PHYS1200
PHYSICS II
SPRING 2006
HOMEWORK SOLUTIONS
CLASS 7.
28.35,
29.30, 29.36, 29.87;
Q30.02
28.35.
a)
The magnetic force on the wire must produce an upward force equal to the weight of
the wire if the tension in the leads is to equal zero.
The magnetic force is described
by the equation,
B
L
i
F
B
×
=
.
Since
L
and
B
are perpendicular to each other, the
magnitude of the magnetic force is given by,
F
B
= iLB
. Then, we must have,
iLB = mg
,
so
T)
m)(0.440
620
.
0
(
)
m/s
kg)(9.8
0130
.
0
(
2
=
=
LB
mg
i
i
= 0.467 A
b)
B
L
i
F
B
×
=
. According to the right hand rule for the vector product, the current must
be directed from
left to right
to produce an upward force with the magnetic field
given.
From left to right
29.30.
Since all the currents are parallel, the forces between each pair of wires is attractive.
From the symmetry, it should be clear that the magnitude of the net force on each wire is
the same. Number the wires, starting with the lower left corner and proceeding clockwise.
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 Spring '06
 Stoler
 Physics, Force, Work, Magnetic Field, Wire, Righthand rule

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