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Ferromagnets and Electromagnets
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OpenStax
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†
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1 Learning Objectives
By the end of this section, you will be able to:
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•
•
•
Dene ferromagnet.
Describe the role of magnetic domains in magnetization.
Explain the signicance of the Curie temperature.
Describe the relationship between electricity and magnetism.
The information presented in this section supports the following AP
tices:
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®
learning objectives and science prac-
2.D.3.1 The student is able to describe the orientation of a magnetic dipole placed in a magnetic eld
in general and the particular cases of a compass in the magnetic eld of the Earth and iron lings
(S.P. 1.2)
2.D.4.1 The student is able to use the representation of magnetic domains to qualitatively analyze the
magnetic behavior of a bar magnet composed of ferromagnetic material. (S.P. 1.4)
4.E.1.1 The student is able to use representations and models to qualitatively describe the magnetic
surrounding a bar magnet.
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properties of some materials that can be aected by magnetic properties of other objects in the system.
(S.P. 1.1, 1.4, 2.2)
2 Ferromagnets
Only certain materials, such as iron, cobalt, nickel, and gadolinium, exhibit strong magnetic eects. Such
materials are called
ferromagnetic,
after the Latin word for iron,
ferrum.
A group of materials made
from the alloys of the rare earth elements are also used as strong and permanent magnets; a popular one
is neodymium.
Other materials exhibit weak magnetic eects, which are detectable only with sensitive
instruments. Not only do ferromagnetic materials respond strongly to magnets (the way iron is attracted to
magnets), they can also be
magnetized themselvesthat is, they can be induced to be magnetic or made
into permanent magnets.
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Figure 1:
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An unmagnetized piece of iron is placed between two magnets, heated, and then cooled, or
simply tapped when cold. The iron becomes a permanent magnet with the poles aligned as shown: its
south pole is adjacent to the north pole of the original magnet, and its north pole is adjacent to the
south pole of the original magnet. Note that there are attractive forces between the magnets.
When a magnet is brought near a previously unmagnetized ferromagnetic material, it causes local magnetization of the material with unlike poles closest, as in Figure 1.
previously unmagnetized material to the magnet.)
Figure 2.
The regions within the material called
(This results in the attraction of the
What happens on a microscopic scale is illustrated in
domains
act like small bar magnets.
Within domains,
the poles of individual atoms are aligned. Each atom acts like a tiny bar magnet. Domains are small and
randomly oriented in an unmagnetized ferromagnetic object. In response to an external magnetic eld, the
domains may grow to millimeter size, aligning themselves as shown in Figure 2(b). This induced magnetization can be made permanent if the material is heated and then cooled, or simply tapped in the presence
of other magnets.
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Figure 2:
domains.
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(a) An unmagnetized piece of iron (or other ferromagnetic material) has randomly oriented
(b) When magnetized by an external eld, the domains show greater alignment, and some
grow at the expense of others. Individual atoms are aligned within domains; each atom acts like a tiny
bar magnet.
Conversely, a permanent magnet can be demagnetized by hard blows or by heating it in the absence of
another magnet. Increased thermal motion at higher temperature can disrupt and randomize the orientation
and the size of the domains. There is a well-dened temperature for ferromagnetic materials, which is called
the
K
Curie temperature, above which they cannot be magnetized.
(770
º
The Curie temperature for iron is 1043
C), which is well above room temperature. There are several elements and alloys that have Curie
temperatures much lower than room temperature and are ferromagnetic only below those temperatures.
3 Electromagnets
Early in the 19th century, it was discovered that electrical currents cause magnetic eects. The rst signicant
observation was by the Danish scientist Hans Christian Oersted (17771851), who found that a compass
needle was deected by a current-carrying wire. This was the rst signicant evidence that the movement
of charges had any connection with magnets.
Electromagnetism is the use of electric current to make
electromagnets. Electromagnets are employed
magnets. These temporarily induced magnets are called
for everything from a wrecking yard crane that lifts scrapped cars to controlling the beam of a 90-kmcircumference particle accelerator to the magnets in medical imaging machines (See Figure 3).
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Figure 3:
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Instrument for magnetic resonance imaging (MRI). The device uses a superconducting cylin-
drical coil for the main magnetic eld. The patient goes into this tunnel on the gurney. (credit: Bill
McChesney, Flickr)
Figure 4 shows that the response of iron lings to a current-carrying coil and to a permanent bar magnet.
The patterns are similar. In fact, electromagnets and ferromagnets have the same basic characteristicsfor
example, they have north and south poles that cannot be separated and for which like poles repel and unlike
poles attract.
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Figure 4:
Iron lings near (a) a current-carrying coil and (b) a magnet act like tiny compass needles,
showing the shape of their elds. Their response to a current-carrying coil and a permanent magnet is
seen to be very similar, especially near the ends of the coil and the magnet.
note:
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Figure 5:
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Compass needles and a bar magnet.
In Figure 5, a series of tiny compass needles are placed in an external magnetic eld. These dipoles
respond exactly like the iron lings in Figure 4. For needles close to one pole of the magnet, the
needle is aligned so that the opposite pole of the needle points at the bar magnet. For example,
close to the north pole of the bar magnet, the south pole of the compass needle is aligned to be
closest to the bar magnet.
As an experimenter moves each compass around, the needle will rotate in such a way as to orient
itself with the eld of the bar magnet at that location. In this way, the magnetic eld lines may be
mapped out precisely.
The strength of the magnetic eld depends on the medium in which the magnetic eld exists.
Some substances (like iron) respond to external magnetic elds in a way that amplies the external
magnetic eld. The magnetic permeability of a substance is a measure of the substance's ability to
support or amplify an already existing external magnetic eld. Ferromagnets, in which magnetic
domains in the substance align with and amplify an external magnetic eld, are examples of objects
with high permeability.
Combining a ferromagnet with an electromagnet can produce particularly strong magnetic eects.
(See
Figure 6.) Whenever strong magnetic eects are needed, such as lifting scrap metal, or in particle accelerators,
electromagnets are enhanced by ferromagnetic materials. Limits to how strong the magnets can be made are
imposed by coil resistance (it will overheat and melt at suciently high current), and so superconducting
magnets may be employed. These are still limited, because superconducting properties are destroyed by too
great a magnetic eld.
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Figure 6:
An electromagnet with a ferromagnetic core can produce very strong magnetic eects. Align-
ment of domains in the core produces a magnet, the poles of which are aligned with the electromagnet.
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Figure 7 shows a few uses of combinations of electromagnets and ferromagnets. Ferromagnetic materials
can act as memory devices, because the orientation of the magnetic elds of small domains can be reversed or
erased. Magnetic information storage on videotapes and computer hard drives are among the most common
applications. This property is vital in our digital world.
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Figure 7:
An electromagnet induces regions of permanent magnetism on a oppy disk coated with a
ferromagnetic material. The information stored here is digital (a region is either magnetic or not); in
other applications, it can be analog (with a varying strength), such as on audiotapes.
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4 Current: The Source of All Magnetism
An electromagnet creates magnetism with an electric current. In later sections we explore this more quantitatively, nding the strength and direction of magnetic elds created by various currents. But what about
ferromagnets? Figure 8 shows models of how electric currents create magnetism at the submicroscopic level.
(Note that we cannot directly observe the paths of individual electrons about atoms, and so a model or visual
image, consistent with all direct observations, is made. We can directly observe the electron's orbital angular
momentum, its spin momentum, and subsequent magnetic moments, all of which are explained with electriccurrent-creating subatomic magnetism.)
Currents, including those associated with other submicroscopic
particles like protons, allow us to explain ferromagnetism and all other magnetic eects. Ferromagnetism,
for example, results from an internal cooperative alignment of electron spins, possible in some materials but
not in others.
Crucial to the statement that electric current is the source of all magnetism is the fact that it is impossible
to separate north and south magnetic poles.
(This is far dierent from the case of positive and negative
charges, which are easily separated.) A current loop always produces a magnetic dipolethat is, a magnetic
eld that acts like a north pole and south pole pair. Since isolated north and south magnetic poles, called
magnetic monopoles,
are not observed, currents are used to explain all magnetic eects.
If magnetic
monopoles did exist, then we would have to modify this underlying connection that all magnetism is due
to electrical current. There is no known reason that magnetic monopoles should not existthey are simply
never observedand so searches at the subnuclear level continue. If they do
out why not. If they
:
do
not
exist, we would like to nd
exist, we would like to see evidence of them.
Electric current is the source of all magnetism.
Figure 8:
(a) In the planetary model of the atom, an electron orbits a nucleus, forming a closed-current
loop and producing a magnetic eld with a north pole and a south pole. (b) Electrons have spin and can
be crudely pictured as rotating charge, forming a current that produces a magnetic eld with a north
pole and a south pole. Neither the planetary model nor the image of a spinning electron is completely
consistent with modern physics. However, they do provide a useful way of understanding phenomena.
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Explore the interactions between a compass and bar magnet. Discover how you can use a battery
and wire to make a magnet! Can you make it a stronger magnet? Can you make the magnetic eld
reverse?
Figure 9:
Magnets and Electromagnets
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5 Test Prep for AP Courses
Exercise 1
(Solution on p. 15.)
A bar magnet is oriented so that the north pole of the bar magnet points north. A compass needle
is placed to the north of the bar magnet. In which direction does the north pole of the compass
needle point?
(a) North
(b) East
(c) South
(d) West
Exercise 2
Assume for simplicity that the Earth's magnetic north pole is at the same location as its geographic
north pole.
meridian (0
If you are in an airplane ying due west along the equator, as you cross the prime
◦
longitude) facing west and look down at a compass you are carrying, you see that
the compass needle is perpendicular to your direction of motion, and the north pole of the needle
dipole points to your right. As you continue ying due west, describe how and why the orientation
of the needle will (or will not) change.
Exercise 3
(Solution on p. 15.)
Describe how the magnetic domains in an unmagnetized iron rod will respond to the presence of
a strong external magnetic eld.
(a) The domains will split into monopoles.
(b) The domains will tend to align with the external eld.
(c) The domains will tend to orient themselves perpendicular to the external eld.
(d) The domains will tend to align so as to cancel out the external eld.
Exercise 4
Describe what steps must be undertaken in order to convert an unmagnetized iron rod into a
permanently magnetized state. As part of your answer, explain what a magnetic domain is and
how it responds to the steps described.
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Exercise 5
(Solution on p. 15.)
Iron is ferromagnetic and lead is diamagnetic, which means its magnetic domains respond in the
opposite direction of ferromagnets but many orders of magnitude more weakly.
The two blocks
are placed in a magnetic eld that points to the right. Which of the following best represents the
orientations of the dipoles when the eld is present?
Iron
(a)
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(b)
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(c)
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(d)
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Table 1
Exercise 6
A weather vane is some sort of directional arrow parallel to the ground that may rotate freely in
a horizontal plane.
A typical weather vane has a large cross-sectional area perpendicular to the
direction the arrow is pointing, like a One Way street sign. The purpose of the weather vane is
to indicate the direction of the wind. As wind blows past the weather vane, it tends to orient the
arrow in the same direction as the wind.
Consider a weather vane's response to a strong wind.
Explain how this is both similar to and dierent from a magnetic domain's response to an external
magnetic eld. How does each aect its surroundings?
6 Section Summary
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Magnetic poles always occur in pairs of north and southit is not possible to isolate north and south
poles.
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All magnetism is created by electric current.
Ferromagnetic materials, such as iron, are those that exhibit strong magnetic eects.
The atoms in ferromagnetic materials act like small magnets (due to currents within the atoms) and
can be aligned, usually in millimeter-sized regions called domains.
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Domains can grow and align on a larger scale, producing permanent magnets.
magnetized, or induced to be magnetic.
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Such a material is
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Above a material's Curie temperature, thermal agitation destroys the alignment of atoms, and ferromagnetism disappears.
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Electromagnets employ electric currents to make magnetic elds, often aided by induced elds in
ferromagnetic materials.
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Solutions to Exercises in this Module
Solution to Exercise (p. 11)
(a)
Solution to Exercise (p. 11)
(b)
Solution to Exercise (p. 12)
(b)
Glossary
Denition 1: ferromagnetic
materials, such as iron, cobalt, nickel, and gadolinium, that exhibit strong magnetic eects
Denition 2: magnetized
to be turned into a magnet; to be induced to be magnetic
Denition 3: domains
regions within a material that behave like small bar magnets
Denition 4: Curie temperature
the temperature above which a ferromagnetic material cannot be magnetized
Denition 5: electromagnetism
the use of electrical currents to induce magnetism
Denition 6: electromagnet
an object that is temporarily magnetic when an electrical current is passed through it
Denition 7: magnetic monopoles
an isolated magnetic pole; a south pole without a north pole, or vice versa (no magnetic monopole
has ever been observed)
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