Buggé: Magnetism 5 Solutions
Practice Problems
5.1 Represent and reason Two wires are parallel to each other. Wire 1 has electric current going into
the page and wire 2 has electric current coming out of the page. Fill in the table that follows to
emphasize the importance of choosing a source of a field and a system on which the field exerts a
force.
5.2 Represent and reason An electric current travels through a horizontal bar that
hangs from two thin side wires (see the side view at the right). In what direction
should an external magnetic field point so that the magnetic force that the
magnetic field exerts on the bar helps support the bar? Explain.
The magnetic field created by the segment of wire parallel to the rod will
exert a force down the rod. An external magnetic field would have to
point out of the page through the bar such that the force is exerted up.
Buggé: Magnetism 5 Solutions
5.3 Evaluate the solution
The problem: You are playing a video air hockey game in which a hockey puck of mass m with electric
charge +q leaves a velocity selector traveling at speed v on a horizontal frictionless surface. You are to
write an expression for the magnitude B of a magnetic field and decide on its direction so that it bends
the puck in a curving half circle (toward the bottom of the screen) and it hits a target a distance 2R below
the place the puck left the velocity selector.
Solution: The situation is pictured at the right, along with a free-body diagram for the puck as seen in the
plane of its motion at the instant the puck has reached the halfway point around the half circle. The
acceleration direction is indicated.
Front view
Top view
Mathematical Representation
y
∑Fradial = mv2/R
Consequently,
B = mv2/qR and
or
Velocity
selector
qvB sin 900 = mv2/R.
the field points to the left.
Target
on P
x
FB on P
(a) Identify any missing elements or errors in the
solution.
The magnetic field direction.
FS
FE
on P
.
A final expression.
(b) Provide a corrected solution or missing elements if you find errors.
Magnetic field is out of the page.
B = mv/qR
5.4 A duck accumulates a positive charge of 3.0 x 10-8 C while flying north at speed 18 m/s. Earth's
magnetic field at the duck's location has a magnitude of 5.3 x 10-5 T and points in a direction 620
below a horizontal line pointing north. Determine the magnitude and direction of the force exerted
by Earth’s magnetic field on the duck.
F = qvBsinθ = (3.0 x 10-8 C)(18 m/s)(5.3 x 10-5 T)sin(62) = 2.5 x 10-11 N
Buggé: Magnetism 5 Solutions
5.5 Regular problem What happens to a cosmic-ray proton
at a
 flying into the Earth’s atmosphere
7
-5
speed of about 10 m/s? The magnitude of the Earth’s B field is approximately 5 x 10 T. The mass
-27
m of a proton is approximately
 10 kg. Consider three cases: The proton0 enters the Earth’s
atmosphere parallel to the B field, perpendicular to the field, and at a 30 angle relative to the field.
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Buggé: Magnetism 5 Solutions
5.6 Reason Suppose that the rail for a train had an electric current
traveling through it and that the train had a large coil such as
shown at the right. Can the magnetic field produced by the rail
exert a magnetic force on the train that helps support it—like
magnetic levitation? Explain. Hint: The magnetic field strength
decreases with distance from the current-carrying wire.
If the current in the rail is traveling in the
opposite direction as the train pictured above
then the magnetic field will exert a net force on
the train that is upwards because the magnitude
of the magnetic field would decrease as you get
further from the rails.
5.7 Evaluate You work in the complaint office at General Motors. A customer makes a complaint. Fill
in the table that follows to describe the process in other ways and decide how you, an employee in
the complaint office, will respond.
A customer says that while driving through the Earth’s magnetic field, a potential difference was
developed from one end of her car to the other. The car discharged, causing her to lose control and
run off the highway into a ditch. Decide the action you should take. Be sure to use your physics
knowledge to help complete the recommendation—people have much more confidence in decisions
based on physics, don’t they?
a) Sketch the situation.
b) Construct a physical representation (force diagrams, magnetic field diagram, etc).
c) Write a mathematical representation.
Buggé: Magnetism 5 Solutions
d) Evaluate the argument and provide a recommendation.
This is too much charge to put into one place so the customer must think of another excuse
for driving off the road.
5.8 Represent and reason The speed of blood flow in an artery can be measured using our knowledge
of a magnetic field and an electric field. The process is described below in words. Represent the
process in other ways.
Positive and negative ions move with blood through a magnetic field in an artery that is
perpendicular to the blood’s velocity. The magnetic force causes some positive ions to accumulate

on one wall of the artery and negative ions on the other wall. This charge separation causes an E
field that opposes further charge separation. Derive an expression for the speed of the blood in terms
of the electric and magnetic fields.
a) Draw a sketch.
c) Write a mathematical representation.
b) Draw a physical representation.
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