Physics 151
Exam 3
Useful Constants
 = 4 x 10-7 T m/A
c = 3.00x108 m/s
k = 9.0 x109 Nm2/C2
 = 8.85 x 10-12 C2/(N m2)
e = 1.6 x 10-19 C
All magnetic fields have their origin in
(A) moving electric charges.
(B) iron atoms.
(C) permanent magnets.
(D) magnetic domains.
(E) magnetic monopoles.
Spring 1998
1 eV
g
me
mp
=
=
=
=
1.60x10-19 J
9.8 m/s2
9.1 x 10-31 kg
1.67 x 10-27 kg
Magnetic fields do not interact with
(A) stationary electric charges.
(B) moving electric charges.
(C) stationary permanent magnets.
(D) moving permanent magnets.
(E) none of the above.
The magnetic force on a stationary electric charge in a northward directed magnetic field is
(A) directed to the north.
(B) directed to the east.
(C) directed upward.
(D) necessarily zero.
(E) cannot be determined, more information is needed.
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Physics 151
Exam 3
Spring 1998
The magnetic field lines near a long, straight current are in the form of
(A) straight lines parallel to the current.
(B) straight lines perpendicular to the current.
(C) circles around the wire, centered on the current.
(D) spirals around the wire, winding outwards.
An electron enters a magnetic field perpendicularly to B. The electrons’s
(A) motion is unaffected.
(B) direction is changed.
(C) speed is changed.
(D) energy is changed.
An electron enters a magnetic field with motion parallel to B. The electron's
(A) motion is unaffected.
(B) direction is changed.
(C) speed is changed.
(D) energy is changed.
Two parallel wires carrying current in the same direction interact magnetically.
(A) The wires will attract each other.
(B) The wires will repel each other.
(C) The wire experience a torque that tend to turn the wires perpendicular to each other.
(D) The wires will not exert any forces on each other since they are parallel.
Two parallel wires carrying current in opposite directions interact magnetically.
(A) The wires will attract each other.
(B) The wires will repel each other.
(C) The wire experience a torque that tend to turn the wires perpendicular to each other.
(D) The wires will not exert any forces on each other since they are parallel.
Inside a solenoid the magnetic field
(A) is zero.
(B) is uniform.
(C) radiates from the center towards the sides of the solenoid.
(D) oscillates.
In the presence of a northerly magnetic field, a current which is easterly experiences a force
which is
(A) directed north.
(B) directed up.
(C) directed down.
(D) directed east.
(E) zero.
2
Physics 151
Exam 3
Spring 1998
In the presence of a northerly magnetic field, a current which is westerly experiences a force
which is
(A) directed north.
(B) directed up.
(C) directed down.
(D) directed east.
(E) zero.
An electric current is flowing east in a magnetic field directed northward. the direction of the
force is
(A) directed north.
(B) directed south.
(C) directed up.
(D) directd east.
(E) zero.
An ion moves in a circular orbit of radius R in a magnetic field. If the particle’s speed is
doubled, the orbit will
(A) double.
(B) quadruple.
(C) be reduced to half.
(D) be reduced to one-fourth.
A circular loop of wire is in a region of an external magnetic field which is uniform and
decreasing 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 loop carries a current clockwise as shown. The direction of the
magnetic field in the center of the loop is
(A)
(B)
(C)
(D)
(E)
into the page.
out of the page.
towards the top of the page.
towards the bottom of the page.
trick question, there is no magnetic field produced.
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Physics 151
Exam 3
Spring 1998
A loop carries a current counterclockwise as shown. The direction of
the magnetic field in the center of the loop is
(A)
(B)
(C)
(D)
(E)
into the page.
out of the page.
towards the top of the page.
towards the bottom of the page.
trick question, there is no magnetic field produced.
A bar magnet is passed through a coil of wire. The
induced current is greatest when
(A) the magnet moves slowly, so that it is inside
the coil for a long time.
(B) the magnet moves fast, so that it is inside the
coil for a short time.
(C) the north pole enters the coil first.
(D) the south pole enters the coil first.
N
S
An electromotive force is induced within a conductor whenever
(A) the conductor is in the presence of a magnetic field.
(B) the conductor is in the presence of a changing magnetic field (producing eddy
currents, e.g.).
(C) the conductor has a component of velocity perpendicular to the magnetic field.
(D) the conductor has a component of velocity parallel to the magnetic field.
(E) both (B) and (C) above.
The alternating current in the secondary coil of a transformer is induced by
(A) a varying electric field.
(B) a varying magnetic field.
(C) the iron core of the transformer.
(D) motion of the primary coil.
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Physics 151
Exam 3
Spring 1998
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.
A transformer has 100 turns in its primary winding and 300 turns on its secondary. If the power
input to the transformer primary is 60W, then the power output to the secondary is
(A) 20W.
(B) 60W.
(C) 180W.
(D) 540W.
Based upon the law of reflection, the height of the smallest mirror in which a 6 foot tall man can
see himself is
(A) 6 feet, no matter where what the distance to the mirror.
(B) 3 feet, no matter where what the distance to the mirror.
(C) 3 feet, but only if the distance to the mirror is 6 feet.
(D) can be any size, if he is far enough from the mirror.
In vacuum, the speed of light
(A) depends upon its frequency.
(B) depends upon its wavelength.
(C) is a universal constant of nature.
(D) is zero, since no wave can travel in vacuum.
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Physics 151
Exam 3
Spring 1998
Problems
Two parallel wires 1.00 m apart carry currents of 12.0 A in the same direction.
(a) Find the magnetic field magnitude at a point between the two wires at a point 0.25 m from
one wire and 0.75 m from the other.
(b) Find the magnetic field magnitude at a point 0.25 m beyond one wire which will be 1.25 m
from the other wire.
B for part (b)
I1
B for part (a)
I2
Two parallel wires 100 mm apart carry currents of 8.0 A in opposite directions.
(a) Find the magnetic field magnitude at a point between the two wires at a point 25 mm from
one wire and 75 mm from the other.
(b) Find the magnetic field magnitude at a point 25 mm beyond one wire which will be 125 mm
from the other wire.
B for part (b)
I1
B for part (a)
I2
6
Physics 151
Exam 3
Spring 1998
Two parallel wires 200 mm apart carry currents of 5.0 A in the same direction.
(a) Find the magnetic field magnitude at a point between the two wires at a point 50 mm from
one wire and 150 mm from the other.
(b) Find the magnetic field magnitude at a point 50mm beyond one wire which will be 250 mm
from the other wire.
B for part (b)
I
B for part (a)
I
A wire 2.0 m long is perpendicular to a magnetic field 0f 0.20 T.
(a) What is the force on the wire if it carries a current of 15 A.
(b) What is the force if the wire is parallel to the field.
A proton (charge = +e) is moving at 5.0 x 106 m/s through the center of a long solenoid.
perpendicular to the (uniform) field within the solenoid. The solenoid has 80 turns/cm and
carries a current of 40 A.
(a) Find the magnetic field within the solenoid.
(b) Find the magnitude of the magnetic force on the proton.
(c) What is the radius of the circular path in the magnetic field?
A proton (charge = +e) is moving at 2.0 x 107 m/s through the center of a long solenoid.
perpendicular to the field within the solenoid. The solenoid has 50 turns/cm and carries a current
of 5 A.
(a) Find the magnetic field within the solenoid.
(b) Find the magnitude of the magnetic force on the proton.
(c) What is the radius of the circular path in the magnetic field?
What is the radius of the circular motion of a charged particle moving in a field of .2T if
(a) the particle is an electron travelling at 2.0x106 m/s
(a) the particle is a proton travelling at 2.0x106 m/s
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Physics 151
Exam 3
Spring 1998
A transformer has 50 turns in its primary winding and 100 turns in its secondary winding. The
primary voltage is 120 V and a resistance of 5  is placed across the secondary.
(a) What is the potential difference across the secondary?
(b) What is the current delivered by the secondary?
(c) What is the power delivered by the secondary
(d) What current is delivered to the primary?
(e) What power is delivered to the primary?
A transformer has 400 turns in its primary winding and 100 turns in its secondary winding. The
primary voltage is 120 V and a resistance of 5  is placed across the secondary.
(a) What is the potential difference across the secondary?
(b) What is the current delivered by the secondary to the resistor?
(c) What is the power delivered by the secondary
(d) What current is delivered to the primary?
(e) What power is delivered to the primary?
A potential difference of 1.8V is found between the ends of a 2.0-m wire moving perpendicular
to a magnetic field at a speed of 20 m/s. What is the magnitude of the magnetic fields?
A car is traveling at 60 km/hr in a region where the horizontal component of the earth's magnetic
field has magnitude 3.0x10-5 T. Find the emf induced in the car's vertical radio antenna, which is
1.2 m long.
A beam of light in air strikes a glass surface at an angle of 60º , and is refracted at an angle of
35º. What is the index of refraction of the glass?
Crown glass can be used to separate out white light into its component colors (dispersion) since
its index of refraction changes with wavelength. The index of refraction varies from 1.51 for red
light to 1.53 for blue light. A beam of light is incident (from air) upon a crown glass surface at
an angle of 30. What is the range of angles for the refracted light?
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