Download Test #3 – Electromagnetism

Physics 12A
Electromagnetism Test
Name: ___________________________
Section I: Multiple Choice Questions (1 point each)
There is only one correct answer per question. Circle your answer.
1) For a wire 30 cm long, carrying a
current of 2.0 A, and experiencing a
perpendicular magnetic force of 1.2 N,
the magnetic field in the area must be
a) 20 T
b) 18 T
c) 2.0 T
d) 0.020 T
5) An electron is moving to the left across the page,
through a magnetic field directed into the page.
The force on the electron causes it to be
deflected in the direction of
a) the top of the page
b) the bottom of the page
c) out of the page
d) into the page
2) If conventional current flows from left
to right in the wire shown in the
diagram, what is the direction of the
magnetic field at point P?
6) In a motor, a coil in the armature is wound
around a soft iron core. The purpose of the soft
iron core is to
a) Cause the torque on the coil to remain in the
same direction
b) Change from AC to DC
c) Oppose the applied potential difference and
reduce the current in the coil
d) Strengthen and concentrate the magnetic
field through the coil
P
a)
b)
c)
d)
e)
towards bottom of paper
toward top of paper
to the right
into paper
out of paper
3) If a solenoid carries current which is
coming down at the front of the coil as
shown in the diagram below, the north
pole of the magnet is located
a) Behind the coil
b) In front of the coil
c) To the right of the coil
d) To the left of the coil
I
4) One part that a galvanometer has that a
motor does not is
a) An armature
b) A split ring commutator
c) A coil of wire
d) A spring
7) A split ring commutator is used in the DC motor
a) To stop the armature from turning all the
way around
b) To keep the armature going all the way
around
c) To change the magnitude of the current
d) To change the direction of the permanent
magnet’s magnetic field
8) Lenz’s law states that
a) a coil of wire subjected to a changing
magnetic field will induce a current to
oppose the change
b) the right hand rule for a solenoid indicates
where the north pole can be found
c) the right hand rules are used for
conventional current and the left hand rules
for electron flow
d) never touch a lens with your fingers
Physics 12A
9) In the space below, draw the magnetic
field around the wire, which is carrying
conventional current directly towards
you.
15) For an ideal step up transformer, the number of
turns in the secondary winding is ______ the
number of turns in the primary winding.
a) more than
b) less than
c) equal to
d) not enough info

10) A step up transformer is used to
a) Increase the available power
b) Increase the current
c) Increase the voltage
d) Increase the resistance
11) Consider the situation indicated by the
diagram below. In what direction would
current be induced in the loop of wire?
a) Clockwise
b) Counterclockwise
c) Out of the page
d) No current induced
v








 B
12) In the figure below, what is the direction
of the magnetic field from wire A at the
location of wire B?
a) left
b) right
c) out of the page
d) into the page
A
14) The maximum force on a wire in a magnetic
field occurs when the angle between the
magnetic field and the current is
a) 0
b) 45
c) 90
d) 180
B
13) What is the direction of the force ON
wire B in the preceding question?
a) right
b) left
c) out of page
d) into page
16) For an ideal transformer, the amount of ____ in
the primary coil and the secondary coil is equal.
a) Voltage
b) Current
c) Windings of wire
d) Power
17) If, in this room, there was a magnetic field
directed toward the left side of the page, a
straight wire lying along the right edge of the
test paper, carrying a conventional current of
2.0 A directed from the bottom of the page
toward the top of the page would experience a
force ___ the page.
a)
b)
c)
d)
directed toward the right side of
directed into
directed out of
directed toward the top edge of
18) A beam of protons with speed vo enters a
uniform magnetic field moving perpendicular to
the magnetic field. The protons then move in a
circle of radius Ro. If the speed of the protons
was decreased, the radius of the circle on which
they move would
a)
b)
c)
d)
increase
decrease
remain constant
be impossible to predict
Physics 12A
19) A long straight overhead conductor
carries a steady conventional current
from the north toward the south. You
stand directly below the wire. The
direction of the magnetic field at the
location of your head is ___.
a)
b)
c)
d)
east.
west.
north.
south.
e) upward.
f) downward
g) @#$*&%$???!!!
h) so much for engineering
20) Consider a magnetic field uniform over a large
volume of space directed perpendicular to and
into the page. An electron entering this field
moving initially along a line parallel to the top
edge of this page from your left toward
your right would experience a force ___ the
page.
a)
b)
c)
d)
e)
f)
directed toward the right side of
directed perpendicular to and into
directed perpendicular to and out of
directed toward the top edge of
directed toward the bottom edge of
directed toward the left side of
Section 2: Problems
Show all steps and formulas, use correct units and significant figures, if you expect to get full
marks. Be neat and concise.
1) Consider the diagram below. If the magnetic field strength is 1.1 T in the direction shown, the
length of the wire is 18 cm as shown, and the wire is moved at a speed of 9.5 m/s, determine
the magnitude and direction of the induced current in the wire. The resistance of the loop of
wire is 0.13  and V=IR. (5 pts)


v





 B




18 cm
Physics 12A
2) A certain transformer has 300 turns in the primary and 1250 turns in the secondary. It is only
80% efficient. If a primary voltage of 15 V is applied, determine the output voltage. (3 pts)
3) A force of 5.6010-16 N [up] acts on an unknown particle traveling at 90 degrees to a
magnetic field. The particle travels 2.54105 m [South] in 4.5 s and the field is
3.1010-2 T [West], how many electrons has the particle lost or gained? (7 pts)
Physics 12A
Section 3: Essay (5 points)
Choose ONLY ONE of the following questions to answer. Use complete sentences, proper
grammar and spelling. Include as much information as possible, but be specific and stay on topic.
1) Explain how a galvanometer works using at least one diagram, plus word explanations. Use
appropriate vocabulary learned in class.
OR
2) Explain how Lenz’s law can be used to determine the direction of induced current in a loop of
wire in at least two different situations. Also explain what Lenz’s law has to do with
conservation of energy (hint: consider the magnet & copper pipe experiment).