Mr. Abineri,
The explanation you give your students, about the force exerted on a
superconducting sample in an external magnetic field, may be helped by the
logic or the diagram in Physics for Scientists and Engineers, 6th edition,
chapter 32 problem 79, page 1031, by Raymond Serway and John Jewett. (It
is the same problem number in the fifth edition.) This problem allows a
clear identification of the existence and the direction of the force
acting on a superconducting bar with one end in a uniform external field
and the other end in a field-free region. Note that the sample does not
exert a force on itself, but feels a force exerted by the external field
only, acting on the supercurrents in its surface. The problem suggests
calculating the strength of the force from the energy density of the
external magnetic field, by pointing out that a problem six chapters
earlier showed that this method works for finding the force an electric
field exerts on a perfect dielectric.
I recommend showing the students early in their study of electromagnetism
that a light conducting ball feels a force of attraction into a region of
stronger electric field. We can account for it like this: The metal shell
prevents penetration of the external electric field inside the volume it
encloses, by having charge rearrange itself on the surface of the metal
until E = 0 within. The charge separation is ordinarily called
polarization, but we could invent the term 'supercharger' for the metal,
as well as calling it a perfect dielectric. In a nonuniform external
electric field, we show that the net force on the metal sample is in the
direction toward which the field is increasing--the direction of the
gradient. Then later in the course the same steps show the origin of the
force on a superconductor in a magnetic field, in the direction opposite
the gradient.
-- Ralph McGrew Broome Community College, Binghamton, New York
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