Path of Charge in Magnetic Field
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?
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.
Department of Physics and Astronomy
University of Wyoming
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Update: June 2012