Eddy currents are induced currents
set up in a piece of metal when
it moves through a nonuniform
magnetic field. For example, consider
the flat metal plate swinging
at the end of a bar as a pendulum,
as shown in Figure CQ20.9. (a) At
position 1, the pendulum is moving
from a region where there is
no magnetic field into a region
where the field B→
is directed into
the paper. Show that at position 1
the direction of the eddy current is
counterclockwise. (b) At position 2, the pendulum is moving
out of the field into a region of zero field. Show that the direction
of the eddy current is clockwise in this case. (c) Use righthand
rule number 2 to show that these eddy currents lead to
a magnetic force on the plate directed as shown in the figure.
Because the induced eddy current always produces a retarding
force when the plate enters or leaves the field, the swinging
plate quickly comes to rest.
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