Download Magnetic Effects

Survey
yes no Was this document useful for you?
   Thank you for your participation!

* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project

Document related concepts

Mathematics of radio engineering wikipedia , lookup

Giant magnetoresistance wikipedia , lookup

Superconductivity wikipedia , lookup

Magnetic core wikipedia , lookup

Galvanometer wikipedia , lookup

Transcript
1-
r
Magnetic
Effects
1-
f
1
magnetism (MAG-ne-tizm)
n.: the property
associated with clmrged objects in motion, which
give rise to afield of force.
MAGNETISM
19.1 Magnetic
Materials
Physicists believe that all
magnetic phenomena result from forces between electric
charges in motion. Vast quantities of electric energy are
now generated as a consequence of relative motion between electric conductors and magnetic fields. Electric
energy is transformed into mechanical energy by relative
motion between electric currents and magnetic fields. The
function of many electric measuring instruments depends
on the relationship between electricity and magnetism.
The basic theory of electric generators and motors is presented in Chapter 20. Electric measuring instruments are
discussed later in this chapter. Before undertaking
the
study of magnetic effects of electric currents, we shall examine the magnetic properties of substances and learn of
the nature of magnetism and magnetic fields.
Deposits of a magnetic iron are were discovered many
centuries ago by the Greeks in a section of Turkey. The
region was then known as Magnesia and the are was
called magnetite. Deposits of magnetite are found in the
Adirondack Mountains of New York and in other regions
of the world. Pieces of magnetite are known as natural
magnets. A suspended piece of magnetite aligns itself with
the magnetic field of the earth. These natural magnets,
known as lodestones (leading stones), were first used as
magnetic compasses during the twelfth century.
A few materials, notably iron and steel, are strongly attracted by magnets; cobalt and nickel are attracted to a
o
BJECTIVE~
. Discuss the domain theory of
magnetism.
. Describe the nature of the
magnetic force.
. Describe a magnetic monopole.
Discuss the techniques for
mapping magnetic fields.
. Study magnetic induction.
. Describe the earth's magnetic
field.
. Define the link between moving
charges and magnetic fields of
force.
.
.
Study the magnetic
field
produced by the current in a
straight wire and a solenoid.
. Describe electric meters and
their use in d-c circuits.
465
466
The Latin word for iron is "ferrum"; thus the name "ferromagnetic."
Alnico (AI Ni Co) consists mainly
of aluminum, nickel, and cobalt
plus iron,
.
Figure 19-1. Iron filings attracted
to a lodestone give evidence of
the field of force surrounding the
natural magnet.
The orbiting eIeciron produces 1/
magnetic field because, in this
motion, it constitutes an electric
current.
CHAPTER
19
lesser degree. These substances are said to have ferromagnetic properties. Special alloys such as permalloy and alnico
have extraordinary
ferromagnetic
properties.
Physicists
have shown much interest in the structure of materials
possessing the property of ferromagnetism.
Today very strong magnets are made from ferromagnetic substances. Alnico magnets may support a weight of
over 1000 times that of the magnets themselves. Ferromagnetic substances are commonly referred to simply as
"magnetic substances."
Materials are commonly classified as magnetic or nonmagnetic. Those that do not demonstrate the strong ferromagnetism of the Iron Family of metals are said to be
"nonmagnetic."
However, if these materials are placed in
the field of a very strong magnet, some are observed to be
slightly repelled by the magnet while others are very
slightly attracted.
Zinc, bismuth, sodium chloride, gold, and mercury are
a few of the substances that are feebly repelled; they are
diamagnetic. The property of diamagnetism is an important
concept in the modern theory of magnetism, as we shall
see in Section 19.2.
Wood, aluminum,
platinum, oxygen, and copper(IJ)
sulfate are examples of substances that are very slightly
attracted by a strong magnet. Such materials are paramagnetic, and this magnetic behavior is called paramagnetism.
19.2 The Domain Theory of Magnetism
William Gilbert's report on his experiments with natural magnets,
published in 1600, probably represents the first scientific
study of magnetism. In the years that followed, discoveries by Coulomb, Oersted, and Ampere added to our
knowledge of the behavior of magnets and the nature of
magnetic forces. Physicists believe, however, that it is
only within this century that they have begun to understand the true nature of magnetism. The present view is
that the magnetic properties of matter are electric in origin
and result from the movements of electrons within the
atoms of substances. Since the electron is an electrically
charged particle, this theory suggests that magnetism is a
property of a charge in motion. If so, we can account for the
energy associated with magnetic forces by using known
laws of physics.
Two kinds of electron motion are important in this modern concept of magnetism. First, an electron revolvingabout
the nucleus of an atom imparts a magnetic property to the atom
structure. See Figure 19-2. When the atoms of a substance
are subjected to the magnetic force of a strong magnet, the
r'
MAGNETIC
467
EFFECTS
force affects this magnetic property, opposing the motion
of the electrons. The atoms are thus repelled by the magnet. This is diamagnetism.
If the electron's only motion
were its movement about the nucleus, all substances
would be diamagnetic. Diamagnetic repulsion is quite feeble in its action on the total mass of a substance.
The second kind of motion is that of the electron spinning on its
own axis. Each spinning electron acts asa tiny permanentmagnet. Opposite spins are designated as + and - spins; electrons spinning in opposite directions tend to form pairs
and so neutralize their magnetic character. See Figure
19-3. The magnetic character of an atom as a whole may be
weak because of the mutual interaction between the electron spins.
Figure 19-2. Revolving electrons
impart a magnetic property to the
atom.
,
,
Figure 19-3. Ferromagnetism in
matter from the spin of electrons.
Magnetic properties are associated with both kinds of
electron motion. The atoms of some substances may possess permanent magnet characteristics because of an imbalance between orbits and spins. These atoms act like
tiny magnets, called dipoles, and are attracted by strong
magnets. Substances in which this attractive effect exceeds
the diamagnetism common to an atoms show the property
of paramagnetism.
In the atoms of ferromagnetic substances there are unpaired electrons whose spins are oriented in the same
way. The common metals iron, cobalt, and nickel and the
rare earth elements gadolinium and dysprosium
show
strong ferromagnetic properties. Some alloys of these and
other elements, as well as certain metallic oxides called
ferrites, also exhibit strong ferromagnetic properties.
The inner quantum levels, or shells, of the atom structures of most elements contain only paired electrons. The
highest quantum level, or outer shell, of each of the noble
gases (except helium) consists of a stable octet of electrons
made up of four electron pairs. The atoms of other elements achieve this stable configuration by forming chemical bonds. Only in certain transition elements that have
Each iron atom lias four
inner-shell
electrons.
unpaired
468
CHAPTER
19
incomplete inner shells do unpaired electrons result in ferromagnetic properties. The electron configuration of the
iron atom, Figure 19-4, shows four unpaired electrons in
the third principal quantum level. The similarly oriented
spins of these electrons, enhanced by the influence of
nearby
atoms in the metallic crystal, account for iron's
26eC'ft'lnSwit\
strong
ferromagnetism.
.
'
From the preceding discussion, it would seem that
(
ectrpnswill
-sJ)!n
every piece of iron should behave as a magnet. However,
such is not the case. Atoms are grouped in microscopic
magnetic regions called domains. The atoms in each domain are magnetically polarized parallel to a crystal axis.
In a polycrystalJine specimen, ordinarily these axes (and
the domains) are oriented in all possible directions. The
Figure 19.4. The iron atom has
domains effectively cancel one another and the net magstrong ferromagnetic properties.
netism is essentially zero. In Figure 19-5 the polarity of
each domain in an unmagnetized
material is represented
by an arrow.
When a ferromagnetic material is placed in an external
magnetic field, two effects occur. The domains more favorably oriented in this magnetic field may increase in size
at the expense of less favorably oriented adjacent domains. Other domains may rotate in order to become more
favorably oriented with respect to the external field. The
material becomes magnetized. If the domain boundaries
remain extended to some degree even after the external
magnetizing force is removed, the material is said to be
"permanently"
magnetized. When the direction of magnetization of a magnetic domain is rotated by an external
magnetic field, it must be understood that the material of
Figure 19-5. The domains of an
un magnetized ferromagnetic subthe domain does not change its position in the specimen.
stance are polarized along the
It is only its direction of magnetization
that changes.
crystal axes. Dots and plus signs
When the temperature
of a ferromagnetic material is
represent arrows going out of and
raised above a certain critical value, the domain regions
into the page, respectively.
disappear and the material becomes paramagnetic. This
temperature is known as the Curie point. It is usually lower
than the melting point of the substance. The Curie points
for some ferromagnetic substances are given in Table 19-1.
When a single crystal of iron is sprinkled with colloidal
Table 19-1
particles of iron oxide, the microscopic domains become
CURIE POINTS OF
FERROMAGNETIC
ELEMENTS
visible. Using this technique, physicists are able to photograph magnetic domains and observe the effects of exterElement
nal magnetic fields on them. Typical photomicrographs
of
magnetic
domains
are
shown
in
Figure
19-6.
iron
no°c
cobalt
A recent magnet technology that makes use of a group
1131°C
nickel
358°C
of ferromagnetic substances known as terrifes yields strong
16QC
gadolinium
hard magnets with unique properties. Ferrites are iron oxides combined with oxides of other metals such as manga-
o
\~
0
1-
MAGNETIC EFFECTS
469
Figure 19-6. Photomicrographs of
magnetic domains.
nese, cobalt, nickel, copper, or magnesium. The combined
oxides are powdered,
formed into the desired shape
under pressure, and fired. The ferrites have very high
electric resistance, a property that is extremely important
in some applications of ferromagnetic materials. The originallodestone,
commonly called magnetic iron oxide, is a
material of this type. Chemically it is a combination of
iron(II) oxide, FeO, and iron(IlI) oxide, FeZ03' Its formula
is considered to be Fe(FeOzh.
like poles repel
19.3 Force Between Magnet Poles
The fact that iron
filings cling mainly to the ends of a bar magnet indicates
that the magnetic force acts on the filings primarily in
these regions, or poles; it does not mean that the middle
r'
region of the magnet is unmagnetized.
The pole that
points toward the north when the magnet is free to swing
about a vertical axis is commonly called the north-seeking
pole, or N pole. The opposite pole, which points toward
the south, is called the south-seeking pole, or 5 pole.
Suppose a bar magnet is suspended as shown in Figure
19-7. When the N pole of a second magnet is brought near
the N pole of the suspended magnet, the two repel each
other. A similar action is observed with the two S poles.
When the S pole of one magnet is placed near the N pole
of the other magnet, they attract each other. Such experiments show that like poles repel and unlike poles attract.
Magnets usually have two well-defined poles-one
N
and one S. Sometimes long bar magnets acquire more
than two poles, and an iron ring may have no poles at all
when magnetized. Physicists have long speculated about
the existence of single-pole magnetic particles called
monopoles. Known magnetic poles, however, always come
in pairs called dipoles. The most elementary magnet has an
S pole and an N pole. If cut in half, each half is found to be
dipolar. A magnet has an S pole for every N pole. An
isolated N pole of unit strength is sometimes assumed in
"thought" experiments. A unit pole may bethought of as one
that repels an exactly similar pole, placed one centimeter away,
with a force of 10--" N.
Figure 19-7. Like poles repel. Unlike poles attract.
Experimental evidence of the possible existence of magnetic
monopoles has been reported but
not verified. Physicists believe
that proof of the existence of
monopoles could help verify some
of the basic concepts of physics.
170
CHAPTER
The quantitative expression for
Coulomb's law of magnetism is
M1M2
F = k
. Compare this
d2
equation with those in Sections
3.11 and 16.8.
The first quantitative study of the force between two
magnetic poles is generally credited to Coulomb. He
found this magnetic force governed by the same inversesquare relationship that applies to gravitational force and
electrostatic force. Coulomb's law of magnetism is; The
force between two magnetic poles is directly proportional to the
product of the strengths
Glass
S!~___Needlefloalstoright
of the poles and inversely
Figure 19-8. The path followed by
independent N pole.
IAI
IBI
Figure 19-9. (A) The path taken by
an independent N pole in a magnetic field suggests a line of flux.
(8) Magnetic flux about a bar
magnet.
proportional
to
the square of the distance between them. The force is one of
repulsion or attraction, depending on whether the magnetic poles are alike or different.
Fields of Force In Section 16.9 we described the electric field of force near an electrically
charged object. Electric forces are not the only forces that
act on charged particles. Sometimes we observe the effect
of a force that is both perpendicular and proportional to the
velocity of a moving charge. This force identifies a magnetic
field. A dipole magnet in such a region of space experiences a torque. We speak of a magnetic field in the space
around a bar magnet in the same way we speak of an
electric field around a charged rod. Furthermore, we can
represent a magnetic field by lines of flux, just as we represented an electric field by lines of force.
The behavior of our imaginary independent N pole in a
magnetic field can be approximated by using a magnetized
darning needle as illustrated in Figure 19-8. The needle is
supported by cork so that it floats with the N pole extended below the surface of the water. The 5 pole is far
enough removed to have negligible influence on the
movement of the needle. A bar magnet placed under the
glass dish with its N pole near the needle causes the floating magnet to move along a path that approximates the
path an isolated N pole would follow.
The path of an independent
N pole in a magnetic field
suggests a line of flux. A line of flux is a line so drawn that a
tangent to it at any point indicates the direction of the magnetic
field. Flux lines are assumed to emerge from a magnet at
the N pole and to enter the magnet at the S pole. Every
flux line is a closed path running from S pole to N pole
within the magnet. See Figure 19-9.
The lines of flux perpendicular to a specified area in the
magnetic field are collectively called the magnetic flux, for
which the Greek letter q:, (phi) is used. The unit of magnetic flux is the weber (Wb).
The magnetic flux density, B, is the number of flux lines per
unit area that permeates the magnetic field. The flux density B
is a vector quantity; the direction of B at any point in the
magnetic field is the direction of the field at that point.
19.4 Magnetic
the floating magnet in this experiment is approximately that of an
19
B~
"'fA
f
471
MAGNETIC EFFECTS
Flux density is expressed in webers per square meter
(Wb/m2). The flux density determines the magnetizing force
at any point in the magnetic field. The weber per meter- is
also called the tesla (T).
= 1 tesla
1 weber/meter2
The measurement
of these quantities is in Section 19.9.
Flux lines drawn to indicate how tiny magnets would
behave when placed at various points in a magnetic field
provide a means of mapping the field. A line drawn tangent to a flux line at any point indicates the direction a
very small magnet would assume if placed there. An arrowhead can be added to the tangent line to indicate the
direction in which the N pole of the tiny magnet would
point, thus giving the direction of the magnetic field, and
the B vector, at that point.
Using a suitable scale of flux lines per unit area perpendicular to the field, the flux density, B, at any point can be
illustrated. Selection of a number of lines to represent a
unit of magnetic flux is arbitrary. Usually, one flux line per
square meter represents a flux density of 1 Wb/m2. In this
sense, one line of flux is a weber.
The magnetic field near a single bar magnet is suggested
by the pattern formed by iron filings sprinkled
on
a glass plate laid over the magnet. A photograph of this
field pattern is shown in Figure 19-10. Using a similar
technique, the magnetic fields near the unlike poles and
near the like poles of two bar magnets are illustrated in
Figure 19-11. Observe that the magnetic force acting on
the two unlike poles is one of attraction and that acting
on the two like poles is one of repulsion. Figure 19-12
similarly illustrates an end-on view of the magnetic field
between the poles of a horseshoe magnet.
19.5 Magnetic
Permeability
In Section 19.4 we described the effect of a magnetic field of force on iron filings
and on a magnetized needle as experienced through glass
,)\' '~:'
Imaginary lines of magnetic flux
are useful for mapping magnetic
fields.
Figure19-10. Iron filings near a
single bar magnet.
",'",
'/ " I''i'~~!?!/::"7",,:-::-=,~- ~:~
"'''" ~(\~)): J ',1'~'17:
'1
I
l~
;<:'-::;:'~'>J'\\\\','I,:
\\I\\
fl, ,
,:{,
'
' :,
''';:,..,"___~" ::" , ":'\\'\il'
, , ,
,
!
~.:£"c::->-'
''-''",,
/.!'.:, ' j, ",, / ~ "~'\\\
"" \ Ib',//.
::--C~"
,"
'
. \
"'
'
\
\1"
I
/'j.../- "
:..,-.,.."""'-"'-...:
"'-...:,
,,,,\,
I'..>,"', :' :;,' ,(1'.":--' -"'-.-.'-"-.,:_" -'.:>"\.,...,~'" . ~ ,'.'I '/ .~, ,,',
~:---.
!'\""
.,. 'C(' '~
'~'
:. \~\"
1;';f/~"-;-c-2?~'
~,~
~/ ~
~~ "
'
,
.
,
'-'
.
-"
~:.::~~~~-:,¥.::-
~~
~
~
,-
,,~~~*~,.,:}tMi~%\RI'I~~~~:';-;'~':"~:T
',f::.:::;..'
~~~~':S:::-c-",
'[;~~--;.~:
-'-'
~'~'-::-~:.'
"
,
...',~,,\\'iW..i
:';:t'C~~~
~~~~, ~~-:.~;~,~;;;;,1'f:i'f,.,~!i;,,:,:
~ ,~:7-~~~-:.
.,,0.-00,.
t.,-'
..- _~~
.~"/
-"'-'i)'.
'
.,'\;\:~ \~'
''\'~..:'-.,
':"
,(,;,:',';
~:;
:;;@.~i:
~:;
:,:, !.:~',~(\{;~~.~~2\~'~l
~~?:~-~
...,"
/,//'/-'
,/; I"'\
. ,.;.,
,
,.'"......
rif
'_"
;
"j
',..,'
:/.
-
, '." ,
,
r \\
II.
"'/'-
/:
,,",/./'
"('"/~."
,t,..;,V.
r
',I
'.'i
'..
'~('0~~--?!';:'~':
~
~"
"\\.
1 'I'
1' .' ',' .\'
;"/;, ;..-,I;/),! ::-\,;<~,\"i..'\:>'
L,-::riI(.':-/-;
.
:::"':"'.,I','u,
'.~1, .,,,,.,.,,, \,
:'>:..",-",_~,~ .>_',./
.',_
"''__
1,!
i,
'..'."'"
,'.-'(.:..,
--':'
-.
,.,
' ' ,.,"
,
...l\ j""." ,:
',,'
,,
"""\,"
"',.\,.".":
'\\;'\.,_.
,'C,~",' I. ~.:
""
;:~':-"""
c>_.-,
,'.CO',.,. ~'.~..\ "_-"'...~_
_
",...",'iZ"':'-c,'~_-y_:~.
''j''<,',.. '-'I"
":,~__
CHAPTER 19
472
Figure 19-11.
Iron filings are
shown near unlike poles of two
bar magnets in (A) and near their
like poles in (B).
16)
IAI
IA)
Figure 19-12. (A) Iron filings near
the poles of a horseshoe magnet,
end view. (B) An idealized drawing
of (A) showing lines of flux.
In a practical sense, all materials
except those that are ferromagnetic can be considered magnetically inert. In a magnetic fie/d,
they behave just like air.
16)
and water. Nonmagnetic materials in general are transpar-
ent to magnetic flux; that is, their effect on the lines of flux
is not appreciably different than that of air. Theproperty of a
material by which it changes the flux density in a magnetic field
from the value in air is calledits permeability, f.L. Permeability
is a ratio of flux densities and is without dimension. The
permeability of empty space is taken as unity and that of
air as very nearly the same. The permeabilities of diamagnetic substances are slightly less than unity; permeabilities
of paramagnetic substances are slightly greater than unity.
Permeabilities of ferromagnetic materials are many times
that of air.
If a sheet of iron covers a magnet, there is little magnetic
field above the sheet. The flux enters the iron and follows
a path within the iron itself. Similarly, an iron ring placed
between the poles of a magnet provides a better path than
air for the magnetic flux. This effect is illustrated in Figure
19-13. The flux density in iron is greater than it is in air;
therefore, iron is said to have a high permeability. The
permeabilities of other ferromagnetic substances are also
very high.
MAGNETIC EFFECTS
473
Magnetic
lines~
c
;
c
f
,
,,
Suppose a bar of soft iron lies in a magnetic field, as in
Figure 19-14. Because of the high permeability of the iron,
the field is distorted and the magnetic flux passes through
the iron in preference to the air. Under these circumstances the soft iron bar becomes a magnet with end A as
the 5 pole and end B as the N pole. Such a bar is said to be
magnetized by induction. Magnetism produced in a ferromagnetic substanceby the influellceof a magneticfield is called
induced magnetism.
A
Figure 19-13. At left. magnetic
flux crosses the air gap between
the poles of a magnet. At right,
magnetic flux follows the soft iron
ring, which is more permeable
than air.
,
~
If the magnetic field is removed by withdrawing the two
bar magnets, most of the induced magnetism will be lost.
Magnets produced by induction are known as temporary
magnets. A piece of hardened steel is not so strongly magnetized by induction but retains a greater residual magnetism when removed from the induction field.
There is no significant difference in the process if the
iron bar in Figure 19-14 is brought into contact with one of
the magnet poles. The magnetization process is somewhat
more efficient due to the reduction of the air gap. See Figure 19-15.
19.6 Terrestrial
Magnetism
Suppose the earth contained a great bar magnet. See Figure 19-16. It would produce a magnetic field similar to its actual field. Over most
of the earth's populated surface the north-seeking pole of
a compass points northward. Although it is the south pole
of our fictitious magnet that attracts the N pole of the compass, the pole region is conventionally called the north
magnetic pole because it is located in the northern hemisphere. Similarly, the pole region in the southern hemisphere is called the south magnetic pole.
The earth's magnetic axis does not coincide with its
polar (geographic) axis, but is inclined to the polar axis at a
small angle. The north magnetic pole, at latitude 73°N
and longitude 100oW, is about 2000 km (1200 miles) south
Figure 19-14. An iron bar magnetized by induction.
,
Figure 19-15. The nail becomes
magnet by induction. Are the
tacks also magnets?
a
474
CHAPTER
Magnetic
field____
19
Electric
current
Figure 19-16.
The magnetic field
of the earth may be produced
by
electric currents within its fluid
core. The field is oriented as
though the earth contained a
large magnet passing through its
center, with the magnetic axis
slightly inclined with respect to
the axis of the earth.
of the north geographic
pole. The south magnetic
pole is
located in Antarctica near the Ross Sea. Thus from most
locations on the earth, the N pole of the compass needle
does not point to the true geographic north. At any surface location the angle between
magnetic
north and the
true north is called the declination, or variation. In the region of Los Angeles the compass variation is about 15°E.
That is to say a compass needle points about 15° to the east
of true north. In the region of Boston the variation is about
15°W. Cincinnati, Ohio, is located very near the line of
zero declination. Here the compass needle points to the
true north and the variation is 0°.
A compass needle mounted on a horizontal axis and
provided with a means of measuring the angle the needle
makes with the horizontal plane is called a dipping needle.
At certain places on the earth's surface, about midway between the magnetic poles, the angle of dip is zero and the
needle is horizontal. A line drawn through a succession of
such points identifies the magnetic equator. The angle of dip
is 90° at the magnetic pole. The dip, or deviation between
the equilibrium position of a dipping needle and the horizontal, is known as the magnetic inclination.
In 1600 the English physicist William Gilbert (1540?1603) published his scientific treatise De magnete, which
deals with the magnetism of the earth. This is one of the
earliest publications on the experimental treatment of a
scientific topic. Gilbert inferred that the earth behaved as a
large magnet because the interior consisted of permanently magnetic material. Today scientists believe the core
of the earth is too hot to be a permanent magnet and is
fluid rather than solid.
,
"
MAGNETIC
475
EFFECTS
The German physicist Karl Friedrich Gauss (1777-1855)
showed that the magnetic field of the earth must originate
inside the earth. In 1939 the American theoretical physicist
Walter M. Elsasser suggested that the earth's magnetic
field results from electric currents generated by the flovv of
matter in the earth's fluid core. See Figure 19-16. Today
physicists believe that the magnetic field is due primarily
to electric currents within the earth, but they have not yet
established the origin of these currents.
c
c
f
f
Electric current loops inside the
enrth are responsiblefor its magnetic field.
f
19.7 The Magnetosphere
Because space vehicles now
travel to the outer limits of the earth's atmosphere and
beyond, there is a growing interest in a region of the outer
atmosphere known as the magnetosphere.
Located beyond
200 km, the magnetosphere
is the region in which the
motion of charged particles is governed primarily by the
magnetic field of the earth. At lower altitudes, where
the density of the atmosphere is much greater, the motion
of charged particles is controlled largely by collisions.
The magnetosphere
on the side facing the sun extends
beyond the earth's surface approximately 57000 km, or
about 10 earth radii. On the side away from the sun, the
magnetosphere
probably extends outward for hundreds
of earth radii. See Figure 19-17. The elongated shape results from the influence of the onrushing solar wjnd, or
solar plasma. The solar wind, consisting mainly of protons
and electrons emitted by the sun, compresses the magnetosphere on the side nearest the sun.
In 1958 regions of intense radiation were discovered
within the magnetosphere by a team of physicists headed
f
r
I
I
I
,
Figure 19-17. The magnetosphere
of the earth. The overall radiation
regions are shown in color. The
inner and outer Van Allen belts of
intense radiation are the dark regions ranging outward to approximately 4 earth radii.
f
,
r
I
Solar
I
I
wind
~~J\cE'''h
"-
I
VanAllen
radiation bells
~
~
r-.
"
"
10
5
0
5
0
15
"
"
Earth
30
30
40 radii
CHAPTER 19
476
Auroral displays
over the polar
regions are related to fhe escape of
energeticparticles from fhe radiation belts.
QUESTIONS:
by Dr. J. A. Van Allen (b. 1914). These regions, now
known as the Van Allen radiation belts, contain energetic
protons and electrons trapped by the earth's magnetic
field. Those trapped in the inner belts probably originate
in the earth's atmosphere; those trapped in the outer belts
probably have their origin in the sun. When these intense
radiation belts were first discovered, scientists were concerned about the serious threat they appeared to present
to space travel. Today, astronauts journeying into outer
space are able to pass quickly through these regions with
adequate protection from the Van Allen radiation.
GROUP A
1. (a) What are natural magnets called?
(b) Is this term appropriate? Explain.
2. Describe the two electron motions that
affect an object's magnetic properties.
3. What is a magnetic domain?
4. Why are iron atoms so strongly affected by magnetic fields?
5. When a magnetized steel needle is
heated strongly in a bunsen burner
flame, it becomes demagnetized.
Explain why.
6. What indicates that a piece of iron is
magnetic-its
attraction to or repul.
sian from another piece of iron?
7. Describe the magnetosphere.
8. (a) What is the minimum number of
poles for a magnet? (b) Can a magnet
have three poles? Explain.
9. (a) When you break a magnet in half,
how many poles does each piece
have? (b) How small would the pieces
have to be for this not to be true?
10. What is the difference between the
angles of declination and inclination?
GROUP B
11. VVhat is the difference behveen a paramagnetic and a ferromagnetic
material?
12. What do we mean when we say a
piece of paper is magnetically transparent?
13. Why does a very strong magnet attract both poles of a weak magnet?
14. (a) How does solar wind affect the
shape of the earth's magnetic field?
(b) Name two other effects of charged
atmospheric particles on the earth's
magnetic field.
15. A strong magnet in a junkyard can
lift a car; what does this tell you
about the relative strength of the
magnetic and gravitational forces on
the car?
16. What happens on a subatomic level
when a magnet attracts a steel
needle?
17. If a small magnet is repeatedly
dropped, it becomes demagnetized.
Explain what is happening subatomically.
IS. Compare and contrast the effects of
electrostatic and magnetic forces.
19. Sir William Gilbert believed the earth
contained an iron core that was a
huge permanent magnet. Give some
evidence to refute this theory.
20. What happens when you pass a magnet across a computer floppy disk?
ELECTROMAGNETISM
19.8 The Link Between an Electric Current and Magnetism It can be easily demonstrated that electrostatic
charges and stationary magnets have no effect on one an-
c
,
f'
,
f
r
f
f
"
MAGNETIC EFFECTS
477
other. However, in 1820 Hans Christian Oersted (er-stet)
(1777-1851), a Danish physicist and professor of physics at
the University of Copenhagen, observed that a small compass needle is deflected when brought near a conductor
carrying an electric current. This was the first evidence of a
long-suspected
link between electricity and magnetism.
Oersted discovered that forces exist behveen a magnet
and electric charges in monon. His famous experiment is
so significant that a brief description of it is in order.
A dry cell, compass, switch, and conducting wire are
arranged as shown in Figure 19-19(A). With the switch
open, a straight section of the conductor is supported
above the compass in the vertical plane of the compass needle. In Figure 19-19(B) the dry-cell connection is such that
the electron flow will be from north to south. When the
switch is closed, the N pole of the compass is deflected
toward the west. When the drv-cell connections are reversed so electron flO\\' is from south to north, the N pole
of the compass is deflected to the east, as in Figure 1919(C). It is evident that a magnetic field exists in the rcgimz
near the conductor when the circuit is closed. Furthermore, the
direction of the field is dependent 011the direction of the current
Figure 19-18. Hans Christian Oersted studied medicine before becoming professor of physics at the
University of Copenhagen in 1806.
Several years before he performed
his famous experiment.
he predicted that a link between electricity and magnetism would be
found.
ill the conductor.
Figure 19-19. The Oersted experiment as viewed from above. In
,-r-
I
/11\
"
r:
~-1;:
Sw
,I
-.-J
(AJ
,-,---
,BJ
each
j
diagram
is located
the
compass
needle
below the conductor.
,-I
Sw
~
(OJ
If the experiment is repeated with the conductor placed
below the compass needle, the compass deflection is opposite to that in the first experiment. This suggests, but does
not prove, that the magnetic field encircles the conductor.
19.9 Magnetic
after Oersted's
Field and a Charge in Motion
discovery, the French physicist
Shortly
Ampere
determined the shape of the magnetic field about a conductor carrying a current. He had discovered that forces
exist between two parallel conductors in an electric circuit.
If the two currents are in the same direction, the force is
one of attraction; the force is one of repulsion if the currents are in opposite directions. See Figure 19-21.
In a quantitative sense, two long, straight, parallel conductors of length I separated by a distance d and carrying
Figure 19-20. Andre Ampere, the
French physicist for whom the unit
of electric current is named, did
fundamental
work in electromagnetism.
CHAPTER 19
478
currents hand 12will each experience a force F of magnitude
2k 11112
F~
d
The constant
k is exactly 10-7 N/A2. If I} and 12 are expressed in amperes and land d in meters, the force F is
given in newtons.
(~,)x(m'~'A)=N
"\ ,-
,-I'
I
,-
1
j
.
'11
F
11
Figure 19-21.
l'
1
\
,-,
\
F
$.
$.
Forces between
parallel currents (A) in the same
direction and (8) in opposite di-
,A,
rections.
,s,
Because these attractive and repulsive forces between
current-carrying
conductors
are directly proportional
to
the currents
in the conductors,
they provide
a precise
method of defining the unit of current, the ampere. In this
sense, the ampere may be defined as the current in each of
two long parallel conductors spaced one meter apart that causes
a magnetic force of 2 x 10-7 newton per meter length of conductor.
Following this scheme,
charge (an ampere-second)
of electric
the coulomb as a quantity of
may be defined as the quantity
chargethat passesa given point on a cOtlductor in one
second when the conductor carries a constant current of one
ampere.
Ampere investigated the magnetic fields about conductors to find an explanation of the magnetic forces. Suppose
a heavy copper wire passes vertically through the center of
a horizontal sheet of stiff cardboard. When the ends of the
vertical conductor are connected to a dry cell, iron filings
sprinkled over the surface of the cardboard form a pattern
of concentric circles around the conductor. See Figure 1922. If a small compass is placed at successive points
around a circle of filings, the needle always comes to rest
MAGNETIC
479
EFFECTS
,
,
,-I
I)
j
I
f
f
I
f
l
Figure 19-22. The magnetic field
encircling a current in a straight
conductor.
Sw
tangent to the circle and with the same tangential orientation of its N pole.
H the direction of current in the vertical conductor. is
reversed, the compass needle again becomes aligned tangent to the circle of filings, but with its N-pole orientation
reversed. From these observations we conclude that a magnetic j1:eldencircles an electric charge in 1110tion.The lines of
flux are closed concentric circles in a plane perpendicular
to the conductor with the axis of the conductor as their
center. The direction of the magnetic field is everywhere
tangent to the flux and is dependent on the direction of
the current.
Ampere devised a rule, known today as Ampere's rule,
for determining the direction of the magnetic field around
a current in a straight conductor when the direction of the
electron flow is known.
Ampere's rule for a straight conductor: Grasp the conductor in
the left hand with the thumb extended in the direction of
the electron flow. The fingers then will circle the conductor in the direction of the magnetic flux:. See Figure
19-23.
The flux density, B, also called the magnetic inductiOll, at
any point in the magnetic field of a long straight conductor
carrying a current, l, is directly proportional to the current in
the coltductor and inversely proportional to the radial distance, r,
of the point from the conductor.
B
~
2k-
MagI/die phenomena are interpreted ill tenns of the forces associated with electric charges in
motia/I.
The left-hand rule for a straight
cUllductor illdicates the direction
of the magnetic flux surrounding
the conductor.
I
[
r
The constant k again is 10-7 N/A2. When I is given in amperes and r is in meters, B is expressed in newtons per
ampere meter, which is equivalent to webers per square
meter.
In Section 19.4 flux density is defined in terms of the
lines of flux per unit area that permeate the magnetic field.
Electron (e-j
flow
Magnetic
field
Figure
19-23.
Ampere's rule for a
straight conductor.
CHAPTER 19
480
In this sense the expression
Thus
'"
for flux density
is B = 1>/A.
~ BA
When B is expressed in newtons per ampere meter, the
unit for 1>, the weber, can be shown to be 1 newton-meter
per ampere:
N
Wb=
(
A'm
)
x(m2)=-
N'm
A
Whether 1> is expressed in webers or newton meters per
ampere and B is expressed in webers per square meter or
newtons per ampere meter is a matter of convenience in
each situation.
Observe that the definition of B given in Section 19.4 is
based "On the force exerted on an isolated unit pole. An
isolated pole exists only in the fiction of a thought experiment; consequently,
measurements
based on this definition lack precision. The more practical definition given
above involves quantities that can be measured precisely
and is therefore generally preferred.
Figure 19-24. The magnetic field
through a current loop.
19.10 Magnetic
Field and a Current Loop
Keeping
Ampere's rule in mind, let us consider a loop in a conductor carrying a current. The magnetic flux from all segments
of the loop must pass through the inside of the loop in the
same direction; that is, the face of the loop must show polarity.
See Figure 19-24.
,-I
j
N
,-I
j
(A)
A magnetic lube is an imaginary
tube bounded by magnetic lines.
It a/waifs links the current that
prodl/ces the field.
(B)
This loop magnet can be made stronger if the flux density can be increased. Because the magnetic field around a
conductor varies with the current, the flux density can be
increased by increasing the magnitude of the current in
I
[
c
r
f
t
I
I
r
MAGNETIC
481
EFFECTS
the conducting loop, by forming addihonalloops
in the
conductor, or by both.
A linear coil of such conducting loops takes the form of a
helix and is called a sole/wid. The cylindrical column of air
inside the loops, extending the length of the coil, is called
the core. When a current is in a solenoid, the core of each
turn (loop) becomes a magnet; the core of the solenoid is a
magnetic tube through which practically all the magnetic
flux passes. See Figure 19-25.
Because a solenoid conducting an electric current has
the magnetic properties of a bar magnet, its polarity can be
determined by means of a compass. However, the magnetic flux in the core of the solenoid is derived from the
magnetic field of each turn of the conductor. Thus Ampere's rule is modified to adapt it to this special case of the
solenoid.
Ampere's rule for a solenoid: Grasp the coil in the left hand
with the fingers circling the coil in the direction of the
electron flow. The extended thumb will point in the direction of the N pole of the core. See Figure 19-26.
19.11 The Electromagnet
A solenoid with a core of air,
wood, or some other nonmagnetic material does not produce a very strong electromagnet because the permeability
of all nonmagnetic substances is essentially equal to that of
air-unity.
Substitution of such materials for air does not
appreciably change the flux density.
Soft iron, on the other hand, has a high permeability. If
an iron rod is substituted for air as the core material, the
flux density is greatly increased. Strong electromagnets
therefore have ferromagnetic cores with high permeability. For a given core material, the strength of the electromagnet depends on the magnitude of the current and the
number of turns. In other words, its strength is determined by the number of ampere-turns.
19.12 The Galvanometer
Suppose we form a wire loop
in a vertical plane, place a compass needle (free to rotate in
a horizontal plane) in the center of it, and then introduce a
current into the loop. The needle will be deflected. If we
increase the number of turns sufficiently, even a feeble
current will produce a deflection of the needle. Such a
device, called a galvanoscope, may be used to detect the
presence of an electric current or to determine its direction. A simple galvanoscope is shown in Figure 19-28.
A more versatile instrument for detecting feeble currents is the galvanometer, the essential parts of which are
shown in Figure 19-29. A coil of wire wound on a soft iron
core is pivoted on jeweled bearings between the poles of a
I
.~
I
I
.~
I
s
Figure 19-25. Magnetic field
about a solenoid.
s
N
.1
Figure 19-26. Ampere's rule for a
solenoid.
Figure 19-27. A superconducting
electromagnet. In operation, the
eight-foot cylinder is immersed in
liquid helium (-232°C). At this
temperature the niobium-titanium
strips imbedded in the copper
coils lose all electric resistance
and the magnet produces a force
field up to 5000 times greater
than that of the earth.
j.~
..~
CHAPTER 19
482
Figure 19-28.
scope.
Because a magnetic field exerts
forces on moving charges, it exerts torques on current-carrying
coils.
Scale
Permanent
magnet
permanent horseshoe magnet. The coil becomes a magnet
when current is in it. The instrument then has two magnets: a permanent horseshoe magnet in a fixed position
and an electromagnet free to turn on its axis. Electric connections to the coil are made through two control springs
(not shown), one above and one below the coil. These
coiled springs also restrain the rotational motion of the coil
so that the attached pointer returns to the zero scale position when no current is present in the coil. This zero position is often located at the midpoint of the scale, as in
Figure 19-29.
When there is a current in the movable coil, its core is
magnetized. The poles of the core are then attracted and
repelled by the poles of the pennanent magnet. A torque
acts upon the coil and the coil rotates in an attempt to align
its plane perpendicular to the line joining the poles of the
permanent magnet. As the coil rotates, however, it does
work against the two control springs. Its final position is
reached when the torque acting on it is just neutralized by
the reaction of the springs. Since the permanent field flux
is constant, the torque on the coil is proportional to the
current in it. We may assume, for small movements of the
coil, that the reaction of the springs is proportional to
the deflection angle. When the coil reaches its equilibrium
position, these two opposing torques are equal, and the
deflection angle of the coil is therefore proportional to the
current in it.
The scale of the galvanometer is marked at intervals on
either side of the zero center. Readings are made on this
scale by means of a small, lightweight pointer attached to
the coil. For a coil current in one direction, the needle deflection is to the left. If the current direction is reversed,
the needle is deflected to the right.
The galvanometer is a sensitive instrument for detecting
feeble currents of the order of microamperes. For transla~
tion of a reading into absolute current values, the current
sensitivity of the specific instrument must be known. Current sensitivity is usually expressed in microamperes per
scale division.
The pointer deflection, d, of a galvanometer is proportional to the current, 1M, in the coil
1M 0:: d
kd
[M
or
~
Coil
and
19-29. The basic components of a moving-coil galva-
Figure
nometer.
where k is the current
scale division.
k
=
1M
d
sensitivity
in microamperes
per
'-
,
f
r
r
r
I
MAGNETIC EFFECTS
483
The moving coil of the galvanometer
has
resistance.
By
Ohm's law, a potential difference appears across the resistance of the meter when a current is in the coil. We can
express the voltaxe sensitivity of the instrument
since it
must be equal to the product of the meter resistance
and
the current per scale division.
Voltage sensitivity
=
where RM is the resistance of the meter movement. Volt.
age sensitivity is given in microvolts per scale division.
If provision is made to prevent excessive currents from
entering the coil, the galvanometer
can be adapted for
service as either a d-c ammeter or d-c voltmeter.
The following examples illustrate calculations involving
the galvanometer.
Figure19-30. Sensitive galvanometers use a mirror to indicate the
position of a suspended coil by
producing an image of a scale
viewed through a telescope or by
reflecting a beam of light onto a
scale
EXAJIPLE
A galvanometer has a current sensitivity of 2.50' x 10'~jJ..A,
and the scale goes from - 50' to 0 to 50, with 0' at the very center. Calculate the current in amperes that is passing through the coil of the instrument when the reading is 18.5 on either side of zero.
.r
2500 I'-A
50'.0'divisions
k=
d
= 18.5
"'!I" Bnsic equation
Unknown
Cin'r~
-
.
i
1M =
I"
kd
divisions
Solution
I
..
Working- equatIOn:
(
[M
=
kd
=
(2500 I'-A)(lO-O AII'-A)(18.5
~.Odiv
EXAMPI.F If the meter resistance in the previous
what is the voltage across the fully deflected meter?
div)
= 9.25
X
10-4 A
example is ~~
I
Ir'!!!!. JI""'."__
( .Ivcn
"~'
r
k = 50'.0' ,uA/div
d = 50.0' div
RM 10,0 !l
~
='-=''''--"JI
Unknown
V"
Basi(~ e(IlJation~
VM=lMRM
1M=kd
!"!'!I
iI
oJ;
CHAPTER 19
484
Solution
Working equation:
~
~
(50.0I'Aldiv)(IO-' AII'A)(50.0 div)(lO.O fJ)
2.50 x 10-' V
PIL\(:TI(]~ IJIl()BL~MS 1. What magnitude of current will produce a
full-scale deflection of a galvanometer with exactly 40 scale divisions and
a sensitivity of 30.0 fLA per scale division?
Ans. 1.20 x 10-3 A
2. What is the voltage across the fully deflected galvanometer in Problem
17 (The meter resistance is 8.50 n.)
Ans. 1.02 x 10-2 V
19.13 The d-c Voltmeter
The potential
difference
across a galvanometer is quite small even when the needle
is fully deflected. If a galvanometer is to be used to measure voltages of ordinary magnitudes, we must convert it to
a high-resistance instrument. The essential parts of a d-c
voltmeter are shown in Figure 19-31.
d-cvoltmeter
1.5
,.0
2.0
0'
Electrons which flow through
the
voltmeter pass through the rear
coiled control spring and into the
windingsonthemovabJecoil
Coils of high
resistance wire
-,"-"il
c'ilc__
E
"
[7.,
'
iL'3J.N :"1
,'''iI~:
Electrons from the movable coil
windings pass through the front
control spring, and through
highresistancecoilstothe
positive terminal
the
The high resistance coils permit only a few electrons
to flow through the movable coil of a voltmeter
Figure 19-31. The construction of
a d-c voltmeter showing the high
resistance in series with the windings of the movable coil.
If a high resistance is added in series with the moving
coil, most of the potential drop appears across this series
resistor, or multiplier. Since a voltmeter is connected in parallel
with the part of a circuit across which the potential differ-
,
I
MAGNETIC EFFECTS
485
ence is to be measured, a high resistance prevents an appreciable loading effect. By the proper choice of resistance,
the meter can be calibrated to read any desired voltage.
Suppose we convert the galvanometer used in the examples in Section 19.12 to a voltmeter reading 15.0 volts
on full-scale deflection. The current required for full deflection has been found to be 2.50 x 10-3 ampere, and the
resistance of the meter coil is 10.0 ohms. We must determine the value of the resistor Rs to be placed in series with
the moving coil. Figure 19-32 illustrates this problem.
Since RM and Rs (of Figure 19-32) are in series,
Then
V = IMRM + IMRs
V
Rs =
RM
1M
Rs
15.0 V
~
2.50 x 10-3 A
-
10.0
n
Rs = 5990 fi, value of the series resistor
Observe that the total resistance between the terminals
of the meter is 60'00 ohms.
The voltmeter sensitivity is frequently expressed in
terms of ohms per volt. When the ohms-per-volt sensitivity
of a voltmeter is known, we can quickly estimate the loading effect it will have when placed across a known resistance component of a circuit. For example, at 400 ohms
per volt, our meter, which reads from 0 to 15 volts, has
6000 ohms between the terminals. If it were placed across
resistances greater than about 600 ohms, the loading effect
would result in serious meter errors.
19.14 The d-c Ammeter
We could use the basic galvanometer in Section 19.12 as a microammeter by calibrating
the graduated scale to read directly in microamperes.
However, the meter would not be useful in circuits in
which the current exceeded 2500 microamperes.
Current
in the resistance of a galvanometer coil produces ]2R heating, and an excessive current would burn out the meter.
To convert the galvanometer to read larger currents, an
alternate (parallel) low-resistance path for current, called a
sllunt, must be provided across the terminals. By the
proper choice of shunt resistance, we can calibrate the
meter to read over the required range of current magnitudes. See Figure 19-33.
Suppose we wish to convert the same galvanometer to
an ammeter reading 10.0 amperes full scale. As before, the
current required for the full deflection of the moving coil
is 2.50 x 10-3 ampere and the resistance is 10.0 n. We
Galvanometer
mav.bl.coil
Seri.'r...i'tar
.
RM =10.051
1M =0.002
50A
Figure 19-32. Converting a galvanometer to a voltmeter.
486
CHAPTER 19
,
4
.
Electrons from the movable
coilwindingspasslhrough
the front control spring and
The few electrons which flow
through the coil of the ammeter
pass througl1 the rear CDiled
conlrol spring and into the
windings on ttle movable coil
theconn~tingwiret()the
positiveterminaJ
Mostoftheelectronsentering
the ins!rument Ilow through
Ihe low resislance shunt
Figure 19-33. The construction of
a d-c ammeter showing the low
resistance in parallel with the
windings of the movable coil.
must determine the resistance of the shunt
across the coil. Figure 19-34 applies.
Since RM and Rs are in parallel,
IMRM
But
Galvanometer
moving coil
Then
R s --
Rs
"
'"
Shunt,e<istQr
Figure 19.34. Converting a galvanometer to an ammeter.
=
Is
=
Rs
=
= IsRs
IT
- 1M
IMRM
IT
2.50 X 10-3 A x 10.0
-
to be used
-
1M
n
10.0 A 0.002 50 A
0.002 50 H, the value for the shunt resistor
The total resistance of the ammeter is the equivalent
value for 10.0 ohms and 0.002 50 ohm in parallel; it will be
less than 0.002 50 ohm. This exercise demonstrates clearly
why an ammeter must be connected in seriesin a circuit, and
why it does not materially alter the magnitude of current
in the circuit.
19.15 The Ohmmeter
An ohmmeter provides a convenient means of measuring the resistance of a circuit
component. A basic ohmmeter circuit is shown in Figure
19-35. Its accuracy limitation is approximately the same as
r
f
f
I-
487
MAGNETIC EFFECTS
that of the voltmeter-ammeter method of measuring resistance; actually, it is a modified version of this method.
The ohmmeter must be used only on a completely deenergized circuit.
The ohmmeter circuit of Figure 19-35 shows a milliammeter requiring 1 mA for full-scale deflection. With an emf
of 4.5 volts, by Ohm's law, 4500 ohms of resistance will
provide 1 mA of current when terminals A-B are shortcircuited. A fixed resistor, R2, of 4000 ohms and a rheostat
Rl1 of 0-1000 ohms are provided.
To use this ohmmeter, A and B are short-circuited and
RI is adjusted to give full deflection. If the emf is 4.5 V, RI
will be set at 500 ohms. The pointer position at full-scale
deflection is now marked as zero ohm (0 H). The rest position of the pointer is the open-circuit position with infinite
p:
~A
400012
R,
~_0-1000'1
4.5V-=-
'mA
(full ~cale)
~
A
Figure 19.35. A basic ohmmeter
circuit.
resistance between A and B. This position is marked as
cc u. Other resistance calibrations may be made from
Ohm's law applications. For example, 4500 ohms between
A and B will mean a total of 9000 ohms in the circuit and
0.5 mA of current. This mid-scale position of the pointer
can be marked 4500 n. The meter, when recalibrated, will
read the resistance between terminals A-B.
Each time the ohmmeter is used, it is first shorted across
A-B and R1 is adjusted
to "zero"
the meter.
This operation
calibrates the meter and accommodates any decrease in
the terminal voltage of the battery with age. The resistance
RI allows the ohmmeter to be used until jg drops below 4.0
volts.
QUESTIONS:
GROUP A
5. Upon what factors does the strength
1. What did Oersted discover?
2. A conductor carrying a current is arranged so that electrons flow in one
segment from north to south. If a
compass is held over this segment of
the wire, in what direction is the needle deflected?
3. Describe a simple experiment to show
the nature of the magnetic field about
a straight conductor carrying a current.
4. Suppose an cIectron flow in a
con-
ductor passing perpendicularly
through this page is represented by a
dot inside a small circle when the direction of flow is up out of the page.
What is the direction of the magnetic
flux about this current?
of an electromagnet depend?
prevents the movable coil of a
galvanometer from aligning its magnetic field parallel to that of the permanent magnet each time a current is
in the coil?
7. (a) Why is it necessary that an amme~
ter be a low-resistance instrument?
(b) Why must a voltmeter be a high~
resistance instrument?
6. What
GROUP B
8. 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 such that the electron flow is
clockwise when viewed from end A
B
488
9.
10.
11.
12.
13.
CHAPTER
toward end B. How will the coil align
itself in the earth's magnetic field?
A stream of electrons is projected
horizontally to the right. A straight
conductor carrying a current is supported paralJel to the electron stream
and above it. (a) What is the effect on
the electron stream if the direction of
the current in the conductor is from
left to right, (b) if the current is
reversed?
If the conductor in Question 9 is replaced by a magnet with a downward
magnetic field, what is the effect on
the electron stream?
Why might the potential difference
indicated by a voltmeter placed across
a circuit load be different from the
potential difference with the meter
removed?
Suppose the resistance of a highresistance load is to be determined
using the voltmeter-ammeter
method.
Considering the design characteristics
of ammeters and voltmeters, how
would you arrange the meters in the
circuit to reduce the error to a minimum? Draw your circuit diagram and
justify your arrangement.
Assume that the resistance of a lowresistance load is to be determined
using the voltmeter-ammeter
method.
How would you arrange the meters
in this circuit to reduce the error to a
minimum? Draw your circuit diagram
and justify your arrangement.
PROBLEMS:
GROUP A
1. Two parallel conductors 2.0 m long
and 1.a m apart and carrying equal
currents experience a total force of
1.6 x 10-6 N. What magnitude of current is in each conductor?
2. An ammeter that has a resistance of
0.01 ohm is connected in a circuit and
indicates a current of 10 amperes. A
shunt having a resistance of 0.001 ohm
19
is then connected across the meter terminals. What is the new reading on
the meter? Assume the introduction of
the shunt does not affect the total circuit current.
3. A galvanometer has a zero-center scale
with 20.0 divisions on each side of
zero. The pointer deflects 15.0 scale
divisions when a current of 375 J1.A is
in the movable coil. (a) What is the
current sensitivity of the meter?
(b) What current will produce a fullscale deflection?
GROUP R
4. A galvanometer
has a resistance
of
50.0 n and requires 75.0 mA to produce a full-scale deflection. What resistance must be connected in series
with the galvanometer in order to use
it as a voltmeter for measuring a maximum of 300.0 V?
5. A galvanometer movement has a resistance of 2.5 ohms and when fully
dgflected has a potential difference of
50 millivolts across it. What shunting
resistance is required to enable the instrument to be used as an ammeter
reading 7.5 amperes full scale?
6. A repulsive force of 9.6 x 10-4 N is
experienced by each of two parallel
conductors 5.0 m long when a current
of 3.2 A is in each conductor. By what
distance are they separated?
PHYSICS ACTIVITY
Try to measure the magnetic declination
(if any) where you live. On a clear night,
go outside with a compass and compare
the direction of the compass needle with
the direction of the North Star. If possible, conduct the experiment in an open
space to avoid interference from local
magnetic fields.
I
MAGNETIC EFFECTS
Magnetite, a magnetic iron ore, is a
natural magnet. Metals of the Iron Family
and special metallic alloys and oxides are
strongly attracted by magnets; they have
ferromagnetic properties. Very strong
magnets are made from ferromagnetic
substances. Materials that are not ferromagnetic are commonly said to be nonmagnetic. Ferromagnetic materials have
high permeabilities. Nonmagnetic materials in general are transparent to magnetic
flux. These materials may be very feebly
diamagnetic or paramagnetic. Magnetism
is explained by the domain theory.
Coulomb's law for magnetism is a
quantitative expression for the force acting between two magnetic poles. A magnetic field, and its influence on a fictitious
N pole, shows similarities to an electric
field and its influence on a positive test
charge.
1-
1-
I
VOCABUURY
ammeter
Ampere's rule
Coulomb's law of
magnetism
Curie point
declination
diamagnetism
dipping needle
..
.....
SUMMARY
489
...
A charge in motion is surrounded by a
magnetic field. The core of a coil carrying
an electric current becomes a magnet.
Strong electromagnets are produced by
winding a conducting coil around a ferromagnetic core. The strength of an electromagnet depends on the number of turns
of coil and the magnitude of the current
in the coil.
The galvanometer is the basic meter for
d-c measurements.
The galvanometer can
be calibrated as a voltmeter by placing a
high resistance in series with the galvanometer coil. It can be calibrated as an
ammeter by placing a very low resistance
shunt across the galvanometer coil. An
ohmmeter requires a source of emf, an
adjustable resistance, and a sensitive
ammeter. It is essentially a voltmeterammeter method of measuring the resistance of a circuit component.
.-
I.U.
domain
electron pair
ferromagnetism
flux density
galvanometer
induced magnetism
line of flux
magnetic induction
magnetosphere
ohmmeter
paramagnetism
permeability
solar wind
solenoid
voltmeter