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Re: comprehending electric/magnetic interactions



OK -- I had a serious Sr. moment: Since the potential is constant the
gradient is zero. Is this above the level of Turner's class?

bc

Bernard Cleyet wrote:

"A small current carrying loop placed into
this region is not attracted to one pole or the
other, because at no place is its energy lower
than any other place. (This assumes the axis of
the loop is aligned with the magnetic field. )"


Roger's very good use of the energy principle in this example illustrates its power. However, I think, it is inappropriate for a beginning Physics class.

I am unsure as to whether the energy principle is proven or the result of its success in many examples. If proven, is the proof accessible to a beginning class? If not, has the class been given many examples of its use?

In this particular case, I think showing the balance of forces would be more more convincing to the students. The electrical analog [e. dipole in a uniform E field] could be discussed also.

bc


Roger Haar wrote:



Hi,

Just a couple of half considered thoughts:

If you look in most text books one finds a
discussion of the torque on a magnetic dipole but
nothing about the force between two magnets. Why,
because this second situation potentially ugly,
because it involve how quickly the field spreads
out and weakens.

Consider a region containing a uniform magnetic
field (field lines are a constant distance apart.
. .) A small current carrying loop placed into
this region is not attracted to one pole or the
other, because at no place is its energy lower
than any other place. (This assumes the axis of
the loop is aligned with the magnetic field. )

I think that without knowing it or accepting it,
Gary has basically rediscovered the above.


Thanks
Roger Haar


Gary Turner wrote:




Could anyone help me explain this problem that arose during a discussion
of magnetic field interactions.

Consider a simple current loop. It will produce a magnetic field and, if
brought into an opposing magnetic field (in such a way that the external
field is perpendicular to the plane of the loop), should be repelled by
it. Now, square the loop up and take three of the sides out of the region
of magnetic field. Now all that remains is a current-carrying wire in a
magnetic field, which will be pushed to the side (F=qvxB).

Indeed, a simple F=qvxB calculations shows that the loop will be
either "stretched" or "compressed" depending on the direction of the
current instead of being attracted towards, or repelled from, the source
of the external magnetic field.

It seems that the one side remaining in the field should still have a
repulsive force - if it does not apply to any one side, how can it apply
to the whole - but that would imply that there exists a force parallel to
the magnetic field in addition to the qvxB. This would have to hold
equally for a long conductor just sitting in a magnetic field.

Anyone out there considered this?