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CHAPTER 36
MAGNETISM
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REVIEW
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• TeacherEXPRESS
• Virtual Physics Lab 33
36 REVIEW
Concept Summary
••••••
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think! Answers
36.1 Yes, just as every coin has two sides, a “head”
     and a “tail.” (Some “trick” magnets have more
     than two poles.)
36.4 Domains align in the individual filings, caus-
     ing them to act like tiny compasses. The poles
     of each “compass” are pulled in opposite
     directions, producing a torque that twists each
     filing into alignment with the external mag-
     netic field.
36.7 Newton’s third law, which applies to all forces
     in nature.
36.8 A galvanometer and a motor are similar in
     that they both employ coils positioned in
     magnetic fields. When current passes through
     the coils, forces on the wires rotate the coils.
     The fundamental difference is that the maxi-
     mum rotation of the coil in a galvanometer
     is one half turn, whereas in a motor the coil
     (armature) rotates through many complete
     turns. In the armature of a motor, the current
     is made to change direction with each half
     turn of the armature.
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Like poles repel; opposite poles attract.
The direction of the magnetic field
outside a magnet is from the north to
the south pole.
A magnetic field is produced by the
motion of electric charge.
Permanent magnets are made by simply
placing pieces of iron or certain iron al-
loys in strong magnetic fields.
An electric current produces a magnetic
field.
A moving charge is deflected when it
crosses magnetic field lines but not when
it travels parallel to the field lines.
Since a charged particle moving through
a magnetic field experiences a deflecting
force, a current of charged particles mov-
ing through a magnetic field also experi-
ences a deflecting force.
The principal difference between a gal-
vanometer and an electric motor is that
in an electric motor, the current is made
to change direction every time the coil
makes a half revolution.
A compass points northward because
Earth itself is a huge magnet.
••••••
Key Terms
magnetic pole
 (p. 721)
magnetic field
 (p. 722)
magnetic domain
   (p. 724)
electromagnet
   (p. 726)
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ASSESS
36 ASSESS
Check Concepts
Section 36.1
••••••
Check Concepts
9. Why will heating or dropping a magnet on a
   hard surface weaken it?
Section 36.5
1. What do electric charges have in common
   with magnetic poles?
2. What is a major difference between electric
   charges and magnetic poles?
Section 36.2
10. What is the shape of the magnetic field that
    surrounds a current-carrying wire?
11. If a current-carrying wire is bent into a
    loop, why is the magnetic field stronger
    inside the loop than outside?
Section 36.6
3. What is a magnetic field, and what is its
   source?
Section 36.3
12. What must a charged particle be doing in
    order to experience a magnetic force?
13. With respect to an electric and a magnetic
    field, how does the direction of a magnetic
    force on a charged particle differ from the
    direction of the electric force?
14. What role does Earth’s magnetic field play
    in cosmic ray bombardment?
Section 36.7
4. Every spinning electron is a tiny magnet.
   Since all atoms have spinning electrons, why
   are not all atoms tiny magnets?
5. What is so special about iron that makes
   each iron atom a tiny magnet?
Section 36.4
15. How does the direction in which a current-
    carrying wire is forced when in a magnetic
    field compare with the direction that mov-
    ing charges are forced?
Section 36.8
6. What is a magnetic domain?
7. Why do some pieces of iron behave as mag-
   nets, while other pieces of iron do not?
8. How can a piece of iron be induced into
   becoming a magnet? For example, if you
   place a paper clip near a magnet, it will itself
   become a magnet. Why?
16. How do the concepts of force, field, and
    current relate to a galvanometer?
CHAPTER 36
MAGNETISM
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36 ASSESS
Section 36.9
(continued)
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Think and Rank
17. Why is it important that the current in the
    armature of a motor that uses a permanent
    magnet periodically change direction?
18. What is meant by magnetic declination?
19. According to most geophysicists, what is the
    probable cause of Earth’s magnetic field?
20. What are magnetic pole reversals, and what
    evidence is there that Earth’s magnetic field
    has undergone pole reversals throughout its
    history?
21. The electromagnets A, B, and C are shown
    below. Note the number of turns of wire
    and the current in each. The iron cores are
    identical. Rank the electromagnets in mag-
    netic field intensity at point P (just outside
    each electromagnet), from strongest to
    weakest.
A
2 amp
P
B
4 amp
P
C
4 amp
P
Think and Rank
•••••
22. Three pairs of electromagnets made with
    identical iron cores are shown below. Note
    the number of turns of wire, each of which
    carries the same current.
Suppose you wrap some wire into a coil around
an iron bar. When the coil carries a current,
you have an electromagnet. The polarity of the
electromagnet can be found using your right
hand: When you curl your right-hand fingers
in the direction of the current, your thumb
points to the north pole. (The opposite end of the
electromagnet is the south pole.)
S
N
A
B
C
a. Rank the electromagnet pairs A, B, and C
   in terms of the force between the electro-
   magnets, from most attractive to most
   repulsive.
b. Rank the electromagnet pairs A, B, and C
   in terms of the magnitude of the mag-
   netic field at the exact midpoint between
   the electromagnets.
Rank each of the following sets of scenarios in
order of the quantity or property involved. List
them from left to right. If scenarios have equal
rankings, separate them with an equal sign.
(e.g., A B)
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Think and Explain
•••••
23. Since every iron atom is a tiny magnet, why
    aren’t all iron materials themselves magnets?
24. “An electron always experiences a force in
    an electric field, but not always in a mag-
    netic field.” Defend this statement.
25. What kind of field surrounds a stationary
    electric charge? A moving electric charge?
26. Why can iron be made to behave as a mag-
    net while wood cannot?
27. If you place a chunk of iron near the north
    pole of a magnet, attraction will occur. Why
    will attraction also occur if you place the
    iron near the south pole of the magnet?
28. A friend tells you that a refrigerator door,
    beneath a layer of white-painted plastic, is
    made of aluminum. How could you check
    to see if this is true (without any scraping)?
29. Since iron filings are not themselves mag-
    nets, by what mechanism do they align
    themselves with a magnetic field as shown
    in Figure 36.6?
30. We know a compass points northward
    because Earth is a giant magnet. Will the
    northward-pointing needle point northward
    when the compass is brought to the South-
    ern Hemisphere?
31. A strong magnet and a weak magnet attract
    each other. Which magnet exerts the stron-
    ger force—the strong one or the weak one?
    (Could you have answered this using what
    you learned in Chapter 6?)
32. 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.
33. Why will the magnetic field strength be fur-
    ther increased inside a current-carrying coil
    if a piece of iron is placed in the coil?
34. Can an electron be set into motion with a
    magnetic field? With an electric field?
    Explain.
35. In what direction relative to a magnetic
    field does a charged particle travel in order
    to experience maximum magnetic force?
    Minimum magnetic force?
36. When a current-carrying wire is placed in a
    strong magnetic field, no force acts on the
    wire. What orientation of the wire is likely?
37. A magnetic field can deflect a beam of
    electrons, but it cannot do work on them to
    speed them up. Why? (Hint: Consider the
    direction of the force relative to the direc-
    tion in which the electrons move.)
CHAPTER 36
MAGNETISM
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36. 
36 ASSESS
(continued)
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38. A cyclotron is a device for accelerating
    charged particles to high speeds in circu-
    lar orbits of ever-increasing radius. The
    charged particles are subjected to both an
    electric field and a magnetic field. One of
    these fields increases the speed of the par-
    ticles, and the other field holds them in a
    circular path. Which field performs which
    function?
39. By present-day custom, the direction of
    electric current is the direction in which
    positive charge flows. The direction of a
    magnetic field surrounding a current-
    carrying wire can be shown by what is called
    the “right-hand rule.” Wrap your right-hand
    fingers around a current-carrying wire in
    such a way that your thumb points in the
    direction of the electric current; then your
    fingers point in the direction of the mag-
    netic field. Suppose instead that the current
    direction were defined as the direction in
    which electrons move in the wire. Why then
    would a “left-hand rule” be used for identi-
    fying magnetic field direction?
40. Pigeons have multiple-domain magnetite
    magnets within their skulls that are con-
    nected through a large number of nerves
    to the pigeon’s brain. How does this aid the
    pigeon in navigation? (Magnetic material
    also exists in the abdomens of bees.)
41. How do the input and output parts of a
    generator and a motor compare?
42. Your friend says that if you crank the shaft
    of a DC motor manually, the motor be-
    comes a DC generator. Do you agree or
    disagree? Defend your position.
43. Residents of northern Canada are bombard-
    ed by more intense cosmic radiation than
    are residents of Mexico. Why is this so?
Current
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Activities
44. What changes in cosmic ray intensity at
    Earth’s surface would you expect during pe-
    riods in which Earth’s magnetic field passed
    through a zero phase while undergoing pole
    reversals? (A speculation supported by fossil
    evidence is that the periods of no protective
    magnetic field may have been as effective in
    changing life forms as X-rays have been in
    the famous heredity studies of fruit flies.)
45. Earth’s core is probably composed of iron
    and nickel, excellent metals for making
    permanent magnets. Why is it unlikely that
    Earth’s core is a permanent magnet?
47. Find the direction and dip of Earth’s mag-
    netic field lines in your locality. Magnetize
    a large steel needle or straight piece of steel
    wire by stroking it a few dozen times with
    a strong magnet. Run the needle or wire
    through a cork in such a way that when the
    cork floats, the thin magnet remains hori-
    zontal (parallel to the water surface). Float
    the cork in a plastic or wooden container
    of water. The needle will point toward the
    magnetic pole. Then press unmagnetized
    common pins into the sides of the cork per-
    pendicular to the needle. Rest the pins on
    the rims of a pair of drinking glasses so that
    the needle or wire points toward the mag-
    netic pole. It should dip in line with Earth’s
    magnetic field.
Activities
••••••
46. An iron bar can be easily magnetized by
    aligning it with the magnetic field lines of
    Earth and striking it lightly a few times with
    a hammer. This works best if the bar is tilted
    down to match the dip of Earth’s field. The
    hammering jostles the domains so they can
    better fall into alignment with Earth’s field.
    The bar can be demagnetized by striking it
    when it is in an east–west direction.
CHAPTER 36
MAGNETISM
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