the field will experience the stronger force, so flipping the orientation of the dipole will change a repulsion to an attraction and vice versa).

Now, a dipole magnet in an external magnetic field will re-orient to be pulled in the direction of increasing intensity of the field, if it is allowed to re-orient. What this means after all the dust clears is that the dipole will be attracted to either pole of an external dipole magnetic field, just as two magnets gently pushed toward each other will tend to cling together, even if one or both of them must flip to do so.

So, a spinning charged particle with a magnetic moment will be attracted to either pole of a dipole magnet. This holds for any value of its electric charge. It will not be attracted to one pole and repelled by another.




First of all: charges spiral when they are moving perpendicular to a uniform magnetic field. In fact, if the magnetic fields were all there was to it, the charges would not spiral, but instead move in unending, unchanging circles. The spiraling is due to slowing by friction. When a charge moves parallel to a magnetic field, the field exerts no charge on it.

A static magnetic field does not do any work on a charge it affects--that is, it can only change its direction. It cannot speed it up or slow it down. Magnetic north and south poles neither attract nor repel electric charges. The most they can do is deflect them.

Richard Barrans
Department of Physics and Astronomy
University of Wyoming



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