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Unformatted text preview: Physics 53! Summer 2011 Quiz 5
Solutions Check the best answer. 1.! A small disk magnet is dropped from rest vertically down three equally long
empty cylindrical tubes, in succession. Tube A is made of a non-conductor, tube B
of aluminum, and tube C of copper. It takes time T for the magnet to emerge from
the bottom of the tube. Which of the following is wrong? TA < TB < TC . [This is correct, so the next one must be wrong.] !
√! TA < TC < TB . ! It makes no difference which pole of the magnet is on top. ! The retardation of the fall is caused by eddy currents induced in the tube
by the changing B-ﬁeld as the magnet passes by. Check T or F depending on whether the statement is true or false. 2.! A positive point charge moving parallel to the current in a long wire is repelled
from the wire.
T √ F [Attracted; like parallel currents.] 1 Physics 53! 3.! Summer 2011 A conducting ring rests on a thin glass
plate, underneath which is a solenoid
(represented in the drawing by the
cylinder). The solenoid is connected
to a battery by a switch, which is
initially open; when the switch is
closed a large current ﬂows in the
solenoid, producing a strong B-ﬁeld with lines emerging from its top.
a.! When the switch is closed the ring jumps vertically off the glass plate.
Explain this in terms of the induced magnetic moment of the ring. b. ! Suppose the solenoid is wound so the B-ﬁeld lines go in at its top. Would
the ring behave differently when the switch is closed? Explain. c.! Suppose the ring is placed at rest on the plate while the switch is closed,
and then the switch is opened. What will the ring do? Explain. a.! The increase in the upward B-ﬁeld induces a current in the ring to produce a
downward ﬁeld and magnetic moment. This moment is opposite to the applied
ﬁeld, so there is a force toward weaker ﬁeld, i.e., upward, making the ring jump. b. ! Now the current in the ring produces an upward ﬁeld and moment, still opposite
to the applied ﬁeld, so the force is the same as in (a) and the ring jumps again. c.! Now the induced moment is parallel to the applied ﬁeld, so the ring is attracted to
stronger ﬁeld, i.e., downward. Because of the plate it does not move. 2 ...
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- Summer '09