Download 202b399

Physics 202
Test Bank III
Fall 1999
___ The motion of a charged particle in the presence of a (nearly uniform) magnetic field tends to be
(A) helical (i.e. spiral), circulating about the magnetic field lines.
(B) straight line, except when the charges bounce off of a magnetic field line as in magnetic
mirrors.
(C) rapidly slowing as the frictional effect of the magnetic force quickly uses up the particles
kinetic energy.
(D) rapidly increasing in speed as the particle is accelerated by the magnetic force.
(E) completely unaffected by the magnetic field.
___ A negative charge moves south through a magnetic field directed north. The particle will be
deflected
(A) North.
(B) Up.
(C) Down.
(D) East.
(E) not at all.
___ In order to use magnetic forces to levitate (i.e. magnetic force is upward against the force of gravity)
a horizontal wire carrying a current towards the east, the magnetic field must be directed
(A)
Up.
(B)
Down.
(C)
North.
(D)
East.
(E)
South.
___ The force on an electron traveling eastward through a northward directed magnetic field is
(A) south.
(B) north.
(C) west.
(D) down.
(E) up.
___ A negative charge moves south through a magnetic field directed north. The particle will be
deflected
(A) North.
(B) Up.
(C) Down.
(D) East.
(E) not at all.
1
Physics 202
Test Bank III
___ The features (a) and (b) of the magnetization versus applied
magnetic field plot at right are
(A)
hysteresis and permanent magnetization, respectively.
(B)
saturation and permanent magnetization, respectively.
(C)
hysteresis and saturation, respectively.
(D)
permanent magnetization and hysteresis, respectively.
(E)
none of the above.
Fall 1999
M
(a)
B0
(a)
(b)
2
Physics 202
Test Bank III
Fall 1999
___ The magnetic field of a long straight wire carrying a current into the page has field lines given by
B
I
I
B
(B)
(A)
B
I
I
B
(D)
(C)
(E) There is no magnetic field unless the current is changing.
___ 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.
___ A jet with a wingspan of 20m travels west at 1000 m/s through a region near the north magnetic
pole where the magnetic field is 40x10-6 T downward. The magnitude of the motional emf
induced across the wingspan is
(A)
0 V.
(B)
2x10-9 V.
(C)
2x10-3 V
(D)
.8 V.
(E)
none of the above.
___
Two long parallel wires are separated by .05 m and both carry a current of 10 A in the opposite
directions. The force exerted on a 1m section of one wire is
(A)
4x10-3 N, towards the other wire.
(B)
4x10-3 N, away from the other wire.
(C)
4x10-4 N, along the wire, in the direction of the current.
(D)
4x10-4 N, along the wire, in the opposite direction of the current.
(E)
none of the above.
___ The equation:
(A)
(B)
(C)
(D)
0I
is useful for a finite length of current carrying wire when
2 r
when the distance from the wire is much larger than the distances from the ends of the
wire.
when the distance from the wire is much smaller than the distances from the ends of the
wire.
when the wire is carrying very large currents.
when the wire is carrying very small currents.
3
Physics 202
(E)
Test Bank III
Fall 1999
when ever the instructor needs it to be.
4
Physics 202
Test Bank III
Fall 1999
___ A bar magnet is passed through a coil of wire. The induced current is
greatest when
(A) the magnet moves quickly, so that it is inside the coil for a short
time.
(B) the magnet moves slowly, so that it is inside the coil for a long
time.
(C) the north pole enters the coil first.
(D) the south pole enters the coil first.
(E) never (no current is induced since the coil is not moving).
N
S
___ Which property is associated with ferromagnetic materials?
(A)
Strong increase of magnetic field within the material.
(B)
Hysteresis.
(C)
Saturation.
(D)
Permanent magnetization.
(E)
all of the above.
___ The equation:
(A)
(B)
(C)
(D)
 B dA  0
indicates that there is no magnetic field (B = 0).
indicates that there is no magnetic charge (no isolated poles).
indicates that superconductivity is occurring.
indicates that Gauss's law does not work for magnetic fields.
___ A circular loop of wire is in a region of magnetic field which 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.
___ A circular loop of wire is in a region of magnetic field which is
uniform, directed into the page and increasing in strength with
time,.
(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.
5
Physics 202
Test Bank III
Fall 1999
[The same set of answers applies to the following 3 questions]
___ Which of the following is Ampere's law, which allows us to calculate magnetic fields in situations
with a great deal of symmetry (such as around a long straight wire) ?
___ A displacement current (i.e. a changing electric field) creates a magnetic field :
___ How to create electric fields from changing magnetic flux:
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) .
___ 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.
___ The magnetic field at the center of the circular loop (shown at right) carrying a
current clockwise will be directed
(A) into the page.
(B) out of the page.
(C) clockwise.
(D) counterclockwise.
(E) there is no magnetic field unless the current is changing.
I
___ A circular loop of wire is in a region of magnetic field, which is uniform and constant, directed into
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.
6
Physics 202
Test Bank III
Fall 1999
7
Physics 202
Test Bank III
Fall 1999
___ Lenz's Law, which characterizes induced currents in terms of a resistance to change in magnetic
flux, was characterized by Dr. Gallis as
(A) electromagnetic friction.
(B) electromagnetic inertia.
(C) electromagnetic temperature.
(D) electromagnetic hocus pocus.
8
Physics 202
Test Bank III
Fall 1999
Part II Problems
A strip of potassium 2.0 cm wide and 1mm thick carrying a current of 100 A produces a Hall emf with
magnitude 223 V in a magnetic field of 5.00 T.
(a) What is the density n of
B
free electrons in potassium?
(b) What is the magnitude of
the drift velocity of the
electrons?
If the magnetic field is then
decreased to 2.50 T,
(c) What is the Hall emf?
(d) What is the density n of
free electrons in potassium?
t
w
I
Charges are accelerated by an accelerating potential of 80 kV (of appropriate polarity for positive or
negative charges) into a region of uniform magnetic field (directed out of the page).
If the charges are electrons,
Accelerating potential V
(parallel plates)
(A) What is the speed of the electrons as they enter the
magnetic field?
(B) Indicate the path of the electrons on the diagram at
right (label the path e).
(C) What is the radius of the circular path taken by the
electrons as they travel in the uniform magnetic field?
If the charges are protons:
(D) What is the speed of the protons as they enter the
magnetic field?
(E) Indicate the path of the protons on the diagram at
right (label the path p).
(F) What is the radius of the circular path taken by the
protons as they travel in the uniform magnetic field?
Region of uniform magnetic field B
9
Physics 202
Test Bank III
Fall 1999
A mass spectrometer is constructed as shown by allowing particles to enter a velocity selector (with
crossed Electric and Magnetic fields and then entering a region of uniform magnetic field only. The
electric field within the velocity selector is 1x106 V/m and the magnetic field with both the velocity
selector and mass spectrometer is .2T, directed out of the page (see diagram). The ions are deflected by
the magnetic field, and traverse a semicircle of radius R, at the end of which they are detected
(a) In the figure at right, sketch in the trajectory of
the ions within the magnetic field.
velocity
selector
(b) For both C12 and C14 (two isotopes of
carbon), calculate the speed v of the ions as they
leave the accelerating potential and 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
uniform magnetic field
THROUGH OUT THESE CALCULATIONS.
B
(c) How far apart are the endpoints of the
semicircular trajectories?
The figure is an end view of two long parallel wires perpendicular to the xy plane, each carrying a
current I, the top is coming out of the page, the bottom is going into 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
10
Physics 202
Test Bank III
Fall 1999
The long straight wire in the figure shown carries a current of 20.0 A. The rectangular loop whose long
edges are parallel to the wire carries a current of 8.00 A. The loop is 10 cm long, 4 cm wide and the left
side of the loop is located 2 cm from the long straight wire. Find the magnitude and direction of the net
magnetic force exerted on the loop by the magnetic field of the wire.
I =8A
I = 20 A
A conducting bar moves on conducting rails as shown. There is a uniform magnetic with magnitude
.4 tesla directed into the page. The bar is pushed to the right at a constant speed of 25 m/s. The
resistance (which completes the loop) is 2 .
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)
11
Physics 202
Test Bank III
Fall 1999
A square loop of wire with resistance R is moved at a constant speed v across a uniform magnetic field
confined to a square region whose sides are twice the length of the square loop. (a) In the space
provided, graph the magnetic flux through the loop as a function of the position of the loop (referenced
to the front of the loop). The maximum flux has been determined for you. (b) In the second space
provided make a qualitative graph the induced current as a function of position.
(c) Determine the maximum current in terms of B, L, v, and R. (this will determine the limits of the
second graph.
2L
L
=
=
B
v

2L
-L
0
L
2L
3L
2L
-L
0
L
2L
3L
-L
0
L
2L
3L
B L2


0
I
0
12