suspended object of mass M. The uniform magnetic field
has a magnitude B, and the distance between the rails is !.
The rails are connected at one end by a load resistor R.
Derive an expression that gives the horizontal speed of the
bar as a function of time, assuming that the suspended
object is released with the bar at rest at t " 0. Assume no
friction between rails and bar.
67.
A solenoid wound with 2 000 turns/m is supplied with
current that varies in time according to I "
(4A) sin(120,t), where t is in seconds. A small coaxial
circular coil of 40 turns and radius r " 5.00 cm is located
inside the solenoid near its center. (a) Derive an expres-
sion that describes the manner in which the emf in the
small coil varies in time. (b) At what average rate is energy
delivered to the small coil if the windings have a total resis-
tance of 8.00 (?
68.
Figure P31.68 shows a stationary conductor whose shape is
similar to the letter e. The radius of its circular portion is
a " 50.0 cm. It is placed in a constant magnetic field of
0.500 T directed out of the page. A straight conducting
rod, 50.0 cm long, is pivoted about point O and rotates
with a constant angular speed of 2.00 rad/s. (a) Deter-
mine the induced emf in the loop POQ. Note that the area
of the loop is #a
2
/2. (b) If all of the conducting material
has a resistance per length of 5.00 (/m, what is the
induced current in the loop POQ at the instant 0.250 s
after point P passes point Q?
69.
A betatron accelerates electrons to energies in the MeV
range by means of electromagnetic induction. Electrons in
a vacuum chamber are held in a circular orbit by a
magnetic field perpendicular to the orbital plane. The
magnetic field is gradually increased to induce an electric
field around the orbit. (a) Show that the electric field is in
the correct direction to make the electrons speed up.
(b) Assume that the radius of the orbit remains constant.
Show that the average magnetic field over the area
enclosed by the orbit must be twice as large as the
magnetic field at the circumference of the circle.
70.
A wire 30.0 cm long is held parallel to and 80.0 cm above a
long wire carrying 200 A and resting on the floor (Fig.
P31.70). The 30.0-cm wire is released and falls, remaining
parallel with the current-carrying wire as it falls. Assume
that the falling wire accelerates at 9.80 m/s
2
and derive an
equation for the emf induced in it. Express your result as a
function of the time t after the wire is dropped. What is
the induced emf 0.300 s after the wire is released?
A long, straight wire carries a current that is given by
I " I
max
sin(*t 2 4) and lies in the plane of a rectangular
coil of N turns of wire, as shown in Figure P31.9. The quanti-
ties I
max
, *, and 4 are all constants. Determine the emf
induced in the coil by the magnetic field created by the
current in the straight wire. Assume I
max
"
50.0 A,
* "
200, s
%
1
, N " 100, h " w " 5.00 cm, and L " 20.0 cm.
72.
A dime is suspended from a thread and hung between the
poles of a strong horseshoe magnet as shown in Figure
P31.72. The dime rotates at constant angular speed *
71.
Problems
1001
R
M
m
B
!
g
Figure P31.66
The moving
straight rod
P
Q
O
B
out
θ
a
Figure P31.68
30.0 cm
80.0 cm
I = 200 A
Figure P31.70
ω
N
S
Figure P31.72