Download PHY 104 Exam #3 Magnetism, magnetic Forces and

PHY 104 Exam #3 Magnetism, magnetic Forces and Electromagnetic Induction
1) A solenoid is connected to a battery as shown in the
figure to the right and a bar magnet is placed nearby and
fixed in place but able to pivot about its center. In what
direction will the bar magnet rotate in response to the
magnetic field created by the solenoid? Explain your
answer by describing, in words, the magnetic polarity of
the solenoid.
S
N
+
_
2) Uranium atoms are ionized by intense heating and injected, with
E and B
B
various velocities, into a velocity selector/mass spectrometer
x x x x x x
    
illustrated to the right, for the purpose of separating the less
fissionable 238U from the more fissionable 235U atoms for the
x x N/C
x x x x
E1 = 50,000
    
(alleged) purpose of producing fuel for a nuclear fission reactor.
B1 = .01T
The electric field and magnetic field strengths are given in the
B2 = 1.5 T
diagram.
    
A) What velocity ions will emerge from the velocity selector
and enter the turning field?
B) What will be the radius of curvature of the trajectory of a singly ionized 235U ion?
    
C) What will be the difference in cm of the radius of curvature of the trajectories of 235U and 238U ions?
1
3) A long wire carrying a 5.00 A current penetrates into a region of a
0.44 T magnetic field as shown below. Using the dimensions in the
figure determine the magnitude and direction of the net force on the
wire.
X
1
X
2
X


X





X X


B field
5.00 A
30 cm
X
X
50 cm
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
4) A current in a long straight wire carries a current of 2.0 A. A second wire runs parallel to the first wire but
is 3 cm above the first wire. What is the direction and magnitude of the current in the second wire that will
just cancel the magnetic field created by the first wire at a point 9 cm below the X
first Xwire?
X X X X X
5) A solenoid is to be used to insert and withdraw a vertical iron rod from a lock
mechanism, thus allowing a door to be electronically locked and unlocked. The
solenoid has 200 turns and a length of 10 cm. The graph below displays the
empirical relationship between the force on the iron rod and the strength of the
magnetic field of the solenoid. If the mass of the iron rod is 500 g, what current
is needed to lift the rod from the lock? Assume that there is no friction holding
the
rod
in
the
lock

Solenoid













Fe
Bar
Bar





mechanism.
6) A strong magnet with its north pole oriented downward is falling through a vertical copper
pipe at a constant velocity. The magnet’s weight is opposed by magnetic forces from eddy
currents created in the copper pipe. Using physical principles discussed in class, describe, step
by step, how an eddy current that appears below the falling magnet creates a force on the
magnet that opposes its weight.
S
N
7) A metal triangular coat hanger (30 cm wide and 15 cm tall) hangs in your closet such that the plane of the
hanger is oriented in an east-west direction. The horizontal component of the magnetic field of the Earth is
perpendicular to the coat hanger, but the net magnetic field, whose strength is 5.7 x 10-5 T, is dipping 30
degrees below the horizontal as it passes through the coat hanger. If, in a historically unprecedented event,
the magnetic field of the Earth were to suddenly vanish in a time period of 1 μs, what would be the induced
emf in the coat hanger?
Looking North

B northward but dipping downward at a 30 angle
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X

B northward but
Looking East
Hanger seen
sideways
dipping downward at a
30 angle
Horizontal
15 cm
X
X
X
30 cm
X
X
X
8) A rectangular loop of wire with dimensions 5 cm by 10 cm as indicated in the figure moves with a velocity
of 15 cm/s towards, into and through a region of constant magnetic field whose intensity is 0.8 T.
Calculate the magnitude of the induced emf in the coil as it moves (A) towards, (B) into and (C) through
the magnetic field. Justify your answers using Faraday’s Law. Emf’s that create clockwise currents are
positive and emf’s that create counter-clockwise currents are negative.

B  0.8 T
10 cm
5 cm
15 cm/s
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
9) A circular coil of wire with diameter 10.0 cm and resistance of 1.25 Ohms is in a constant 5 x 10-5 T
magnetic field so that the plane of the coil is perpendicular to the B field. The coil is “flipped” through an
angle of 180 degrees in a time period of 0.1 s.
A) What is the change in the magnetic flux through the coil after a 180 degree flip? (Hint: You would get
the same change in flux if the magnetic field were turned 180 degrees)
B) If the induced current in the loop is 0.63 mA, how many turns are there in the coil? Round your result
to the nearest integer.