Conceptual Physics

Paul G. Hewitt

Chapter 24

Magnetism - all with Video Answers

Educators

Chapter Questions

Problem 1

Many dry cereals are fortified with iron, which is added to the cereal in the form of small iron particles. How might these particles be separated from the cereal?

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Problem 2

In what sense are all magners electroma gnets?

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Problem 3

All atoms have moving electric charges. Why, then. aren't all materials magnetic?

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Problem 4

To make a compass, point an ordinary iron nail along the direction of the Earth's magnetic field (which, in the Northern Hemisphere, is angled downward as well as northward) and repeatedly strike it for a few seconds with a hammer or a rock. Then suspend it at its center of gravity by a string. Why does the act of striking magnetize the nail?

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Problem 5

If you place a chunk of iron near the north pole of a magnet, atbaction will occur. Why will attraction also occur if you place the same iron near the south pole of the magnet?

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Problem 6

Do the poles of a horseshoe magnet attract each other? If you bend the magnet so that the poles get closer together, what happens to the force between the poles?

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Problem 7

Why is it inadvisable to malke a horseshoe magnet from a flexble material?

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Problem 8

What land of force field surrounds a stationary electric cherge? What additional field surrounds it when it moves?

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Problem 9

Your study buddy claims that an electron always experiences a force in an electric field, but not always in a magnetic field. Do you agree? Why or. why not?

Averell Hause

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Problem 10

What is different about the magnetic poles of common refrigerator magnets and those of common bar magnets?

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Problem 11

A friend tells you that a refrigerator door, beneath its layer of white-painted plastic, is made of aluminum. How could you check to see if this is true (without any scraping)?

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Problem 12

Why will a magnet attract an ondinary nail or paper clip but not a wooden pencil?

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Problem 13

Will either pole of a magnet attract a paper clip? Explain what is happening inside the attracted paper clip. (Hint: Consider Figure 24.13.)

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Problem 14

Why aren't permanent magnets really permanent?

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Problem 15

One way to make a compass is to stick a magnetized needle into a piece of cork and ficat it in a glass bowl full of water. The needie will align itself with the horizontal component of the Earth's magnetic field. Since the
north pole of this compass is attracted northward, will the needle float toward the north side of the bowl? Defend your answer.

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Problem 16

A "dip needle" is a small magnet mounted on a horiaontal axis so that it can swivel up or down (like a compass tumed on its side). Where on Earth will a dip needle point most nearly vertically? Where will it point most nearly horizontally?

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Problem 17

In what direction would a compass needle point, if it were frec to point in all directions, when located at the Earth's north magnetic pole in Carada?

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Problem 18

What is the net magnetic force on a compass needle? By what mechanism does a compass needle align with a magnetic field?

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Problem 19

Since the iron filings that align with the magneric field of the bar magnet shown in Figure $24.2$ are not solely litule individual magnets, by whar mechanism do they align themselves with the field of the magnet?

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Problem 20

The north pole of a compass is atracted to the north magnetic pole of the Earth, yet like poles repel, Can you resolve this apparent dilemma?

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Problem 21

We know that a compass points northward because Earth is a giant magnet. Will the northward-pointing needle point northward when the compass is. brought to the Southern Hemisphere?

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Problem 22

Your friend says that, when a compass is taken across the equator, it turns around and points in the opposite direction. Your other friend says that this is not true, that people in the Southern Hemisphere use the south magnetic pole of the compass to point toward the nearest pole. You're on, what do you say?

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Problem 23

In what position can a current-camying loop of wire be located in a magnetic field so that it doesn't tend to rotate?

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Problem 24

Magnet $A$ has twice the magnetic field strength of magnet $B$ (at equal distance) and, at a certain digance, ic pulls on magriet $\mathrm{B}$ with a force of $50 \mathrm{~N}$. With how much force, then, does magnet $B$ pull on magnet A?

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Problem 25

In Figure 24.15 we see a magnet exerting a force on a current-carrying wire. Does a current-camying wire exert a force on a magnet? Why or why not?

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Problem 26

A strong magnet attracts a paper clip to itself with a certain force. Does the paper clip exert a force on the strong magnet? If not, why not? If so, does it exert as much force on the magnet as the magnet exerts on it? Defend your answers.

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Problem 27

A current-camying wire is in a north-south orientation. When a compass needle is placed below or above it, in what direction does the compass needle point?

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Problem 28

A loudspeaker consists of a cone attached to a current cairrying coil located in a magnctic ficld. What is the relationship between vibrations in the current and vibrations of the cone?

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Problem 29

Will a superconducting magnet use less clectric power than a traditional copper-wire electromagnet or the same amount of power? Defend your answer.

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Problem 30

When iron-hulled naval ships are built, the location of the shipyard and the orientation of the ship in the shipyard are recorded on a brass plaque permanently actached to the ship. Why?

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Problem 31

Can an electron at rest in a magnetic ficld be set into motion by the magnetic field? What if it were at rest in an electric field?

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Problem 32

A beam of electrons passes through a magnetic field without being deflected. What can you conclude about the orientation of the beam relative to the magnetic field? (Neglect any other fields.)

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Problem 33

A cyclotron is a device for accelerating charged particles to high speed as they follow an expanding spiral path. The charged particles are subjected to both an electric field and a magnetic field. One of these fields increases the speed of the charged particles, and the other field causes them to follow a curved path. Which field performs which function?

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Problem 34

A proton moves in a circular path perpendicular to a constant magnetic field. If the field strength of the magnet is increased, does the diameter of the circular path increase, decrease, or remain the same?

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Problem 35

A beam of high-energy protons emerges from a. cyclotron. Do you suppose that there is a magnetic field associated with these particles? Why or why not?

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Problem 36

A magnet can exert a force on a moving charged particle, but it cannot change the particle's lanetic energy. Why not?

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Problem 37

A magnetic field can deflect a beam of electrons, but it cannot do work on the electrons to change their speed. Why?

Averell Hause

Carnegie Mellon University

Problem 38

Two charged particles are projected into a magnetic field that is perpendicular to their velocities. If the particles are deflected in opposite directions, what does this tell you about them?

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Problem 39

Inside a laboratory room there is said to be either an electric field or a magnetic field, but not both. What experiments might be performed to establish what Iand of field is in the room?

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Problem 40

Why do astronauts keep to aluitudes beneath the Van Allen radiation belts when doing space walles?

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Problem 41

Residents of northem Canada are bombarded by more intense cosmic radiation than residents of Mexico. Why is this so?

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Problem 42

Whar changes in cosmic-ray intensity at the Earth's surface would you expect during periods in which the Earth's magnetic field passed through a zero phase While undergoing pole reversals?

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Problem 43

In a mass spectrometer (Figure $34.14)$, ions are directed into a magnetic field, where they curve and strike a detector. If a variety of singly ionized atoms travel at the same speed through the magnetic field, would you expect them all to be deflected by che same amount, or would different ions be bent different amounts? Defend your ansWer,

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Problem 44

One way to shield a habitat in outer space from cosmic rays is with an absorbing blanket of some kind, which would function much like the atmosphere that protects the Earth. Speculate on second way for shielding the habitar that would also be similar to the Earth's natural shielding

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Problem 45

If you had two bars of iron one magnetized and one unmagnetized - and no other matenals at hand, how could you detemine which bar was the magnet?

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Problem 46

Historically, replacing dirt roads with paved roads reduced friction on vehicles. Peplacing paved roads with steel rails reduced friction further. What recerit step reduces rail friction of vehicles? What friction remains after rail friction is eliminated?

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Problem 47

Will a pair of parallel current-carrying wires exert forces on each other?

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Problem 48

What is the magnetic effect of placing two wires with equal but oppositely directed currents close together or twisted about each other?

Averell Hause

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Problem 49

When a current is passed through a helically coiled spring, the spring contracts as if it's compressed. What's your explarution?

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Problem 50

When preparing to undergo an MRU scan, why are patients advised to remove eyeglasses, watches. jewelry, and other metal objects?

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