Chapter 8 Review
Key Terms
cosmic rays
comprised of particles that originate mainly from outside the solar system and reach Earth
cyclotron
device used to accelerate charged particles to large kinetic energies
dees
large metal containers used in cyclotrons that serve contain a stream of charged particles as their speed is increased
gauss
unit of the magnetic field strength;
Hall effect
creation of voltage across a currentcarrying conductor by a magnetic field
helical motion
superposition of circular motion with a straightline motion that is followed by a charged particle moving in a region of magnetic field at an angle to the field
magnetic dipole
closedcurrent loop
magnetic dipole moment
term
of the magnetic dipole, also called
magnetic field lines
continuous curves that show the direction of a magnetic field; these lines point in the same direction as a compass points, toward the magnetic south pole of a bar magnet
magnetic force
force applied to a charged particle moving through a magnetic field
mass spectrometer
device that separates ions according to their chargetomass ratios
motor (dc)
loop of wire in a magnetic field; when current is passed through the loops, the magnetic field exerts torque on the loops, which rotates a shaft; electrical energy is converted into mechanical work in the process
north magnetic pole
currently where a compass points to north, near the geographic North Pole; this is the effective south pole of a bar magnet but has flipped between the effective north and south poles of a bar magnet multiple times over the age of Earth
righthand rule1
using your right hand to determine the direction of either the magnetic force, velocity of a charged particle, or magnetic field
south magnetic pole
currently where a compass points to the south, near the geographic South Pole; this is the effective north pole of a bar magnet but has flipped just like the north magnetic pole
tesla
SI unit for magnetic field:
velocity selector
apparatus where the crossed electric and magnetic fields produce equal and opposite forces on a charged particle moving with a specific velocity; this particle moves through the velocity selector not affected by either field while particles moving with different velocities are deflected by the apparatus
Key Equations
Force on a charge in a magnetic field  
Magnitude of magnetic force  
Radius of a particle’s path in a magnetic field 

Period of a particle’s motion in a magnetic field 

Force on a currentcarrying wire in a uniform magnetic field 

Magnetic dipole moment 

Torque on a current loop 

Energy of a magnetic dipole 

Drift velocity in crossed electric and magnetic fields 

Hall potential 

Hall potential in terms of drift velocity 

Chargetomass ratio in a mass spectrometer 

Maximum speed of a particle in a cyclotron 

Summary
8.1 Magnetism and Its Historical Discoveries
 Magnets have two types of magnetic poles, called the north magnetic pole and the south magnetic pole. North magnetic poles are those that are attracted toward Earth’s geographic North Pole.
 Like poles repel and unlike poles attract.
 Discoveries of how magnets respond to currents by Oersted and others created a framework that led to the invention of modern electronic devices, electric motors, and magnetic imaging technology.
8.2 Magnetic Fields and Lines
 Charges moving across a magnetic field experience a force determined by The force is perpendicular to the plane formed by and
 The direction of the force on a moving charge is given by the right hand rule 1 (RHR1): Sweep your fingers in a velocity, magnetic field plane. Start by pointing them in the direction of velocity and sweep towards the magnetic field. Your thumb points in the direction of the magnetic force for positive charges.
 Magnetic fields can be pictorially represented by magnetic field lines, which have the following properties:
 The field is tangent to the magnetic field line.
 Field strength is proportional to the line density.
 Field lines cannot cross.
 Field lines form continuous, closed loops.
 Magnetic poles always occur in pairs of north and south—it is not possible to isolate north and south poles.
8.3 Motion of a Charged Particle in a Magnetic Field
 A magnetic force can supply centripetal force and cause a charged particle to move in a circular path of radius
 The period of circular motion for a charged particle moving in a magnetic field perpendicular to the plane of motion is
 Helical motion results if the velocity of the charged particle has a component parallel to the magnetic field as well as a component perpendicular to the magnetic field.
8.4 Magnetic Force on a CurrentCarrying Conductor
 An electrical current produces a magnetic field around the wire.
 The directionality of the magnetic field produced is determined by the right hand rule2, where your thumb points in the direction of the current and your fingers wrap around the wire in the direction of the magnetic field.
 The magnetic force on currentcarrying conductors is given by where is the current and is the length of a wire in a uniform magnetic field
8.5 Force and Torque on a Current Loop
 The net force on a currentcarrying loop of any plane shape in a uniform magnetic field is zero.
 The net torque on a currentcarrying loop of any shape in a uniform magnetic field is calculated using where is the magnetic dipole moment and is the magnetic field strength.
 The magnetic dipole moment is the product of the number of turns of wire the current in the loop and the area of the loop or
8.6 The Hall Effect
 Perpendicular electric and magnetic fields exert equal and opposite forces for a specific velocity of entering particles, thereby acting as a velocity selector. The velocity that passes through undeflected is calculated by
 The Hall effect can be used to measure the sign of the majority of charge carriers for metals. It can also be used to measure a magnetic field.
8.7 Applications of Magnetic Forces and Fields
 A mass spectrometer is a device that separates ions according to their chargetomass ratios by first sending them through a velocity selector, then a uniform magnetic field.
 Cyclotrons are used to accelerate charged particles to large kinetic energies through applied electric and magnetic fields.
Answers to Check Your Understanding
8.1 a.
b.
c.
d.
8.2 a.
toward the south; b.
8.3 a. bends upward; b. bends downward
8.4 a. perpendicular ; b. antialigned
8.5 a.
b.
8.6
Conceptual Questions
8.2 Magnetic Fields and Lines
1. Discuss the similarities and differences between the electrical force on a charge and the magnetic force on a charge.
2. (a) Is it possible for the magnetic force on a charge moving in a magnetic field to be zero? (b) Is it possible for the electric force on a charge moving in an electric field to be zero? (c) Is it possible for the resultant of the electric and magnetic forces on a charge moving simultaneously through both fields to be zero?
8.3 Motion of a Charged Particle in a Magnetic Field
3. At a given instant, an electron and a proton are moving with the same velocity in a constant magnetic field. Compare the magnetic forces on these particles. Compare their accelerations.
4. Does increasing the magnitude of a uniform magnetic field through which a charge is traveling necessarily mean increasing the magnetic force on the charge? Does changing the direction of the field necessarily mean a change in the force on the charge?
5. An electron passes through a magnetic field without being deflected. What do you conclude about the magnetic field?
6. If a charged particle moves in a straight line, can you conclude that there is no magnetic field present?
7. How could you determine which pole of an electromagnet is north and which pole is south?
8.4 Magnetic Force on a CurrentCarrying Conductor
8. Describe the error that results from accidently using your left rather than your right hand when determining the direction of a magnetic force.
9. Considering the magnetic force law, are the velocity and magnetic field always perpendicular? Are the force and velocity always perpendicular? What about the force and magnetic field?
10. Why can a nearby magnet distort a cathode ray tube television picture?
11. A magnetic field exerts a force on the moving electrons in a current carrying wire. What exerts the force on a wire?
12. There are regions where the magnetic field of earth is almost perpendicular to the surface of Earth. What difficulty does this cause in the use of a compass?
8.6 The Hall Effect
13. Hall potentials are much larger for poor conductors than for good conductors. Why?
8.7 Applications of Magnetic Forces and Fields
14. Describe the primary function of the electric field and the magnetic field in a cyclotron.
Problems
8.2 Magnetic Fields and Lines
15. What is the direction of the magnetic force on a positive charge that moves as shown in each of the six cases?
16. Repeat previous exercise for a negative charge.
17. What is the direction of the velocity of a negative charge that experiences the magnetic force shown in each of the three cases, assuming it moves perpendicular to
18. Repeat previous exercise for a positive charge.
19. What is the direction of the magnetic field that produces the magnetic force on a positive charge as shown in each of the three cases, assuming
is perpendicular to
?
20. Repeat previous exercise for a negative charge.
21. (a) Aircraft sometimes acquire small static charges. Suppose a supersonic jet has a
charge and flies due west at a speed of
over Earth’s south magnetic pole, where the
magnetic field points straight up. What are the direction and the magnitude of the magnetic force on the plane? (b) Discuss whether the value obtained in part (a) implies this is a significant or negligible effect.
22. (a) A cosmic ray proton moving toward Earth at
experiences a magnetic force of
What is the strength of the magnetic field if there is a
angle between it and the proton’s velocity? (b) Is the value obtained in part a. consistent with the known strength of Earth’s magnetic field on its surface? Discuss.
23. An electron moving at
in a
magnetic field experiences a magnetic force of
What angle does the velocity of the electron make with the magnetic field? There are two answers.
24. (a) A physicist performing a sensitive measurement wants to limit the magnetic force on a moving charge in her equipment to less than
What is the greatest the charge can be if it moves at a maximum speed of
in Earth’s field? (b) Discuss whether it would be difficult to limit the charge to less than the value found in (a) by comparing it with typical static electricity and noting that static is often absent.
8.3 Motion of a Charged Particle in a Magnetic Field
25. A cosmicray electron moves at
perpendicular to Earth’s magnetic field at an altitude where the field strength is
What is the radius of the circular path the electron follows?
26. (a) Viewers of Star Trek have heard of an antimatter drive on the Starship Enterprise. One possibility for such a futuristic energy source is to store antimatter charged particles in a vacuum chamber, circulating in a magnetic field, and then extract them as needed. Antimatter annihilates normal matter, producing pure energy. What strength magnetic field is needed to hold antiprotons, moving at
in a circular path
in radius? Antiprotons have the same mass as protons but the opposite (negative) charge. (b) Is this field strength obtainable with today’s technology or is it a futuristic possibility?
27. (a) An oxygen
ion with a mass of
travels at
perpendicular to a
magnetic field, which makes it move in a circular arc with a
radius. What positive charge is on the ion? (b) What is the ratio of this charge to the charge of an electron? (c) Discuss why the ratio found in (b) should be an integer.
28. An electron in a TV CRT moves with a speed of
in a direction perpendicular to Earth’s field, which has a strength of
(a) What strength electric field must be applied perpendicular to the Earth’s field to make the electron moves in a straight line? (b) If this is done between plates separated by
what is the voltage applied? (Note that TVs are usually surrounded by a ferromagnetic material to shield against external magnetic fields and avoid the need for such a correction.)
29. (a) At what speed will a proton move in a circular path of the same radius as the electron in the previous exercise? (b) What would the radius of the path be if the proton had the same speed as the electron? (c) What would the radius be if the proton had the same kinetic energy as the electron? (d) The same momentum?
30. (a) What voltage will accelerate electrons to a speed of
? (b) Find the radius of curvature of the path of a proton accelerated through this potential in a
field and compare this with the radius of curvature of an electron accelerated through the same potential.
31. An alphaparticle
travels in a circular path of radius
in a uniform magnetic field of magnitude
(a) What is the speed of the particle? (b) What is the kinetic energy in electronvolts? (c) Through what potential difference must the particle be accelerated in order to give it this kinetic energy?
32. A particle of charge
and mass
is accelerated from rest through a potential difference
after which it encounters a uniform magnetic field
If the particle moves in a plane perpendicular to
what is the radius of its circular orbit?
8.4 Magnetic Force on a CurrentCarrying Conductor
33. What is the direction of the magnetic force on the current in each of the six cases?
34. What is the direction of a current that experiences the magnetic force shown in each of the three cases, assuming the current runs perpendicular to
?
35. What is the direction of the magnetic field that produces the magnetic force shown on the currents in each of the three cases, assuming
is perpendicular to
?
36. (a) What is the force per meter on a lightning bolt at the equator that carries
perpendicular to Earth’s
? (b) What is the direction of the force if the current is straight up and Earth’s field direction is due north, parallel to the ground?
37. (a) A dc power line for a lightrail system carries
at an angle of
to Earth’s
field. What is the force on a
section of this line? (b) Discuss practical concerns this presents, if any.
38. A wire carrying a
current passes between the poles of a strong magnet that is perpendicular to its field and experiences a
force on the
of wire in the field. What is the average field strength?
8.5 Force and Torque on a Current Loop
39. (a) By how many percent is the torque of a motor decreased if its permanent magnets lose
of their strength? (b) How many percent would the current need to be increased to return the torque to original values?
40. (a) What is the maximum torque on a
turn square loop of wire
on a side that carries a
current in a
field? (b) What is the torque when
is
?
41. Find the current through a loop needed to create a maximum torque of
The loop has
square turns that are
on a side and is in a uniform
magnetic field.
42. Calculate the magnetic field strength needed on a
turn square loop
on a side to create a maximum torque of
if the loop is carrying
43. Since the equation for torque on a currentcarrying loop is τ = NIAB sin
the units of
must equal units of
Verify this.
44. (a) At what angle
is the torque on a current loop
of maximum? (b)
of maximum? (c)
of maximum?
45. A proton has a magnetic field due to its spin. The field is similar to that created by a circular current loop
in radius with a current of
Find the maximum torque on a proton in a
field. (This is a significant torque on a small particle.)
46. (a) A
turn circular loop of radius
is vertical, with its axis on an eastwest line. A current of
circulates clockwise in the loop when viewed from the east. Earth’s field here is due north, parallel to the ground, with a strength of
What are the direction and magnitude of the torque on the loop? (b) Does this device have any practical applications as a motor?
47. Repeat the previous problem, but with the loop lying flat on the ground with its current circulating counterclockwise (when viewed from above) in a location where Earth’s field is north, but at an angle
below the horizontal and with a strength of
8.6 The Hall Effect
48. A strip of copper is placed in a uniform magnetic field of magnitude
The Hall electric field is measured to be
(a) What is the drift speed of the conduction electrons? (b) Assuming that
electrons per cubic meter and that the crosssectional area of the strip is
calculate the current in the strip. (c) What is the Hall coefficient
?
49. The crosssectional dimensions of the copper strip shown are
by
The strip carries a current of
and it is placed in a magnetic field of magnitude
What are the value and polarity of the Hall potential in the copper strip?
50. The magnitudes of the electric and magnetic fields in a velocity selector are
and
respectively. (a) What speed must a proton have to pass through the selector? (b) Also calculate the speeds required for an alphaparticle and a singly ionized
atom to pass through the selector.
51. A charged particle moves through a velocity selector at constant velocity. In the selector,
and
When the electric field is turned off, the charged particle travels in a circular path of radius
Determine the chargetomass ratio of the particle.
52. A Hall probe gives a reading of
for a current of
when it is placed in a magnetic field of
What is the magnetic field in a region where the reading is
for
of current?
8.7 Applications of Magnetic Forces and Fields
53. A physicist is designing a cyclotron to accelerate protons to onetenth the speed of light. The magnetic field will have a strength of
Determine (a) the rotational period of the circulating protons and (b) the maximum radius of the protons’ orbit.
54. The strengths of the fields in the velocity selector of a Bainbridge mass spectrometer are
and
and the strength of the magnetic field that separates the ions is
A stream of singly charged
ions is found to bend in a circular arc of radius
What is the mass of the
ions?
55. The magnetic field in a cyclotron is
and the maximum orbital radius of the circulating protons is
(a) What is the kinetic energy of the protons when they are ejected from the cyclotron? (b) What is this energy in
? (c) Through what potential difference would a proton have to be accelerated to acquire this kinetic energy? (d) What is the period of the voltage source used to accelerate the protons? (e) Repeat the calculations for alphaparticles.
56. A mass spectrometer is being used to separate common oxygen
from the much rarer oxygen
taken from a sample of old glacial ice. (The relative abundance of these oxygen isotopes is related to climatic temperature at the time the ice was deposited.) The ratio of the masses of these two ions is
to
the mass of oxygen
is
and they are singly charged and travel at
in a
magnetic field. What is the separation between their paths when they hit a target after traversing a semicircle?
57. (a) Triply charged uranium
and uranium
ions are being separated in a mass spectrometer. (The much rarer uranium
is used as reactor fuel.) The masses of the ions are
and
respectively, and they travel at
in a
field. What is the separation between their paths when they hit a target after traversing a semicircle? (b) Discuss whether this distance between their paths seems to be big enough to be practical in the separation of uranium
from uranium
Additional Problems
58. Calculate the magnetic force on a hypothetical particle of charge
moving with a velocity of
in a magnetic field of
59. Repeat the previous problem with a new magnetic field of
60. An electron is projected into a uniform magnetic field
with a velocity of
What is the magnetic force on the electron?
61. The mass and charge of a water droplet are
and
respectively. If the droplet is given an initial horizontal velocity of
what magnetic field will keep it moving in this direction? Why must gravity be considered here?
62. Four different proton velocities are given. For each case, determine the magnetic force on the proton in terms of
and
63. An electron of kinetic energy
passes between parallel plates that are
apart and kept at a potential difference of
What is the strength of the uniform magnetic field
that will allow the electron to travel undeflected through the plates? Assume
and
are perpendicular.
64. An alphaparticle
moving with a velocity
enters a region where
and
What is the initial force on it?
65. An electron moving with a velocity
enters a region where there is a uniform electric field and a uniform magnetic field. The magnetic field is given by
If the electron travels through a region without being deflected, what is the electric field?
66. At a particular instant, an electron is traveling west to east with a kinetic energy of
Earth’s magnetic field has a horizontal component of
north and a vertical component of
down. (a) What is the path of the electron? (b) What is the radius of curvature of the path?
67. Repeat the calculations of the previous problem for a proton with the same kinetic energy.
68. What magnetic field is required in order to confine a proton moving with a speed of
to a circular orbit of radius
?
69. An electron and a proton move with the same speed in a plane perpendicular to a uniform magnetic field. Compare the radii and periods of their orbits.
70. A proton and an alphaparticle have the same kinetic energy and both move in a plane perpendicular to a uniform magnetic field. Compare the periods of their orbits.
71. A singly charged ion takes
to complete eight revolutions in a uniform magnetic field of magnitude
What is the mass of the ion?
72. A particle moving downward at a speed of
enters a uniform magnetic field that is horizontal and directed from east to west. (a) If the particle is deflected initially to the north in a circular arc, is its charge positive or negative? (b) If
and the chargetomass ratio
of the particle is
what is the radius of the path? (c) What is the speed of the particle after it has moved in the field for
? for
?
73. A proton, deuteron, and an alphaparticle are all accelerated through the same potential difference. They then enter the same magnetic field, moving perpendicular to it. Compute the ratios of the radii of their circular paths. Assume that
and
74. A singly charged ion is moving in a uniform magnetic field of
completes
revolutions in
Identify the ion.
75. Two particles have the same linear momentum, but particle
has four times the charge of particle
If both particles move in a plane perpendicular to a uniform magnetic field, what is the ratio
of the radii of their circular orbits?
76. A uniform magnetic field of magnitude
is directed parallel to the
axis. A proton enters the field with a velocity
and travels in a helical path with a radius of
(a) What is the value of
? (b) What is the time required for one trip around the helix? (c) Where is the proton
after entering the field?
77. An electron moving at
enters a magnetic field that makes a
angle with the
axis of magnitude
Calculate the (a) pitch and (b) radius of the trajectory.
78. (a) A
long section of cable carrying current to a car starter motor makes an angle of
with Earth’s
field. What is the current when the wire experiences a force of
? (b) If you run the wire between the poles of a strong horseshoe magnet, subjecting
of it to a
what force is exerted on this segment of wire?
79. (a) What is the angle between a wire carrying an
current and the
field it is in if
of the wire experiences a magnetic force of
? (b) What is the force on the wire if it is rotated to make an angle of
with the field?
80. A
long segment of wire lies along the
axis and carries a current of
in the positive
direction. Around the wire is the magnetic field of
Find the magnetic force on this segment.
81. A
section of a long, straight wire carries a current of
while in a uniform magnetic field of magnitude
Calculate the magnitude of the force on the section if the angle between the