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Physics 212
NAME:_________________________
Exam III
Fall 2001
MC
P
Useful Constants:
k
= 9.0 x109 Nm2/C2

= 8.85 x 10-12 C2/(N m2)
e
= 1.6 x 10-19 C
me = 9.1 x 10-31 kg
1 eV = 1.6x10-19 J
g
= 9.81 m/s2

= 4 x 10-7 N s2/C2
N
E
W
S
Part I: Multiple Choice(4 points. ea.)
Read each question carefully and choose the best answer
___ 1. A particle with a net electrical charge moving through a magnetic field will experience no force if
(A) there is an electric field parallel to the magnetic field.
(B) there is an electric field anti-parallel to the magnetic field.
(C) the particle’s velocity is parallel to the magnetic field.
(D) the particle’s velocity is perpendicular to the magnetic field.
(E) the particle moves in a circle.
___ 2. An electron moves downward through a magnetic field directed north. The electron will be
deflected
(A) North.
(B) Up.
(C) Down.
(D) East.
(E) West.
M
___ 3. The features (a) and (b) of the magnetization versus applied
magnetic field plot at right are
(A) permanent magnetization and hysteresis, respectively.
(B) hysteresis and permanent magnetization, respectively.
(C) hysteresis and saturation, respectively.
(D) saturation and permanent magnetization, respectively.
(E) none of the above.
(a)
B0
(a)
(b)
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Physics 212
Exam III
Fall 2001
___ 4. The magnetic field at the center of a current circulating counter clockwise
will be:
(A)
into the page.
(B)
out of the page.
(C)
clockwise.
(D)
counter clockwise.
(E)
zero, unless the current is changing.
B
I
___ 5. A material which will weaken the net magnetic field (as compared to the same situation without
the material) when an external field is applied is
(A)
paramagnetic.
(B)
ferromagnetic.
(C)
diamagnetic.
(D)
quasimagnetic.
(E)
paranormal.
___ 6. A pair of current carrying wires may be twisted around each other so that
(A) the magnetic fields created by each wire will more effectively cancel.
(B) the magnetic fields created by each wire will more effectively reinforce each other.
(C) the net magnetic force on the wires by earth’s field is stronger.
(D) the magnetic attraction between the wires is stronger.
(E) the wires don’t fly apart from their magnetic repulsion.
___ 7. A bar magnet is passed through a coil of wire. The induced current is
greatest when
time.
(A) the north pole enters the coil first.
(B) the south pole enters the coil first.
(C) the magnet moves slowly, so that it is inside the coil for a long
time.
(D) the magnet moves quickly, so that it is inside the coil for a short
(E) never (no current is induced since the coil is not moving).
N
S
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Physics 212
Exam III
Fall 2001
___ 8. A circular loop of wire is in a region of magnetic field that is uniform and increasing in strength,
directed out of the page.
(A) There will be an induced current, which circulates clockwise.
(B) There will be an induced current, which circulates counter
clockwise.
(C) There is insufficient information to determine the direction of
the induced current flow.
(D) There will not be any induced current.
___ 9. The Lorentz force law :
Qencl
(A)
E dA =
(B)
B dA = 0.
(C)
F = q(E + vB).
(D)
B d = o (
J dA + o d
dt
(E)
E d =  d
dt
B dA ).
o
.
E dA ).
___ 10. All magnetic fields have their origin in
(A) iron atoms.
(B) permanent magnets.
(C) magnetic domains.
(D) moving electric charges.
(E) The origin of magnetic fields cannot be characterized in any simple manner.
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Physics 212
Part II Problems
(15 points each).
Show all work. No work = no credit!
Exam III
Fall 2001
1) A mass spectrometer is constructed as shown by allowing singly ionized atoms to enter a velocity
selector (with crossed Electric and Magnetic fields and then entering a region of uniform magnetic field
only. The electric field is tuned to select a velocity of 1x105 m/s and the magnetic field with both the
velocity selector and mass spectrometer is 0.100 T, directed out of the page (see diagram). As the ions
leave the velocity selector, the ions are deflected by the magnetic field and traverse a semicircle of radius
R, at the end of which they are detected
(A) Determine the magnitude and direction of the
y
Electric field in the velocity selector,
velocity
(a) In the figure at right, sketch in the trajectory of the
selector
ions within the magnetic field.
x
(b) For both C12 and C14 (two isotopes of carbon),
determine the radius R of the semicircular trajectories.
The masses of C12 and C14 are 12u and 14u,
respectively, where 1u = 1.66x10-27 Kg.
KEEP AT LEAST 3 SIGNIFICANT FIGURES
THROUGH OUT THESE CALCULATIONS.
(c) How far apart are the endpoints of the semicircular
trajectories?
uniform magnetic field
B
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Physics 212
Exam III
Fall 2001
2) The figure is an end view of two long parallel wires perpendicular to the xy plane, each carrying a
current I out of the page.
(a) On the diagram, show the contributions of to B from each wire, and the resultant B at the point P
(b) Derive an expression for the magnitude of the resultant B for any point on the x-axis in terms of the
x-coordinate of the point, the y-coordinate of the wire a, and the current I.
(c) Make a graph of the magnitude of B as a function of x.
y axis
I
a
P
x axis
x
a
I
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Physics 212
Exam III
Fall 2001
3 A conducting bar with length .2m moves on conducting rails as shown. There is a uniform magnetic
with magnitude .0500 T directed into the page. The bar is pushed to the right at a constant speed of 12
m/s. The resistance (which completes the loop) is 5 .
a) What is the EMF?
b) What is the size and direction (clockwise or
counter clockwise) of induced current?
c) What is the power dissipated in the EMF?
d) What is the force on the current due to the
R
magnetic field?
v
e) The (mechanical) force which must be applied
to keep the bar moving is equal in size (opposite
direction) to the magnetic force. Using this
information, calculate the mechanical power
which must be delivered to keep the bar moving.
(recall from physics 201 that P =Fv)
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Physics 212
Exam III
Fall 2001
4. A strip of copper 1.0 cm wide and 2mm thick carrying a current of 100 A produces a Hall emf with
magnitude 42.8 V in a magnetic field of 0.800 T.
(a) What is the density n of
B
free electrons in copper?
(b) What is the magnitude of
the drift velocity of the
electrons?
If the magnetic field is then
decreased to 0.300 T,
(c) What is the Hall emf?
(d) What is the density n of
free electrons in copper?
t
w
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