Sabtu, 21 November 2009

Magnetism

The study of magnetism is a science in itself. Electrical and magnetic phe- nomena interact; a detailed study of magnetism and electromagnetism could easily fill a book. Magnetism exists whenever electric charges move relative to other objects or relative to a frame of reference.

Geomagnetism

The Earth has a core made up largely of iron heated to the extent that some of it is liquid. As the Earth rotates, the iron flows in complex ways. This flow gives rise to a huge magnetic field, called the geomagnetic field, that surrounds the Earth.

EARTH’S MAGNETIC POLES AND AXIS

The geomagnetic field has poles, as a bar magnet does. These poles are near, but not at, the geographic poles. The north geomagnetic pole is located
in the frozen island region of northern Canada. The south geomagnetic pole
is in the ocean near the coast of Antarctica. The geomagnetic axis is thus somewhat tilted relative to the axis on which the Earth rotates. Not only this, but the geomagnetic axis does not exactly run through the center of the Earth. It’s like an apple core that’s off center.

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SOLAR WIND

Charged particles from the Sun, constantly streaming outward through the solar system, distort the geomagnetic field. This solar wind in effect
“blows” the field out of shape. On the side of the Earth facing the Sun, the field is compressed; on the side of the Earth opposite the Sun, the field is stretched out. This effect occurs with the magnetic fields around the other planets, too, notably Jupiter.
As the Earth rotates, the geomagnetic field does a complex twist-and- turn dance into space in the direction facing away from the Sun. At and near the Earth’s surface, the field is nearly symmetrical with respect to the geo- magnetic poles. As the distance from the Earth increases, the extent of geomagnetic-field distortion increases.

THE MAGNETIC COMPASS

The presence of the Earth’s magnetic field was noticed in ancient times. Certain rocks, called lodestones, when hung by strings, always orient them- selves in a generally north-south direction. Long ago this was correctly attributed to the presence of a “force” in the air. It was some time before the reasons for this phenomenon were known, but the effect was put to use by seafarers and land explorers. Today, a magnetic compass can still be a valuable navigation aid, used by mariners, backpackers, and others who travel far from familiar landmarks. It can work when more sophisticated navigational devices fail.
The geomagnetic field and the magnetic field around a compass nee- dle interact so that a force is exerted on the little magnet inside the com- pass. This force works not only in a horizontal plane (parallel to the Earth’s surface) but vertically, too, in most locations. The vertical com- ponent is zero at the geomagnetic equator, a line running around the globe equidistant from both geomagnetic poles. As the geomagnetic lat- itude increases toward either the north or the south geomagnetic pole, the magnetic force pulls up and down on the compass needle more and more. The extent of this vertical component at any particular location is called the inclination of the geomagnetic field at that location. You have noticed this when you hold a compass. One end of the needle seems to insist on touching the compass face, whereas the other end tilts up toward the glass.

Magnetic Force

As children, most of us discovered that magnets “stick” to some metals. Iron, nickel, and alloys containing either or both of these elements are known as ferromagnetic materials. Magnets exert force on these metals. Magnets generally do not exert force on other metals unless those metals carry electric currents. Electrically insulating substances never attract mag- nets under normal conditions.

CAUSE AND STRENGTH

When a magnet is brought near a piece of ferromagnetic material, the atoms in the material become lined up so that the metal is temporarily mag- netized. This produces a magnetic force between the atoms of the ferro- magnetic substance and those in the magnet.
If a magnet is near another magnet, the force is even stronger than it is when the same magnet is near a ferromagnetic substance. In addition, the force can be either repulsive (the magnets repel, or push away from each other) or attractive (the magnets attract, or pull toward each other) depend- ing on the way the magnets are turned. The force gets stronger as the mag- nets are brought closer and closer together.
Some magnets are so strong that no human being can pull them apart if they get “stuck” together, and no person can bring them all the way together against their mutual repulsive force. This is especially true of electromag- nets, discussed later in this chapter. The tremendous forces available are of use in industry. A huge electromagnet can be used to carry heavy pieces of scrap iron or steel from place to place. Other electromagnets can provide sufficient repulsion to suspend one object above another. This is called magnetic levitation.

ELECTRIC CHARGE CARRIERS IN MOTION

Whenever the atoms in a ferromagnetic material are aligned, a magnetic field exists. A magnetic field also can be caused by the motion of electric charge carriers either in a wire or in free space.
The magnetic field around a permanent magnet arises from the same cause as the field around a wire that carries an electric current. The responsible

Electricity, Magnetism, and Electronics

factor in either case is the motion of electrically charged particles. In a
wire, the electrons move along the conductor, being passed from atom to atom. In a permanent magnet, the movement of orbiting electrons occurs in such a manner that an “effective current” is produced by the way the elec- trons move within individual atoms.
Magnetic fields can be produced by the motion of charged particles through space. The Sun is constantly ejecting protons and helium nuclei. These particles carry a positive electric charge. Because of this, they pro- duce “effective currents” as they travel through space. These currents in turn generate magnetic fields. When these fields interact with the Earth’s geomagnetic field, the particles are forced to change direction, and they are accelerated toward the geomagnetic poles.
If there is an eruption on the Sun called a solar flare, the Sun ejects more charged particles than normal. When these arrive at the Earth’s geomag- netic poles, their magnetic fields, collectively working together, can disrupt the Earth’s geomagnetic field. Then there is a geomagnetic storm. Such an event causes changes in the Earth’s ionosphere, affecting long-distance radio communications at certain frequencies. If the fluctuations are intense enough, even wire communications and electrical power transmission can be interfered with. Microwave transmissions generally are immune to the effects of geomagnetic storms. Fiberoptic cable links and free-space laser communications are not affected. Aurora (northern or southern lights) are frequently observed at night during geomagnetic storms.

LINES OF FLUX

Physicists consider magnetic fields to be comprised of flux lines, or lines of flux. The intensity of the field is determined according to the number of flux lines passing through a certain cross section, such as a centimeter squared (cm2) or a meter squared (m2). The lines are not actual threads in space, but it is intuitively appealing to imagine them this way, and their presence can be shown by simple experimentation.
Have you seen the classical demonstration in which iron filings are placed on a sheet of paper, and then a magnet is placed underneath the paper? The filings arrange themselves in a pattern that shows, roughly, the
“shape” of the magnetic field in the vicinity of the magnet. A bar magnet has a field whose lines of flux have a characteristic pattern (Fig. 14-1).
Another experiment involves passing a current-carrying wire through the paper at a right angle. The iron filings become grouped along circles



Fig. 14-1. Magnetic flux around a bar magnet.

centered at the point where the wire passes through the paper. This shows
that the lines of flux are circular as viewed through any plane passing through the wire at a right angle. The flux circles are centered on the axis of the wire, or the axis along which the charge carriers move (Fig. 14-2).

POLARITY

A magnetic field has a direction, or orientation, at any point in space near a current-carrying wire or a permanent magnet. The flux lines run parallel to the direction of the field. A magnetic field is considered to begin, or originate
PART 2 Electricity, Magnetism, and Electronics

Fig. 14-2. Magnetic flux produced by charge carriers traveling in a straight line.

at a north pole and to end, or terminate, at a south pole. These poles are not
the same as the geomagnetic poles; in fact, they are precisely the opposite! The north geomagnetic pole is in reality a south pole because it attracts the north poles of magnetic compasses. Similarly, the south geomagnetic pole is
a north pole because it attracts the south poles of compasses. In the case of a permanent magnet, it is usually, but not always, apparent where the magnetic poles are located. With a current-carrying wire, the magnetic field goes around and around endlessly, like a dog chasing its own tail.
A charged electric particle, such as a proton, hovering in space, is an
electric monopole, and the electrical flux lines around it aren’t closed. A
CHAPTER 14 Magnetism 351


positive charge does not have to be mated with a negative charge. The elec-
trical flux lines around any stationary charged particle run outward in all directions for a theoretically infinite distance. However, a magnetic field is different. Under normal circumstances, all magnetic flux lines are closed loops. With permanent magnets, there is always a starting point (the north pole) and an ending point (the south pole). Around the current-carrying wire, the loops are circles. This can be seen plainly in experiments with iron filings on paper.

DIPOLES AND MONOPOLES

You might at first think that the magnetic field around a current-carrying wire is caused by a monopole or that there aren’t any poles at all because the concentric circles apparently don’t originate or terminate anywhere. However, think of any geometric plane containing the wire. A magnetic dipole, or pair of opposite magnetic poles, is formed by the lines of flux going halfway around on either side. There in effect are two such “mag- nets” stuck together. The north poles and the south poles are thus not points but rather faces of the plane backed right up against each other.
The lines of flux in the vicinity of a magnetic dipole always connect the two poles. Some flux lines are straight in a local sense, but in a larger sense they are always curves. The greatest magnetic field strength around a bar magnet is near the poles, where the flux lines converge. Around a current- carrying wire, the greatest field strength is near the wire.




Magnetic Field Strength

The overall magnitude of a magnetic field is measured in units called webers, symbolized Wb. A smaller unit, the maxwell (Mx), is sometimes used if a magnetic field is very weak. One weber is equivalent to 100 mil- lion maxwells. Thus 1 Wb 108 Mx, and 1 Mx 10 8 Wb.


THE TESLA AND THE GAUSS

If you have a permanent magnet or electromagnet, you might see its
“strength” expressed in terms of webers or maxwells. More often, though,

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you’ll hear or read about units called teslas (T) or gauss (G). These units
are expressions of the concentration, or intensity, of the magnetic field within a certain cross section. The flux density, or number of “flux lines per unit cross-sectional area,” is a more useful expressions for magnetic effects than the overall quantity of magnetism. Flux density is customarily denoted
B in equations. A flux density of 1 tesla is equal to 1 weber per meter squared (1 Wb/m2). A flux density of 1 gauss is equal to 1 maxwell per cen- timeter squared (1 Mx/cm2). It turns out that the gauss is equivalent to exactly 0.0001 tesla. That is, 1 G 10 4 T, and 1 T 104 G. To convert from teslas to gauss (not gausses!), multiply by 104; to convert from gauss
to teslas, multiply by 10 4.
If you are confused by the distinctions between webers and teslas or between maxwells and gauss, think of a light bulb. Suppose that a lamp emits 20 W of visible-light power. If you enclose the bulb completely, then
20 W of visible light strike the interior walls of the chamber, no matter how large or small the chamber. However, this is not a very useful notion of the brightness of the light. You know that a single bulb gives plenty of light for
a small walk-in closet but is nowhere near adequate to illuminate a gymna- sium. The important consideration is the number of watts per unit area. When we say the bulb gives off a certain number of watts of visible light, it’s like saying a magnet has an overall magnetism of so many webers or maxwells. When we say that the bulb produces a certain number of watts per unit area, it’s analogous to saying that a magnetic field has a flux den- sity of so many teslas or gauss.



THE AMPERE-TURN AND THE GILBERT

When working with electromagnets, another unit is employed. This is the ampere-turn (At). It is a unit of magnetomotive force. A wire bent into a cir- cle and carrying 1 A of current produces 1 At of magnetomotive force. If the wire is bent into a loop having 50 turns, and the current stays the same, the resulting magnetomotive force becomes 50 times as great, that is, 50 At.
If the current in the 50-turn loop is reduced to 1/50 A or 20 mA, the mag- netomotive force goes back down to 1 At.
A unit called the gilbert is sometimes used to express magnetomotive force. This unit is equal to about 1.256 At. To approximate ampere-turns when the number of gilberts is known, multiply by 1.256. To approxi- mate gilberts when the number of ampere-turns is known, multiply by
0.796.
CHAPTER 14 Magnetism 353


FLUX DENSITY VERSUS CURRENT
In a straight wire carrying a steady direct current surrounded by air or by free space (a vacuum), the flux density is greatest near the wire and dimin- ishes with increasing distance from the wire. You ask, “Is there a formula that expresses flux density as a function of distance from the wire?” The answer is yes. Like all formulas in physics, it is perfectly accurate only under idealized circumstances.
Consider a wire that is perfectly thin, as well as perfectly straight. Suppose that it carries a current of I amperes. Let the flux density (in teslas) be denoted B. Consider a point P at a distance r (in meters) from the wire, as measured along the shortest possible route (that is, within a plane perpendicular to the wire). This is illustrated in Fig. 14-3. The following formula applies:
B 2 10 7 (I/r)

In this formula, the value 2 can be considered mathematically exact to any desired number of significant figures.
As long as the thickness of the wire is small compared with the distance
r from it, and as long as the wire is reasonably straight in the vicinity of the point P at which the flux density is measured, this formula is a good indi- cator of what happens in real life.

PROBLEM 14-1
What is the flux density in teslas at a distance of 20 cm from a straight, thin
wire carrying 400 mA of direct current?

SOLUTION 14-1
First, convert everything to units in the International System (SI). This means
that r 0.20 m and I 0.400 A. Knowing these values, plug them directly into the formula:
B 2 10 7 (I/r)
2.00 10 7 (0.400/0.20)
4.0 10 7 T

PROBLEM 14-2
In the preceding scenario, what is the flux density Bgauss (in gauss) at point P?
SOLUTION 14-2
To figure this out, we must convert from teslas to gauss. This means that we must multiply the answer from the preceding problem by 104:
Bgauss
4.0
10

4.0
10
7 4
10
3 G

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Fig. 14-3. Flux density varies inversely with the distance from a wire carrying direct current.


Electromagnets

Any electric current, or movement of charge carriers, produces a magnetic field. This field can become intense in a tightly coiled wire having many turns and carrying a large electric current. When a ferromagnetic rod, called a core, is placed inside the coil, the magnetic lines of flux are con- centrated in the core, and the field strength in and near the core becomes tremendous. This is the principle of an electromagnet (Fig. 14-4).
Electromagnets are almost always cylindrical in shape. Sometimes the cylinder is long and thin; in other cases it is short and fat. Whatever the
CHAPTER 14 Magnetism

Fig. 14-4. A simple electromagnet.

ratio of diameter to length for the core, however, the principle is always the
same: The flux produced by the current temporarily magnetizes the core.


DIRECT-CURRENT TYPES

You can build a dc electromagnet by taking a large iron or steel bolt (such as a stove bolt) and wrapping a couple of hundred turns of wire around it. These items are available in almost any hardware store. Be sure the bolt is made of ferromagnetic material. (If a permanent magnet “sticks” to the bolt, the bolt is ferromagnetic.) Ideally, the bolt should be at least 3 8 inch
in diameter and several inches long. You must use insulated or enameled wire, preferably made of solid, soft copper. “Bell wire” works well.
Be sure that all the wire turns go in the same direction. A large 6-V
“lantern battery” can provide plenty of dc to operate the electromagnet. Never leave the coil connected to the battery for more than a few seconds at
a time. And do not—repeat, do not—use an automotive battery for this exper- iment. The near-short-circuit produced by an electromagnet can cause the acid from such a battery to violently boil out, and this acid is dangerous stuff.

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Direct-current electromagnets have defined north and south poles, just
like permanent magnets. The main difference is that an electromagnet can get much stronger than any permanent magnet. You should see evidence of this if you do the preceding experiment with a large enough bolt and enough turns of wire. Another difference between an electromagnet and a permanent magnet is the fact that in an electromagnet, the magnetic field exists only as long as the coil carries current. When the power source is removed, the magnetic field collapses. In some cases, a small amount of residual magnetism remains in the core, but this is much weaker than the magnetism generated when current flows in the coil.


ALTERNATING-CURRENT TYPES

You might get the idea that the electromagnet can be made far stronger if, rather than using a lantern battery for the current source, you plug the wires into a wall outlet. In theory, this is true. In practice, you’ll blow the fuse or circuit breaker. Do not try this. The electrical circuits in some buildings are not adequately protected, and a short circuit can create a fire hazard. Also, you can get a lethal shock from the 117-V utility mains. (Do this experi- ment in your mind, and leave it at that.)
Some electromagnets use 60-Hz ac. These magnets “stick” to ferromag- netic objects. The polarity of the magnetic field reverses every time the direction of the current reverses; there are 120 fluctuations, or 60 complete north-to-south-to-north polarity changes, every second (Fig. 14-5). If a per- manent magnet is brought near either “pole” of an ac electromagnet of the same strength, there is no net force resulting from the ac electromagnetism because there is an equal amount of attractive and repulsive force between the alternating magnetic field and the steady external field. However, there
is an attractive force between the core material and the nearby magnet pro- duced independently of the alternating magnetic field resulting from the ac
in the coil.

PROBLEM 14-3
Suppose that the frequency of the ac applied to an electromagnet is 600 Hz
instead of 60 Hz. What will happen to the interaction between the alternating magnetic field and a nearby permanent magnet of the same strength?

SOLUTION 14-3
Assuming that no change occurs in the behavior of the core material, the
situation will be the same as is the case at 60 Hz or at any other ac frequency.

Magnetism


Fig. 14-5. Polarity change in an ac electromagnet.

Magnetic Materials

Some substances cause magnetic lines of flux to bunch closer together than they are in the air; other materials cause the lines of flux to spread farther apart. The first kind of material is ferromagnetic. Substances of this type are, as we have discussed already, “magnetizable.” The other kind of mate- rial is called diamagnetic. Wax, dry wood, bismuth, and silver are examples of substances that decrease magnetic flux density. No diamagnetic material reduces the strength of a magnetic field by anywhere near the factor that ferromagnetic substances can increase it.
The magnetic characteristics of a substance or medium can be quantified in two important but independent ways: permeability and retentivity.


PERMEABILITY

Permeability, symbolized by the lowercase Greek mu ( ), is measured on a scale relative to a vacuum, or free space. A perfect vacuum is assigned, by

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convention, a permeability figure of exactly 1. If current is forced through a
wire loop or coil in air, then the flux density in and around the coil is about the same as it would be in a vacuum. Therefore, the permeability of pure air
is about equal to 1. If you place an iron core in the coil, the flux density increases by a factor ranging from a few dozen to several thousand times, depending on the purity of the iron. The permeability of iron can be as low as about 60 (impure) to as high as about 8,000 (highly refined).
If you use special metallic alloys called permalloys as the core material
in electromagnets, you can increase the flux density, and therefore the local strength of the field, by as much as 1 million (106) times. Such substances thus have permeability as great as 106.
If, for some reason, you feel compelled to make an electromagnet that
is as weak as possible, you can use dry wood or wax for the core material. Usually, however, diamagnetic substances are used to keep magnetic objects apart while minimizing the interaction between them.


RETENTIVITY

Certain ferromagnetic materials stay magnetized better than others. When
a substance such as iron is subjected to a magnetic field as intense as it can handle, say, by enclosing it in a wire coil carrying a high current, there will be some residual magnetism left when the current stops flowing in the coil. Retentivity, also sometimes called remanence, is a measure of how well a substance can “memorize” a magnetic field imposed on it and thereby become a permanent magnet.
Retentivity is expressed as a percentage. If the maximum possible flux density in a material is x teslas or gauss and then goes down to y teslas or gauss when the current is removed, the retentivity Br of that material is given by the following formula:

Br 100y/x
What is meant by maximum possible flux density in the foregoing defi- nition? This is an astute question. In the real world, if you make an elec- tromagnet with a core material, there is a limit to the flux density that can be generated in that core. As the current in the coil increases, the flux den- sity inside the core goes up in proportion—for awhile. Beyond a certain point, however, the flux density levels off, and further increases in current do not produce any further increase in the flux density. This condition is called core saturation. When we determine retentivity for a material, we
CHAPTER 14 Magnetism 359


are referring to the ratio of the flux density when it is saturated and the flux
density when there is no magnetomotive force acting on it.
As an example, suppose that a metal rod can be magnetized to 135 G when it is enclosed by a coil carrying an electric current. Imagine that this is the maximum possible flux density that the rod can be forced to have. For any substance, there is always such a maximum; further increasing the current in the wire will not make the rod any more magnetic. Now suppose that the cur- rent is shut off and that 19 G remain in the rod. Then the retentivity Br is
Br 100 19/135 100 0.14 14 percent
Certain ferromagnetic substances have good retentivity and are excellent for making permanent magnets. Other ferromagnetic materials have poor retentivity. They can work well as the cores of electromagnets, but they do not make good permanent magnets. Sometimes it is desirable to have a sub- stance with good ferromagnetic properties but poor retentivity. This is the case when you want to have an electromagnet that will operate from dc so that it maintains a constant polarity but that will lose its magnetism when the current is shut off.
If a ferromagnetic substance has poor retentivity, it’s easy to make it work as the core for an ac electromagnet because the polarity is easy to switch. However, if the retentivity is high, the material is “magnetically sluggish” and has trouble following the current reversals in the coil. This sort of stuff doesn’t function well as the core of an ac electromagnet.

PROBLEM 14-4
Suppose that a metal rod is surrounded by a coil and that the magnetic flux
density can be made as great as 0.500 T; further increases in current cause no further increase in the flux density inside the core. Then the current is removed; the flux density drops to 500 G. What is the retentivity of this core material?

SOLUTION 14-4
First, convert both flux density figures to the same units. Remember that 1 T
104 G. Thus the flux density is 0.500 104 5,000 G with the current and
500 G without the current. “Plugging in” these numbers gives us this:

Br 100 500/5,000 100 0.100 10.0 percent


PERMANENT MAGNETS

Any ferromagnetic material, or substance whose atoms can be aligned per- manently, can be made into a permanent magnet. These are the magnets

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you played with as a child (and maybe still play with when you use them
to stick notes to your refrigerator door). Some alloys can be made into stronger permanent magnets than others.
One alloy that is especially suited to making strong permanent magnets
is known by the trade name Alnico. This word derives from the chemical symbols of the metals that comprise it: aluminum (Al), nickel (Ni), and cobalt (Co). Other elements are sometimes added, including copper and titanium. However, any piece of iron or steel can be magnetized to some extent. Many technicians use screwdrivers that are slightly magnetized so that they can hold onto screws when installing or removing them from hard-to-reach places.
Permanent magnets are best made from materials with high retentivity. They are made by using the material as the core of an electromagnet for an extended period of time. If you want to magnetize a screwdriver a lit- tle bit so that it will hold onto screws, stroke the shaft of the screwdriver with the end of a bar magnet several dozen times. However, take note: Once you have magnetized a tool, it is practically impossible to completely demagnetize it.


FLUX DENSITY INSIDE A LONG COIL

Suppose that you have a long coil of wire, commonly known as a solenoid,
with n turns and whose length in meters is s. Suppose that this coil carries
a direct current of I amperes and has a core whose permeability is . The flux density B in teslas inside the core, assuming that it is not in a state of saturation, can be found using this formula:
B 4p 10 7 ( nI/s)

A good approximation is
B 1.2566 10 6 ( nI/s)


PROBLEM 14-5
Consider a dc electromagnet that carries a certain current. It measures 20 cm
long and has 100 turns of wire. The flux density in the core, which is known not to be in a state of saturation, is 20 G. The permeability of the core mate- rial is 100. What is the current in the wire?

SOLUTION 14-5
As always, start by making sure that all units are correct for the formula that
will be used. The length s is 20 cm, that is, 0.20 m. The flux density B is 20
G, which is 0.0020 T. Rearrange the preceding formula so it solves for I:
CHAPTER 14 Magnetism 361


B
1.2566
10
B/I
1.2566
10
6 ( nI/s)
6 ( n/s)
I 1 1.2566 10 6 ( n/sB)
5
I 7.9580 10
(sB/ n)

This is an exercise, but it is straightforward. Derivations such as this are subject to the constraint that we not divide by any quantity that can attain a value of zero in a practical situation. (This is not a problem here. We aren’t concerned with scenarios involving zero current, zero turns of wire, permeability of zero, or coils having zero length.) Let’s “plug in the numbers”:
I 7.9580 105 (0.20 0.0020)/(100 100)
7.9580 105 4.0 10 8

0.031832 A 31.832 mA

This must be rounded off to 32 mA because we are only entitled to claim two significant figures.




Magnetic Machines

A solenoid, having a movable ferromagnetic core, can do various things. Electrical relays, bell ringers, electric “hammers,” and other mechanical devices make use of the principle of the solenoid. More sophisticated elec- tromagnets, sometimes in conjunction with permanent magnets, can be used to build motors, meters, generators, and other devices.


A RINGER DEVICE

Figure 14-6 is a simplified diagram of a bell ringer. Its solenoid is an elec- tromagnet. The core has a hollow region in the center, along its axis, through which a steel rod passes. The coil has many turns of wire, so the electromagnet is powerful if a substantial current passes through the coil. When there is no current flowing in the coil, the rod is held down by the force of gravity. When a pulse of current passes through the coil, the rod is pulled forcibly upward. The magnetic force “wants” the ends of the rod,

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Steel plate(ringer)



Fig. 14-6. A bell ringer using a solenoid.

which is the same length as the core, to be aligned with the ends of the core.
However, the pulse is brief, and the upward momentum is such that the rod passes all the way through the core and strikes the ringer plate. Then the steel rod falls back down again to its resting position, allowing the plate to reverberate. Some office telephones are equipped with ringers that produce this noise rather than conventional ringing, buzzing, beeping, or chirping emitted by most phone sets. The “gong” sound is less irritating to some people than other attention-demanding signals.


A RELAY

In some electronic devices, it is inconvenient to place a switch exactly where it should be. For example, you might want to switch a communica-
CHAPTER 14 Magnetism 363


tions line from one branch to another from a long distance away. In wire-
less transmitters, some of the wiring carries high-frequency alternating cur- rents that must be kept within certain parts of the circuit and not routed out
to the front panel for switching. A relay makes use of a solenoid to allow remote-control switching.
A drawing and a diagram of a relay are shown in Fig. 14-7. The movable lever, called the armature, is held to one side by a spring when there is no current flowing through the electromagnet. Under these conditions, termi- nal X is connected to terminal Y but not to terminal Z. When a sufficient






Fig. 14-7. (a) Pictorial drawing of a simple relay. (b) Schematic symbol for the same relay.

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current is applied, the armature is pulled over to the other side. This dis-
connects terminal X from terminal Y and connects X to Z.
There are numerous types of relays, each used for a different purpose. Some are meant for use with dc, and others are for ac; some will work with either ac or dc. A normally closed relay completes a circuit when there is no current flowing in its electromagnet and breaks the circuit when current flows. A normally open relay is just the opposite. (Normal in this sense means “no current in the coil.”) The relay shown in Fig. 14-7 can be used either as a normally open or normally closed relay depending on which contacts are selected. It also can be used to switch a line between two dif- ferent circuits.
These days, relays are used only in circuits and systems carrying extreme currents or voltages. In most ordinary applications, electronic semiconduc- tor switches, which have no moving parts and can last far longer than relays, are preferred.


THE DC MOTOR

Magnetic fields can produce considerable mechanical forces. These forces can be harnessed to do work. The device that converts dc energy into rotat- ing mechanical energy is a dc motor. In this sense, a dc motor is a form of transducer. Motors can be microscopic in size or as big as a house. Some tiny motors are being considered for use in medical devices that actually can circulate in the bloodstream or be installed in body organs. Others can pull a train at freeway speeds.
In a dc motor, the source of electricity is connected to a set of coils producing magnetic fields. The attraction of opposite poles, and the repulsion of like poles, is switched in such a way that a constant torque, or rotational force, results. The greater the current that flows in the coils, the stronger is the torque, and the more electrical energy is needed. One set of coils, called the armature coil, goes around with the motor shaft. The other set of coils, called the field coil, is stationary (Fig. 14-8). In some motors, the field coils are replaced by a pair of permanent mag- nets. The current direction in the armature coil is reversed every half- rotation by the commutator. This keeps the force going in the same angular direction. The shaft is carried along by its own angular momen- tum so that it doesn’t come to a stop during those instants when the cur- rent is being switched in polarity.
CHAPTER 14 Magnetism

Fig. 14-8. Simplified drawing of a dc electric motor. Straight lines represent wires. Intersecting
lines indicate connections only when there is a dot at the point where the lines cross.


THE ELECTRIC GENERATOR

An electric generator is constructed somewhat like a conventional motor, although it functions in the opposite sense. Some generators also can oper- ate as motors; they are called motor/generators. Generators, like motors, are energy transducers of a special sort.
A typical generator produces ac when a coil is rotated rapidly in a strong magnetic field. The magnetic field can be provided by a pair of permanent magnets


(Fig. 14-9). The rotating shaft is driven by a gasoline-powered motor, a turbine, or some other source of mechanical energy. A commutator

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can be used with a generator to produce pulsating dc output, which can be
filtered to obtain pure dc for use with precision equipment.




Magnetic Data Storage

Magnetic fields can be used to store data in various forms. Common media for data storage include magnetic tape and the magnetic disk.


MAGNETIC TAPE

Recording tape is the stuff you find in cassette players. These days, mag- netic tape is largely obsolete, but it is still sometimes used for home enter-


Fig. 14-9. A simple type of ac generator.
Magnetism 367

tainment, especially high-fidelity (hi-fi) music and home video. It also can
be found in some high-capacity computer data storage systems.
The tape consists of millions of particles of iron oxide attached to a plastic or nonferromagnetic metal strip. A fluctuating magnetic field, produced by the recording head, polarizes these particles. As the field changes in strength next
to the recording head, the tape passes by at a constant, controlled speed. This produces regions in which the iron oxide particles are polarized in either direc- tion. When the tape is run at the same speed through the recorder in the play- back mode, the magnetic fields around the individual particles cause a fluctuating field that is detected by a pickup head. This field has the same pat- tern of variations as the original field from the recording head.
Magnetic tape is available in various widths and thicknesses for differ- ent applications. Thick-tape cassettes don’t play as long as thin-tape ones, but the thicker tape is more resistant to stretching. The speed of the tape determines the fidelity of the recording. Higher speeds are preferred for music and video and lower speeds for voice.
The data on a magnetic tape can be distorted or erased by external mag- netic fields. Therefore, tapes should be protected from such fields. Keep magnetic tape away from permanent magnets or electromagnets. Extreme heat also can damage the data on magnetic tape, and if the temperature is high enough, physical damage occurs as well.


MAGNETIC DISK

The era of the personal computer has seen the development of ever-more- compact data storage systems. One of the most versatile is the magnetic disk. Such a disk can be either rigid or flexible. Disks are available in various sizes. Hard disks (also called hard drives) store the most data and generally are found inside computer units. Diskettes are usually 3.5 inches (8.9 cm) in diameter and can be inserted and removed from digital recording/playback machines called disk drives.
The principle of the magnetic disk, on the micro scale, is the same as that of magnetic tape. But disk data is stored in binary form; that is, there are only two different ways that the particles are magnetized. This results
in almost perfect, error-free storage. On a larger scale, the disk works dif- ferently than tape because of the difference in geometry. On a tape, the information is spread out over a long span, and some bits of data are far away from others. On a disk, no two bits are ever farther apart than the diameter of the disk. Therefore, data can be transferred to or from a disk more rapidly than is possible with tape.

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A typical diskette can store an amount of digital information equivalent
to a short novel. Specialized high-capacity diskettes can store the equiva- lent of hundreds of long novels or even a complete encyclopedia.
The same precautions should be observed when handling and storing magnetic disks as are necessary with magnetic tape.





Quiz


Refer to the text in this chapter if necessary. A good score is eight correct. Answers are in the back of the book.
1. The geomagnetic field
(a) makes the Earth like a huge horseshoe magnet.
(b) runs exactly through the geographic poles.
(c) makes a compass work.
(d) makes an electromagnet work.
2. A material that can be permanently magnetized is generally said to be
(a) magnetic.
(b) electromagnetic.
(c) permanently magnetic.
(d) ferromagnetic.
3. The magnetic flux around a straight current-carrying wire
(a) gets stronger with increasing distance from the wire.
(b) is strongest near the wire.
(c) does not vary in strength with distance from the wire.
(d) consists of straight lines parallel to the wire.
4. The gauss is a unit of
(a) overall magnetic field strength.
(b) ampere-turns.
(c) magnetic flux density.
(d) magnetic power.
5. If a wire coil has 10 turns and carries 500 mA of current, what is the magne- tomotive force in ampere-turns?
(a) 5,000
(b) 50
(c) 5.0
(d) 0.02
CHAPTER 14 Magnetism 369


6. Which of the following is not generally observed in a geomagnetic storm?
(a) Charged particles streaming out from the Sun
(b) Fluctuations in the Earth’s magnetic field
(c) Disruption of electrical power transmission
(d) Disruption of microwave propagation
7. An ac electromagnet
(a) will attract only other magnetized objects.
(b) will attract iron filings.
(c) will repel other magnetized objects.
(d) will either attract or repel permanent magnets depending on the polarity.
8. A substance with high retentivity is best suited for making
(a) an ac electromagnet.
(b) a dc electromagnet.
(c) an electrostatic shield.
(d) a permanent magnet.
9. A device that reverses magnetic field polarity to keep a dc motor rotating is
(a) a solenoid.
(b) an armature coil.
(c) a commutator.
(d) a field coil.
10. An advantage of a magnetic disk, as compared with magnetic tape, for data storage and retrieval is that
(a) a disk lasts longer.
(b) data can be stored and retrieved more quickly with disks than with tapes.
(c) disks look better.
(d) disks are less susceptible to magnetic fields.

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