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Bar and Rail Example Problems (Answers at the end)
Question 1
The figure at right shows a wooden cylinder of mass M = 0.25
Kg, a radius R and a length l = 0.1m with a coil of N = 10 turns
of wire wrapped around it longitudinally so that the plane of the
coil is parallel to the inclined surface. The incline is tilted at an
angle of
θ
, and a vertical filed of density B = 0.5T
(perpendicular to the horizontal surface) exists everywhere.
What is the least amount of current required to keep the cylinder
from rolling down the incline? Assume the current,
i
is in the
direction shown and verify whether it is positive or negative (i.e.
reverse direction).
Question 2
A long horizontal, rigidly supported wire carries a current of
i
a
= 100A. Directly above and parallel to it is a
fine rigid wire that has a mass, m = 7.5g/m carrying a current of 20A.
How far above the bottom wire will the second wire "float" if it is to be suspended only by magnetic repulsion?
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 Spring '10
 Linch

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