Holt Physics

Raymond A. Serway, Jerry S. Faughn

Chapter 19

Magnetism - all with Video Answers

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Chapter Questions

Problem 1

What is the minimum number of poles for a magnet?

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Problem 2

When you break a magnet in half, how many poles does each piece have?

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Jheremiah S.

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Problem 3

The north pole of a magnet is attracted to the geographic North Pole of Earth, yet like poles repel. Can you explain this?

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Problem 4

Which way would a compass needle point if you were at the magnetic north pole?

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Problem 5

What is a magnetic domain?

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Problem 6

Why are iron atoms so strongly affected by magnetic fields?

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Problem 7

When a magnetized steel needle is strongly heated in a Bunsen burner flame, it becomes demagnetized. Explain why.

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Problem 8

If an unmagnetized piece of iron is attracted to one pole of a magnet, will it be repelled by the opposite pole?

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Problem 9

Conceptual Questions
In the figure below, two permanent magnets with holes bored through their centers are placed one over the other. Because the poles of the upper magnet are the reverse of those of the lower, the upper magnet levitates above the lower magnet. If the upper magnet were displaced slightly, either up or down, what would be the resulting motion? Explain. What would happen if the upper magnet were inverted?

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Problem 10

Conceptual Questions
You have two iron bars and a ball of string in your possession; one iron bar is magnetized, and one iron bar is not. How can you determine which iron bar is magnetized?

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Problem 11

Conceptual Questions
Why does a very strong magnet attract both poles of a weak magnet?

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Problem 12

Conceptual Questions
A magnet attracts a piece of iron. The iron can then attract another piece of iron. Explain, on the basis of alignment of domains, what happens in each piece of iron.

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Problem 13

Conceptual Questions
When a small magnet is repeatedly dropped, it becomes demagnetized. Explain what happens to the magnet at the atomic level.

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Problem 14

Review Questions
A conductor carrying a current is arranged so that electrons flow in one segment from east to west. If a compass is held over this segment of the wire, in what direction is the needle deflected? (Hint: Recall that current is defined as the motion of positive charges.)

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Problem 15

Review Questions
What factors does the strength of the magnetic field of a solenoid depend on?

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Problem 16

Conceptual Questions
A solenoid with ends marked $A$ and $B$ is suspended by a thread so that the core can rotate in the horizontal plane. A current is maintained in the coil so that the electrons move clockwise when viewed from end $A$ toward end $B .$ How will the coil align itself in Earth's magnetic field?

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Problem 17

Conceptual Questions
Is it possible to orient a current-carrying loop of wire in a uniform magnetic field so that the loop will not tend to rotate?

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Problem 18

Conceptual Questions
If a solenoid were suspended by a string so that it could rotate freely, could it be used as a compass when it carried a direct current? Could it also be used if the current were alternating in direction?

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Problem 19

Review Questions
Two charged particles are projected into a region where there is a magnetic field perpendicular to their velocities. If the particles are deflected in opposite directions, what can you say about them?

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Problem 20

Review Questions
Suppose an electron is chasing a proton up this page when suddenly a magnetic field pointing into the page is applied. What would happen to the particles?

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Problem 21

Review Questions
Why does the picture on a television screen become distorted when a magnet is brought near the screen?

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Problem 22

Review Questions
A proton moving horizontally enters a region where there is a uniform magnetic field perpendicular to the proton's velocity, as shown below. Describe the proton's subsequent motion. How would an electron behave under the same circumstances?

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Problem 23

Review Questions
Explain why two parallel wires carrying currents in opposite directions repel each other.

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Problem 24

Review Questions
Can a stationary magnetic field set a resting electron in motion? Explain.

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Problem 25

At a given instant, a proton moves in the positive $x$ direction in a region where there is a magnetic field in the negative $z$ direction. What is the direction of the magnetic force? Does the proton continue to move along the $x$ -axis? Explain.

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Problem 26

For each situation below, use the movement of the positively charged particle and the direction of the magnetic force acting on it to find the direction of the magnetic field.
(FIGURE CANNOT COPY)

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Problem 27

Conceptual Questions
A stream of electrons is projected horizontally to the right. A straight conductor carrying a current is supported parallel to and above the electron stream.
a. What is the effect on the electron stream if the current in the conductor is left to right?
b. What is the effect if the current is reversed?

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Problem 28

Conceptual Questions
If the conductor in item 27 is replaced by a magnet with a downward magnetic field, what is the effect on the electron stream?

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Problem 29

Conceptual Questions
Two wires carrying equal but opposite currents are twisted together in the construction of a circuit. Why does this technique reduce stray magnetic fields?

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Problem 30

see Sample Problem $A$.
A duck flying due east passes over Atlanta, where the magnetic field of Earth is $5.0 \times 10^{-5}$ T directed north. The duck has a positive charge of $4.0 \times 10^{-8} \mathrm{C} .$ If the magnetic force acting on the duck is $3.0 \times 10^{-11} \mathrm{N}$ upward, what is the duck's velocity?

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Problem 31

see Sample Problem $A$.
A proton moves eastward in the plane of Earth's magnetic equator, where Earth's magnetic field points north and has a magnitude of $5.0 \times 10^{-5}$ T. What velocity must the proton have for the magnetic force to just cancel the gravitational force?

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Problem 32

See Sample Problem $B$.
A wire carries a 10.0 A current at an angle $90.0^{\circ}$ from the direction of a magnetic field. If the magnitude of the magnetic force on a 5.00 m length of the wire is $15.0 \mathrm{N},$ what is the strength of the magnetic field?

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Problem 33

See Sample Problem $B$.
A thin $1.00 \mathrm{m}$ long copper rod in a uniform magnetic field has a mass of $50.0 \mathrm{g} .$ When the rod carries a current of $0.245 \mathrm{A},$ it floats in the magnetic field. What is the field strength of the magnetic field?

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Problem 34

MIXED REVIEW
A proton moves at $2.50 \times 10^{6} \mathrm{m} / \mathrm{s}$ horizontally at a right angle to a magnetic field.
a. What is the strength of the magnetic field required to exactly balance the weight of the proton and keep it moving horizontally?
b. Should the direction of the magnetic field be in a horizontal or a vertical plane?

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Problem 35

MIXED REVIEW
Find the direction of the force on a proton moving through each magnetic field in the four figures below.
(FIGURE CANNOT COPY)

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Problem 36

MIXED REVIEW
Find the direction of the force on an electron moving through each magnetic field in the four figures in item 35 above.

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Problem 37

MIXED REVIEW
In the four figures in item $35,$ assume that in each case the velocity vector shown is replaced with a wire carrying a current in the direction of the velocity vector. Find the direction of the magnetic force acting on each wire.

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Problem 38

MIXED REVIEW
A proton moves at a speed of $2.0 \times 10^{7} \mathrm{m} / \mathrm{s}$ at right angles to a magnetic field with a magnitude of $0.10 \mathrm{T}$. Find the magnitude of the acceleration of the proton.

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Problem 39

A proton moves perpendicularly to a uniform magnetic field, $\mathbf{B}$, with a speed of $1.0 \times 10^{7} \mathrm{m} / \mathrm{s}$ and experiences an acceleration of $2.0 \times 10^{13} \mathrm{m} / \mathrm{s}^{2}$ in the positive $x$ direction when its velocity is in the positive $z$ direction. Determine the magnitude and direction of the field.

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Problem 40

A proton travels with a speed of $3.0 \times 10^{6} \mathrm{m} / \mathrm{s}$ at an angle of $37^{\circ}$ west of north. A magnetic field of 0.30 T points to the north. Determine the following:
a. the magnitude of the magnetic force on the proton.
b. the direction of the magnetic force on the proton.
c. the proton's acceleration as it moves through the magnetic field.
(Hint: The magnetic force experienced by the proton in the magnetic field is proportional to the component of the proton's velocity that is perpendicular to the magnetic field.)

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Jheremiah S.

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Problem 41

In the figure below, a $15 \mathrm{cm}$ length of conducting wire that is free to move is held in place between two thin conducting wires. All the wires are in a magnetic field. When a 5.0 A current is in the wire, as shown in the figure, the wire segment moves upward at a constant velocity. Assuming the wire slides without friction on the two vertical conductors and has a mass of $0.15 \mathrm{kg},$ find the magnitude and direction of the minimum magnetic field that is required to move the wire.

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Problem 42

A current, $I=15 \mathrm{A},$ is directed along the positive $x$ -axis and perpendicular to a uniform magnetic field. The conductor experiences a magnetic force per unit length of $0.12 \mathrm{N} / \mathrm{m}$ in the negative $y$ direction. Calculate the magnitude and direction of the magnetic field in the region through which the current passes.

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Problem 43

A proton moving perpendicular to a magnetic field of strength 3.5 mT experiences a force due to the field of $4.5 \times 10^{-21} \mathrm{N} .$ Calculate the following:
a. the speed of the proton
b. the kinetic energy of the proton Recall that a proton has a charge of $1.60 \times 10^{-19} \mathrm{C}$ and a mass of $1.67 \times 10^{-27} \mathrm{kg}.$

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Problem 44

MIXED REVIEW
A singly charged positive ion that has a mass of $6.68 \times 10^{-27} \mathrm{kg}$ moves clockwise with a speed of $1.00 \times 10^{4} \mathrm{m} / \mathrm{s} .$ The positively charged ion moves in a circular path that has a radius of $3.00 \mathrm{cm} .$ Find the direction and strength of the uniform magnetic field through which the charge is moving. (Hint:
The magnetic force exerted on the positive ion is the centripetal force, and the speed given for the positive ion is its tangential speed.)

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Problem 45

MIXED REVIEW
What speed would a proton need to achieve in order to circle Earth $1000.0 \mathrm{km}$ above the magnetic equator? Assume that Earth's magnetic field is everywhere perpendicular to the path of the proton and that Earth's magnetic field has an intensity of $4.00 \times 10^{-8} \mathrm{T}$. (Hint: The magnetic force exerted on the proton is equal to the centripetal force, and the speed needed by the proton is its tangential speed. Remember that the radius of the circular orbit should also include the radius of Earth. Ignore relativistic effects.)

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Problem 46

MIXED REVIEW
Calculate the force on an electron in each of the following situations:
a. moving at 2.0 percent the speed of light and perpendicular to a 3.0 T magnetic field.
b. $3.0 \times 10^{-6} \mathrm{m}$ from a proton.
c. in Earth's gravitational field at the surface of Earth Use the following: $q_{e}=-1.6 \times 10^{-19} \mathrm{C} ; m_{e}=9.1 \times$
$10^{-31} \mathrm{kg} ; q_{p}=1.6 \times 10^{-19} \mathrm{C} ; c=3.0 \times 10^{8} \mathrm{m} / \mathrm{s} ; k_{C}=$
$9.0 \times 10^{9} \mathrm{N} \cdot \mathrm{m}^{2} / \mathrm{C}^{2}$

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