Module 7
Electricity and Magnetism
A. Electric Charge
How many electrical devices have you used so far today? How many are
operating around you right now? Do you realize that as you read these words, the
information sent to your brain and processed there also relies on electric charges and
electric signals? When you turn or scroll down this page, your brain will communicate to
the muscles in your hand and eyes in the same way. Electricity governs your life in ways
you probably rarely think about.
Even the properties of all matter that surrounds you— the air you breathe, the
water you drink, the clothes that protect and insulate your body—are largely determined
by electrical forces acting in and between atoms. From the latest electronic gizmo you’ve
just acquired to the “glue” that holds matter together, electricity is inextricably woven
into your life. Indeed, most of the material in the remainder of this text is connected to
electricity to one degree or another. So let’s take a closer look at what electricity is.
The word electricity comes from electron, which itself is based on the Greek word
for amber. Amber is a fossil resin that attracts bits of thread, paper, hair, and other things
after it has been rubbed with fur. You may have noticed that a plastic comb can do the
same after you run it through your hair (Figure 7.1). This phenomenon, known as the
amber effect, was documented by the ancient Greeks, but its cause remained a mystery
for more than two millennia. The results of numerous experiments, some conducted by
American scientist and statesman Benjamin Franklin, indicated that matter possessed a
“new” property not connected to mass or gravity. This property was eventually traced to
the atom and is called electric charge.
In many situations, ions are formed on the surface of a substance by the action of
friction. When a piece of amber, plastic, or hard rubber is rubbed with fur, negative ions
are formed on its surface. The contact between the fur and the material causes some of
the electrons in the atoms of the fur to be transferred to some of the atoms on the surface
of the solid. The fur acquires a net positive charge because it has fewer electrons than
protons. Similarly, the amber, plastic, or hard rubber acquires a net negative charge
because it has an excess of electrons.
Combing your hair can charge the comb in the same way. Rubbing glass with silk
causes the glass to acquire a net positive charge. Some of the electrons in the surface
atoms of the glass are transferred to the silk, which becomes negatively charged. Ion
formation by friction is a complicated phenomenon that is still not completely
understood. It is affected by many factors, including what materials are used and how
high the relative humidity is.
B. Electric Force and Coulomb’s Law
The original amber effect illustrates that electric charges can exert forces. You
may have noticed hair being drawn toward a charged comb or “static cling” between
items of clothes removed from a dryer. These are among the most common situations
illustrating this effect—two objects with opposite charges attracting each other (Figure
7.5). The negatively charged comb exerts an attractive force on the positively charged
hair. In addition, two objects with the same kind of charge (both positive or both
negative) repel each other. When two similarly charged combs are suspended from
threads, they push each other apart. Just remember this simple rule: like charges repel,
unlike charges attract.
This force between charged objects is extremely important in the physical world,
particularly at the atomic level. It is this interaction that holds atoms together and makes
it possible for them to exist. In each atom, the positively charged protons in the nucleus
exert attractive forces on the negatively charged electrons. The electric force on each
electron provides the centripetal force that keeps it in its orbit, much as the gravitational
force exerted by the Sun keeps Earth in its orbit. The forces between the atoms in many
compounds arise because opposite charges attract. For example, when salt is formed from
the elements sodium and chlorine, each sodium atom gives up an electron to a chlorine
atom. The resulting ions exert attractive forces on one another because they are
oppositely charged (Figure 7.6). Matter as we know and experience it would not exist
without the electrical force.
Perhaps it is not a surprise that Coulomb’s law has the same form as Newton’s
law of universal gravitation. After all, mass and charge are both fundamental properties
of the particles that comprise matter. We must remember, however, that the
(gravitational) force between two bodies because of their masses is always an attractive
force, whereas the (electrostatic) force between two bodies from their electric charges can
be attractive or repulsive, depending on whether or not they have opposite charges. Also,
all matter has mass and so experiences and exerts gravitational forces, whereas the
electrostatic force normally acts between objects only when there is a net charge on one
or both of them. Generally, when two objects have electric charges, the electrostatic force
between them is much stronger than the gravitational force. For example, the electrostatic
force between an electron and a proton is about 1039 times as large as the gravitational
force between them.
It is possible for a charged object to exert a force of attraction on a second object
that has no net charge. This is what happens when a charged comb is used to pick up bits
of paper or thread. Here the negatively charged comb attracts the nuclei of the atoms and
repels the electrons. The orbits of the electrons are distorted so that the electrons are, on
the average, farther away from the charged comb than the nuclei (Figure 7.8). This results
in a net attractive force because the repulsive force on the slightly more distant,
negatively charged electrons is smaller than the attractive force on the closer, positively
charged protons. The process of inducing a small charge separation (or displacement)
between the nucleus of an atom and its electrons is called polarization.
Some molecules are naturally polarized—that is, they have a net negative charge
displaced to one side of the net positive nuclear charge. They are called polar molecules.
Water molecules have this property. (As we will see in Chapter 8, that’s why microwave
ovens work so effectively.) If a polar molecule is free to rotate—as in a liquid—it will be
attracted to a charged object. Its side with the charge opposite that on the object will turn
toward the object, and the attractive force on that side will be stronger than the repulsive
force on the other side, as with the atoms previously discussed.
The electrostatic force is another example of “action at a distance.” As with
gravitation, the concept of a field is useful. In the space around any charged object, there
is an electric field. This field is the “agent” of the electrostatic force: it will cause any
charged object to experience a force. The electric field around a charged particle is
represented by lines that indicate the direction of the force that the field would exert on a
positive charge. Thus the electric field lines around a positively charged particle point
radially outward, and the field lines around a negatively charged particle point radially
inward.
Perhaps you have had the experience of walking across a carpeted floor and
receiving a shock when you touched a metal doorknob. This is more likely to happen in
winter than in summer because the relative humidity is usually lower then, and
electrostatic charging of your body as you move over (rub against) the carpet takes place
more readily. The shock results from charges flowing between you and the doorknob, and
it may be accompanied by a visible spark. Air normally does not allow charges to flow
through it. A spark occurs when there is an electric field strong enough to ionize atoms in
the air. Freed electrons accelerate in a direction opposite to the direction of the electric
field, and positive ions accelerate in the same direction as the field. The electrons and
ions pick up speed and collide with other atoms and molecules, ionizing them or causing
them to emit light (Figure 7.11). Lightning is produced in this same way on a much larger
scale, as Benjamin Franklin demonstrated using kites, keys, and metal rods in the middle
of the 18th century (see the application feature at the end of this section). In Chapter 10,
we will discuss how atomic collisions cause the emission of light.
Electronic signs that behave like electronically erasable paper make use of electric
fields to form letters and other images. One type, called SmartPaper, consists of millions
of tiny beads between two thin plastic sheets (Figure 7.14). One side of each bead is a
particular color and negatively charged, and the other side is a contrasting color and
positively charged. An electric field exerts opposite forces on the two sides (recall
FigureC7.10), causing the beads to rotate until they are aligned with the field. (As we will
see in Chapter 8, this is just like what a compass needle does in a magnetic field.) Letters
are formed on the electronic paper by selectively applying upward and downward electric
fields at different places so that parts of the display are one color and the rest are the other
color. The type of transistor used most widely in computers and similar devices is the
field-effect transistor (FET). In FETs, an electric field controls the flow of electricity
through the transistor. Electric fields play crucial roles in the operation of liquid crystal
displays (LCDs), the touchpads on laptop computers, and, as noted in the chapter
introduction, the touch-sensitive screens on iPads and iPhones.
C. Superconductivity
An electric current is a flow of charged particles. The cord on an electrical
appliance encloses two separate metal wires covered with insulation. When the appliance
is plugged in and operating, electrons inside each wire move back and forth. Inside older
television picture tubes and computer monitors, free electrons are accelerated from the
back of the unit to the screen at the front. There is a near vacuum inside the video tube so
the electrons can travel without colliding with gas molecules (Figure 7.17). When salt is
dissolved in water, the sodium and chlorine ions separate and can move about just like
the water molecules. If an electric field is applied to the water, the positive sodium ions
will flow one way (in the direction of the field), and the negative chlorine ions will flow
the other way.
A current of 5 amperes in a wire means that 5 coulombs of charge flow through
the wire each second. (Table 7.1 lists some representative currents.) Either positive
charges or negative charges can comprise a current. The effect of a positive charge
moving in one direction is the same as that of an equal negative charge moving in the
opposite direction. Formally, an electric current is represented as a flow of positive
charge. This is because it was originally believed that positive charges moved through
metals. Even after it was discovered that it is negatively charged electrons that flow in a
wire to comprise the current, the convention of defining the direction of current flow as
that which would be associated with positive charges was retained. If positive ions are
flowing to the right in a liquid, then the current is to the right. If negative charges (like
electrons) are flowing to the right, then the direction of the current is to the left.
The ease with which charges move through different substances varies greatly.
Any material that does not readily allow the flow of charges through it is called an
electrical insulator. Substances such as plastic, wood, rubber, air, and pure water are
insulators because the electrons are tightly bound in the atoms, and electric fields are
usually not strong enough to rip them free so they can move. Our lives depend on
insulators: the electricity powering the devices in our homes could kill us if insulators,
like the covering on power cords, didn’t keep it from entering our bodies. An electrical
conductor is any substance that readily allows charges to flow through it. Metals are very
good conductors because some of the electrons are only loosely bound to atoms and so
are free to “skip along” from one atom to the next when an electric field is present. In
general, solids that are good conductors of heat are also good conductors of electricity.
As mentioned before, liquids such as water are conductors when they contain dissolved
ions. Most drinking water has some natural minerals and salts dissolved in it and so
conducts electricity. Solid insulators can become conductors when wet because of ions in
the moisture. The danger of being electrocuted by electrical devices increases
dramatically when they are wet.
What makes a 100-watt lightbulb brighter than a 60-watt bulb? The size of the
current flowing through the filament determines the brightness. That, in turn, depends on
the filament’s resistance. The filament of a 100-watt bulb is thicker than that of a 60-watt
bulb, so its resistance is lower. As we will see in later sections, this means a larger current
normally flows through the 100-watt bulb, so it is brighter. Resistance can be compared
to friction. Resistance inhibits the flow of electric charge, and friction inhibits relative
motion between two substances. In metals, electrons in a current move among the atoms
and in the process collide with them and give them energy. This impedes the movement
of the electrons and causes the metal to gain internal energy. The consequence of
resistance is the same as that of kinetic friction—heating. The larger the current through a
particular device, the greater the heating.
Superconductivity seems too good to be true: electricity flowing through wires
with no loss of energy to heating. Once a current is made to flow in a loop of
superconducting wire, it can flow for years with no battery or other source of energy
because there is no energy loss from resistance. A great deal of the electrical energy that
is wasted as heat in wires could be saved if conventional conductors could be replaced
with superconductors. But the superconducting state for a given material has limitations.
Resistance returns if the temperature is raised above the superconductor’s Tc , if the
current through the substance becomes too large, or if it is placed in a magnetic field that
is too strong.
Widespread practical use of these superconductors is severely limited because
they must be kept cold using liquefied helium. Helium is very expensive and requires
sophisticated refrigeration equipment to cool and to liquefy. Once a superconducting
device is cooled to the temperature of liquid helium, bulky insulation equipment is
needed to limit the flow of heat into the helium and the superconductor. These factors
combine to make the so-called low-Tc superconductors unwieldy or uneconomical except
in certain special applications when there are no alternatives.
But hope for wider use of superconductivity blossomed beginning in 1987 when a
new family of “high-Tc ” superconductors was developed with critical temperatures that
now reach as high as about 140 K. This was an astounding breakthrough because these
materials can be made superconducting through the use of liquid nitrogen (boiling point
77 K). Liquid nitrogen is widely available, is inexpensive to produce compared to liquid
helium, and can be used with much less-sophisticated insulation. However, the new high-
Tc superconductors are handicapped by a couple of unfortunate properties: they are
brittle and consequently are not easily formed into wires, and they aren’t very tolerant of
strong magnetic fields or large electric currents. If these problems can be overcome, a
new revolution in superconducting technology will occur.
D. Electric Circuits and Ohm’s Law
An electric current will flow in a lightbulb, a laptop, or other such device only if
an electric field is present to exert a force on the charges. A conventional flashlight works
because the batteries produce an electric field that forces electrons to flow through the
lightbulb. An electric circuit is any system consisting of a battery or other electrical
power supply, some electrical device such as a lightbulb, and wires or other conductors to
carry the current to and from the device (Figure 7.20). The power supply acts like a
“charge pump”: it forces charges to flow out of one terminal, go through the rest of the
circuit, and flow into the other terminal. Electrons typically move through a circuit quite
slowly, about 1 millimeter per second. In this respect, an electric circuit is much like the
cooling system in a car in which the water pump forces coolant to flow through the
engine, radiator, and the hoses connecting them.
The concepts of energy and work are used to quantify the effect of a power supply
in a circuit. In a conventional flashlight, for instance, the batteries cause electrons to flow
through the bulb’s filament. Because a force acts on the electrons and causes them to
move through a distance, work is done on the electrons by the batteries. In other words,
the batteries give the electrons energy. This energy is converted into internal energy and
light as the electrons go through the lightbulb and heat the filament. This leads to the
concept of electric voltage.
Not all devices remain “ohmic”—that is, obey Ohm’s law—as the voltage applied
to them changes. Often, instead of remaining constant, the resistance of a conductor
changes when the voltage changes. At higher voltages, a larger current flows through the
filament of a lightbulb, so its temperature is also higher. The resistance of the hotter
filament is consequently greater (Figure 7.23). Some semiconductor devices, called
diodes, are designed to have very low resistance when current flows through them in one
direction but very high resistance when a voltage tries to produce a current in the other
direction. Water with salt dissolved in it generally has lower resistance when higher
voltages are applied to it: doubling the voltage will more than double the current. A graph
of V versus I for ordinary tap water is less steep at higher voltages.
In many situations, several electrical devices are connected to the same electrical
power supply. A house may have a hundred different lights and appliances all connected
to one cable entering the house. An automobile has dozens of devices connected to its
battery. There are two basic ways in which more than one device can be connected to a
single electrical power supply—by a series circuit and by a parallel circuit.
In a series circuit, there is only one path for the charges to follow, so the same
current flows in each device (Figure 7.24). In such a circuit, the voltage is divided among
the devices: the voltage on the first device plus the voltage on the second device, and so
on, equals the voltage of the power supply. For example, if three lightbulbs with the same
resistance are connected in series to a 12-volt battery, the voltage on each bulb is 4 volts.
If the bulbs had different resistances, each one’s “share” of the voltage would be
proportional to its resistance.
E. Power and Energy in Electric Currents
Because a battery or other electrical supply must continually put out energy to
cause a current to flow, it is important to consider the power output—the rate at which
energy is delivered to the circuit. The power is determined by the voltage of the power
supply and the current that is flowing. Think of it this way: the power output is the
amount of energy expended per unit amount of time. The power supply gives a certain
amount of energy to each coulomb of charge that flows through the circuit.
What happens to the energy delivered by an electrical power supply? In a
lightbulb, less than 5 percent is converted into visible light, and the rest becomes internal
energy. Even the visible light emitted by a lightbulb is absorbed eventually by the
surrounding matter and transformed into internal energy. (Interior lighting is actually
used to heat some buildings.) Electric motors in hair dryers, vacuum cleaners, and the
like convert about 60 percent of their energy input into mechanical work or energy, while
the remainder goes to internal energy. The mechanical energy is generally dissipated as
internal energy through friction. In a similar way, we can trace the energy conversions in
other electrical devices and the outcome is the same: most electrical energy eventually
becomes internal energy.
Ordinary metal wire converts electrical energy into internal energy whenever
there is a current flowing. You may have noticed when using a hair dryer that its cord
becomes warm. This heating, called ohmic heating, occurs in any conductor that has
resistance, even when the resistance is quite small. The huge cables used to conduct
electricity from power plants to cities are heated by this effect. This heating represents a
loss of usable energy. The temperature that a current-carrying wire reaches from ohmic
heating depends on the size of the current and on the wire’s resistance. Increasing the
current in a given wire will raise its temperature. Many devices utilize this effect. The
resistances of heating elements in toasters and electric heaters are chosen so that the
normal operating current is large enough to heat them until they glow red hot and can
toast bread or heat a room. The filament in an incandescent lightbulb is made so thin that
ohmic heating causes it to glow white hot and emit enough light to illuminate a room.
Ohmic heating is a major consideration in the design of sophisticated integrated
circuit chips. Even though the currents flowing through the tiny transistors are extremely
small, there are so many circuits in such a small space that special steps must be taken to
make sure the heat produced is conducted away. Because a superconductor has zero
resistance, there is no ohmic heating. The overall efficiencies of most electrical devices
could be improved if regular wires could be replaced by superconductors.
Superconducting transmission lines would allow electricity to be carried from a power
plant to a city with no loss of energy. The limitations of currently known
superconductors, however, make such uses impracticable.
F. AC and DC
The electric current supplied by a battery is different from the current supplied by
a normal household wall socket. Batteries supply direct current (DC), and household
outlets supply alternating current (AC). A DC power supply, such as a battery, causes a
current to flow in a fixed direction in a circuit (Figure 7.32). The current flows out of the
positive (1) terminal of the power supply, moves through the circuit, and flows into the
negative (2) terminal of the power supply. If the total resistance in the circuit doesn’t
change, the size of the current remains constant (as long as the battery doesn’t run down).
A graph of the current I versus time t is simply a horizontal line.
Some electric devices (such as lightbulbs) can operate on AC or DC, whereas
others require one or the other. Electric motors and generators must be designed to
operate on or produce either AC or DC. There are devices that can convert an AC voltage
to a DC voltage and vice versa. Batteries can produce direct current only. For this reason,
automobiles have DC electrical systems. (The “alternator” in an automobile generates
AC, which is then converted into DC to be compatible with the battery.)
Alternating current has one distinct advantage over DC: simple, highly efficient
devices called transformers can “step up” or “step down” AC voltages. This makes it
possible to generate AC at a power plant at some intermediate voltage, step it up to a very
high voltage (typically more than 300,000 volts) for economical transmission, and then
step it down again to lower voltages for use in homes and industries. There is no
counterpart of the transformer for DC. Another important use of AC is in electronic
sound equipment. One example: if a 440-hertz tone is recorded on tape and then played
back, the “signal” going to the speaker will be an alternating current with a frequency of
440 hertz.
G. Magnetism
Magnetism was first observed in a naturally occurring ore called lodestone.
Lodestones were fairly common around Magnesia, an ancient city in Asia Minor. Small
pieces of iron, nickel, and certain other metals are attracted by lodestones, much as pieces
of paper are attracted by charged plastic (Figure 8.1). The Chinese were probably the first
to discover that a piece of lodestone will orient itself north and south if suspended by a
thread or floated on water on a piece of wood. The compass revolutionized navigation
because it allowed mariners to determine the direction of north even in cloudy weather. It
was also one of the few useful applications of magnetism up to the 19th century.
Magnets today are made into a variety of sizes and shapes out of special alloys
that exhibit much stronger magnetism than lodestone. All simple magnets exhibit the
same compass effect—one end or part of it is attracted to the north, and the opposite end
or part is attracted to the south. The north-seeking part of a magnet is called its north
pole, and the south-seeking part is its south pole. All magnets have both poles. If a
magnet is broken into pieces, each part will have its own north and south poles. The south
pole of one magnet exerts a mutually attractive force on the north pole of a second
magnet. The south poles of two magnets repel each other, as do the north poles (Figure
8.2). Simply put: like poles repel, unlike poles attract just as with electric charges.
Metals that are strongly attracted by magnets are said to be ferromagnetic. Such
materials have magnetism induced in them when they are near a magnet. If a piece of
iron is brought near the south pole of a magnet, the part of the iron nearest the magnet has
a north pole induced in it, and the part farthest away has a south pole induced in it (Figure
8.3). Once the iron is removed from the vicinity of the magnet, it loses most of the
induced magnetism. Some ferromagnetic metals actually retain the magnetism induced in
them—they become permanent magnets. Common household magnets and compass
needles are made of such metals. Ferromagnetism is also the basis of magnetic data
recording, but more on this later.
As with gravitation and electrostatics, it is useful to employ the concept of a field
to represent the effect of a magnet on the space around it. A magnetic field is produced
by a magnet and acts as the agent of the magnetic force. The poles of a second magnet
experience forces when in the magnetic field: its north pole has a force in the same
direction as the magnetic field, but its south pole has a force in the opposite direction. A
compass can be thought of as a “magnetic field detector” because its needle will always
try to align itself with a magnetic field (Figure 8.4). The shape of the magnetic field
produced by a magnet can be “mapped” by noting the orientation of a compass at various
places nearby. Magnetic field lines can be drawn to show the shape of the field, just as
electric field lines are used to show the shape of an electric field (Figure 8.5). The
direction of a field line at a particular place is the direction that the north pole of a
compass needle at that location points.
Because magnets respond to magnetic fields, the fact that compass needles point
north indicates that Earth itself has a magnetic field. The shape of Earth’s field has been
mapped carefully over the course of many centuries because of the importance of
compasses in navigation. Earth’s magnetic field has the same general shape as the field
around a bar magnet, with its poles tilted about 118 with respect to the axis of rotation.
Because of the tilt of Earth’s “magnetic axis,” at most places on Earth compasses
do not point to true north. For example, in the western twothirds of the United States,
compasses point to the right (east) of true north, whereas in New England compasses
point to the left (west) of true north. The difference, in degrees, between the direction of a
compass and the direction of true north varies from place to place (and with time as well)
and is referred to as the magnetic declination. In parts of Alaska, the magnetic declination
is as high as 258 east. This must be taken into account when navigating with a compass.
Therefore, a compass’s north pole points at Earth’s south magnetic pole. This is
not a physical contradiction: it is a result of naming the poles of a magnet after directions
instead of, say, 1 and 2, or A and B. Some organisms use Earth’s magnetic field to aid in
navigation. Although the biological mechanisms that they employ have not yet been fully
identified, certain species of fish, frogs, turtles, birds, newts, and whales are able to sense
the strength of Earth’s field or its direction (or both). The former allows the animal to
determine its approximate latitude (how far north or south it is) because Earth’s magnetic
field is stronger near the magnetic poles (FigureC8.6). Some migratory species travel
thousands of miles before returning home, guided—at least in part—by sensing Earth’s
magnetic field.
H. Interactions between Electricity and Magnetism
Consider the following items that we usually take for granted: electric motors in
hair dryers, automobiles, computer disc drives, elevators, and countless other devices;
generators that produce most of the electricity we use; speakers, audio and videotape
recorders, and high-fidelity microphones; and the waves that make satellite audio
receivers, cell phones, radar, microwave ovens, medical x-rays, and our eyes work. What
do all of these have in common? They all are possible because electricity and magnetism
interact with each other in basic—and very useful— ways. The word electromagnetic,
which appears dozens of times in this chapter, is perhaps the best indication of just how
intertwined these two phenomena are.
he magnetic field produced is in the shape of circles around the path of the charge
(Figure 8.8). For a steady (DC) current, which is basically a succession of moving
charges in a wire, the field is steady, and its strength is proportional to the size of the
current and inversely proportional to the distance from the wire. (The field is quite weak
unless the current is large. A current of 10 amperes or more will produce a field strong
enough to be detected with a compass; Figure 8.9.) Reversing the direction of the current
in the wire will reverse the directions of the magnetic field lines.
Most applications of this phenomenon use coils—long wires wrapped in the shape
of a cylinder, often around an iron core. The magnetism induced in the iron greatly
enhances the magnetic field of the coil. The magnetic field of such a coil (when carrying
a direct current) has the same shape as the field around a bar magnet. (Figure 8.10 and
compare it to FigureC8.5.) This device is an electromagnet. It behaves just like a
permanent magnet as long as there is a current flowing. One end of the coil is a north
pole, and the other is a south pole. Electromagnets have an advantage over permanent
magnets in that the magnetism can be “turned off” simply by switching off the current.
A coil with a length that is much greater than its diameter is called a solenoid. If
an iron rod is partially inserted into a solenoid with a hollow core, the rod will be pulled
in when the current is switched on: the magnetic pole associated with the coil’s field
nearest the rod induces a magnetic field with the opposite polarity in the rod, thus
exerting an attractive force on it (Figure 8.11). Solenoids are used in common devices for
striking doorbell chimes, opening valves to allow water to enter and to leave washing
machines, withdrawing deadbolts in electric door locks, and engaging starter motors on
car and truck engines.
The polarity of an electromagnet is reversed if the direction of the current is
reversed (FigureC8.10b). An alternating current in a coil will produce a magnetic field that
oscillates: it increases, decreases, and switches polarity with the same frequency as the
current. Such an oscillating magnetic field will cause a nearby piece of iron to vibrate.
The oscillating magnetic field of a coil with AC in it is used in many common devices, as
we shall see in the following sections. This first interaction not only explains how
electromagnets work, but also gives us new insight into permanent magnets as well.
Because electrons in atoms are charged particles in motion about the nucleus, they
produce magnetic fields. Also, the electrons have their own magnetic fields associated
with their spin (more on this in Chapter 12). In any unmagnetized material, the individual
magnetic fields of the electrons are randomly oriented and cancel each other out (Figure
8.13). In ferromagnetic materials, these fields can be aligned with one another by an
external magnetic field; the material then produces a net magnetic field. So we can
conclude that moving electric charges are the causes of magnetic fields even in ordinary
bar and horseshoe magnets.
A stationary electric charge is not affected by a magnetic field, but a moving
charge usually is. Note that this second observation is a logical consequence of the first:
anything that produces a magnetic field will itself be affected by other magnetic fields.
Caveat: If a charge’s velocity or the direction of a current is parallel to that of the
magnetic field or in the opposite direction, the magnetic field does not exert this force. A
curious characteristic of electromagnetic phenomena is that the effects are often
perpendicular to the causes. The direction of the magnetic field from a current-carrying
wire is perpendicular to the direction the current is flowing (FigureC8.8). Similarly, the
force that a magnetic field exerts on a moving charge or on a current-carrying wire is
perpendicular to both the direction of the magnetic field and the direction the charge is
flowing. For example, if a horizontal magnetic field is directed away from you and a wire
is carrying a current to your right, the force on the wire is upward.
The third observed interaction between electricity and magnetism is used by
electric generators. Recall that the first observation tells us that moving charges create
magnetic fields. The third one is a similar statement about moving magnets. The electric
field around a moving magnet is in the shape of circles around the path of the magnet.
This circular electric field will force charges in a coil of wire to move in the same
direction—as a current (Figure 8.17). The process of inducing an electric current with a
magnetic field is known as electromagnetic induction. All that is required is that the
magnet and coil move relative to each other. If the coil moves and the magnet remains
stationary, a current is induced. If the motion is steady in either case, the induced current
is in one direction. If either the coil or the magnet oscillates back and forth, the current
alternates with the same frequency—it is AC.
Another application of this technology is regenerative braking, which is used in
electric and hybrid vehicles. While accelerating and cruising, electric motors turn the
wheels using electricity from batteries. During braking, the motors function as
generators: the wheels turn them, and the electricity that is generated can partially
recharge the batteries. Instead of all of the vehicle’s kinetic energy being converted into
wasted heat—the case with conventional friction brakes—some of it is saved for reuse.
In summary, when electric charges or magnets are in motion, electricity and
magnetism are no longer independent phenomena. The three observations given here are
statements of experimental facts that illustrate this interdependence. They can be
demonstrated easily using a battery, wires, a compass, a large magnet, and a sensitive
ammeter. The fact that electricity and magnetism interact only when there is motion (and
then the effects are perpendicular to the causes) is somewhat startling when compared to,
say, gravitation and electrostatics. As we saw in Chapters 2 and 7, gravitational and
electrostatic forces are always toward or away from the objects causing them, and they
act whether or not anything is moving or changing. These basic yet surprising
interactions between electricity and magnetism are crucial to our modern electrified
society.
I. Principles of Electromagnetism
These two statements summarize the previous observations and also emphasize
the symmetry that exists. In both cases, a “changing” field means that the strength or the
direction (or both) of the field is changing. The first principle can be used to explain the
first observation: as a charge moves past a point in space, the strength of the electric field
increases and then decreases. All the time the direction of the field is changing as well
(Figure 8.19). The effect of this is to cause a magnetic field to be produced. Similarly, the
second principle explains electromagnetic induction.
In addition to being used to change voltages in electrical distribution systems,
transformers are found in a wide variety of electrical appliances. Most electrical
components used in audio systems, calculators, and the like require voltages that are
much smaller than 120 volts. Appliances designed to operate on household AC must
include transformers to reduce the voltage accordingly. High-intensity desk lamps also
use transformers (Figure 8.21). The spark used to ignite gasoline in automobile engines is
generated using a type of transformer called a “coil.” The number of turns in the output
coil is many times the number of turns in the input coil. A spark is produced by first
sending a brief current into the input. A magnetic field is produced that quickly
disappears. This induces a very high voltage (around 25,000 volts) in the output, which is
conducted to the spark plugs to ignite the fuel.
Understanding electromagnetism allows us to better appreciate how the metal
detector introduced at the start of the chapter work. The magnetic pulses are produced by
sending an electric current through a coil of wire for a short period of time. When the
current stops, the magnetic field that was created dies out quickly, and this decreasing
field induces an electric current in the coil. This current is used to monitor how swiftly
the magnetic pulse dies out. Metals are detected because the rapidly changing magnetic
field of each pulse induces electrons in the metal to move—as in the secondary coil in a
transformer—and this current produces an opposite magnetic pulse. This change in the
total magnetic field affects the current induced in the coil. The electronics are designed to
detect any such change and signal an alarm.
J. Applications to Sound Reproduction
A hundred years or so ago, the only people who listened to music performed by
world-class musicians were those few who could attend live performances. Today, people
in the most remote corners of the world can hear concert-quality sound from large home
entertainment systems, pocket-sized or smaller MP3 players, and many devices in
between. The first Edison phonographs were strictly mechanical and did a fair job of
reproducing sound. It was the invention of electronic recording and playback machines
that brought true high fidelity to sound reproduction, however. The sequence that begins
with sound in a recording studio and ends with the reproduced sound coming from a
speaker in your home, headphones or earbuds, or car includes components that use
electromagnetism.
The key to electronic sound recording and playback is first to translate the sound
into an alternating current and then later retranslate the AC back into sound. The first step
requires a microphone, and the second step requires a speaker. Although there are several
different types of microphones, we will take a look at what is called a dynamic
microphone. It consists of a magnet surrounded by a coil of wire attached to a diaphragm
(Figure 8.22). The coil and diaphragm are free to oscillate relative to the stationary
magnet. When sound waves reach the microphone, the pressure variations in the wave
push the diaphragm back and forth, making it and the coil oscillate. Because the coil is
moving relative to the magnet, an oscillating current is induced in it. The frequency of the
AC in the coil is the same as the frequency of the diaphragm’s oscillation, which is the
same as the frequency of the original sound. That is all it takes. This type of dynamic
microphone is also referred to as a moving coil microphone. The alternative is to attach a
small magnet to the diaphragm and keep the coil stationary—a moving magnet
microphone.
The tiny speakers built into earbud earphones now commonly used with iPods and
other portable music devices operate by these same principles. They are made possible by
the use of small but powerful permanent magnets made of an alloy of the elements
neodymium, iron, and boron (NIB). The extreme strength of NIB magnets (which in
some cases can approach that of large medical MRIs) makes them capable of reproducing
a very broad range of frequencies with exceptional fidelity. Coupled with their small size,
this has made them indispensable in the design of compact earphones. Microphones and
speakers are classified as transducers: they convert mechanical oscillation from sound
into AC (microphone), or they convert AC into mechanical oscillation and sound
(speaker). They are almost identical. In fact, a microphone can be used as a speaker, and
a speaker can be used as a microphone. But, as with motors and generators, each is best at
doing what it is designed to do.
Magnetic recording is not limited to sound reproduction. Older television
videocassette recorders (VCRs) record both sound and visual images on magnetic tape.
Computers store information magnetically on tapes, disks, and hard drives. The AC
signals produced by microphones, CD players, and tape playback heads are quite weak.
Amplifiers are used to increase the power of these signals before they are sent to
speakers. Amplifiers also allow the listener to modify the sound by adjusting its loudness
with the volume control and its tone quality with the bass and treble controls.
The superior quality of digital sound comes about because the playback device
looks only for numbers. It can ignore such things as imperfections in the disc or tape, the
weak random magnetization in a tape that becomes tape hiss on cassettes, and the
mechanical vibration of motors that we hear as a rumble on phonographs. A sophisticated
error-correction system can even compensate for missing or garbled numbers. Because
the pickup device in a CD player does not touch the disc, each CD can be played over
and over without the slow deterioration in quality that results from a needle moving in a
phonograph groove or from the constant unwinding and rewinding of a cassette tape over
the recorder heads. This combination of high fidelity and disc durability made the CD
system an immediate hit with consumers.
K. Electromagnetic Waves
As the name implies, EM waves involve both electricity and magnetism. The
existence of these waves was first suggested by 19th century physicist James Clerk
Maxwell while he was analyzing the interactions between electricity and magnetism.
Consider the two principles of electromagnetism stated in Section 8.3. Let’s say that an
oscillating electric field is produced at some place. The electric field switches back and
forth in direction while its strength varies accordingly. This oscillating electric field will
induce an oscillating magnetic field in the space around it. But the oscillating magnetic
field will then induce an oscillating electric field. This will then induce an oscillating
magnetic field and so on in an endless “loop”: the principles of electromagnetism tell us
that a continuous succession of oscillating magnetic and electric fields will be produced.
These fields travel as a wave—an EM wave.
Most electromagnetic waves are transverse waves because the oscillation of both
of the fields is perpendicular to the direction the wave travels, although some EM waves
in plasmas can be longitudinal. Figure 8.28 shows a “snapshot” of a transverse EM wave
traveling to the right. (The three axes are perpendicular to each other.) In this particular
case, the electric field is vertical. As the wave travels by a given point in space, the
electric field oscillates up and down, the way a floating petal oscillates on a water wave.
The magnetic field at the point oscillates horizontally, in and out.
Electromagnetic waves are named and classified according to frequency. In order
of increasing frequency, the groups, or “bands,” are radio waves, microwaves, infrared
radiation, visible light, ultraviolet radiation, x-rays, and gamma rays (g-rays). (Use of the
word radiation instead of waves is not significant here.) Figure 8.29 shows these groups
along with frequency and wavelength scales. This is called the electromagnetic spectrum.
Notice that the groups overlap. For example, a 1017-hertz EM wave could be ultraviolet
radiation or an x-ray. In cases of overlap, the name applied to an EM wave depends on
how it is produced.
We will briefly discuss the properties of each group of waves in the
electromagnetic spectrum—how they are produced, what their uses are, and how they can
affect us. The great diversity of uses of EM waves arises from the variety of ways in
which they can interact with different kinds of matter. All matter around us contains
charged particles (electrons and protons), so it seems logical that EM waves can affect
and be affected by matter. The oscillating electric field can cause AC currents in
conductors; it can stimulate vibration of molecules, atoms, or individual electrons; or it
can interact with the nuclei of atoms. Which sort of interaction occurs, if any, depends on
the frequency (and wavelength) of the EM wave and on the properties of the matter
through which it is traveling—its density, molecular and atomic structure, and so on.
In principle, an electromagnetic wave of any frequency could be produced by
forcing one or more charged particles to oscillate at that frequency. The oscillating field
of the charges would initiate the EM wave. The “lower-frequency” EM waves (radio
waves and microwaves) are produced this way: a transmitter generates an AC signal and
sends it to an antenna. At higher frequencies, this process becomes increasingly difficult.
Electromagnetic waves above the microwave band are produced by a variety of processes
involving molecules, atoms, and nuclei. Note that charged particles are present in all of
these processes. There is one other factor to keep in mind: electromagnetic waves are a
form of energy. Energy is needed to produce EM waves, and energy is gained by
anything that absorbs EM waves. The transfer of heat by way of radiation is one example.
Radio waves, the lowest frequency EM waves, extend from less than 100 hertz to
about 109 Hz (1 billion hertz or 1,000 megahertz; Figure 8.30). Within this range are a
number of frequency bands that have been given separate names— for example, ELF
(extremely low frequency), VHF (very high frequency), and UHF (ultrahigh frequency).
Most frequencies are given in kilohertz (kHz) or megahertz (MHz). Sometimes radio
waves are classified by wavelength: long wave, medium wave, or short wave. As
mentioned earlier, radio waves are produced using AC with the appropriate frequency.
Radio waves propagate well through the atmosphere, which makes them practical for
communication. Lower-frequency radio waves cannot penetrate the upper atmosphere, so
higher frequencies are used for space and satellite communication. Only the very lowest
frequencies can penetrate ocean water.
The next band of EM waves, with frequencies higher than those of radio waves, is
the microwave band. The frequencies extend from the upper limit of radio waves to the
lower end of the infrared band, about 109 –1012 hertz. The wavelengths range from
about 0.3 m to 0.3 mm. One use of microwaves is in communication. Bluetooth and WiFi
signals that interconnect computers, cell phones, and other devices are microwaves. Early
experiments with microwave communication led to the most important use of
microwaves, radar (radio detection and ranging), after the discovery that microwaves are
reflected by the metal in ships and aircraft. As we discussed in Section 6.2, radar is
echolocation using microwaves. The time it takes microwaves to make a round-trip from
the transmitter to the reflecting object and back is used to determine the distance to the
object. Radar systems are quite sophisticated: Doppler radar can determine the speed of
an object moving toward or away from the transmitter by measuring the frequency shift
of the reflected wave. Such radars are essential tools for air traffic control and monitoring
severe weather. During its initial four-year mission to explore Saturn and its environs, the
Cassini spacecraft used imaging radar to penetrate the dense, perpetual smog that
envelopes Titan, Saturn’s largest moon, and to map its surface topology.
Infrared radiation (IR; also called infrared light) occupies the region between
microwaves and visible light in the electromagnetic spectrum. The frequencies are from
about 1012 hertz to about 4 3 1014 hertz (400,000,000 megahertz). The wavelengths of
IR range from approximately 0.3 to 0.00075 millimeters. Infrared radiation is ordinarily
the main component of heat radiation (introduced in Section 5.4). Everything around you
is both absorbing and emitting infrared radiation, just as you are. The warmth you feel
from a fire or heat lamp is the result of your skin absorbing the IR. Infrared radiation is
constantly emitted by atoms and molecules because of their thermal vibration. Absorption
of IR by a cooler substance increases the vibration of the atoms and molecules, thus
raising the temperature.
Visible light is a very narrow band of frequencies of EM waves that happens to be
detectable by human beings. Certain specialized cells in the eye, called rods and cones,
are sensitive to EM waves in this band. They respond to visible light by transmitting
electrical signals to the brain, where a mental image is formed. (The visible ranges of
some animals such as hummingbirds and bees extend into the ultraviolet band. Some
flowers that seem plain to humans are quite attractive to these nectar eaters.) Visible light
is a component of the heat radiation emitted by very hot objects. About 44 percent of the
Sun’s radiation is visible light: it glows white hot. Incandescent lightbulbs produce
visible light in the same way. Fluorescent and neon lights use excited atoms that emit
visible light. In Chapter 10, we will discuss this process and describe how infrared and
ultraviolet light and even x-rays are emitted by excited atoms.
Ultraviolet (UV) radiation, also called ultraviolet light, is a band of EM waves
that begins just above the frequency of violet light and extends to the x-ray band. The
frequency range is from about 7.5 3 1014 hertz to 1018 hertz. Ultraviolet light is also part
of the heat radiation emitted by very hot objects. About 7 percent of the radiation from
the Sun is UV. This part of sunlight is responsible for suntans and sunburns. Ultraviolet
radiation does not warm the skin as much as IR, but it does trigger a chemical process in
the skin that results in tanning. Overexposure leads to sunburn as a short-term effect, and
repeated overexposure during a person’s lifetime increases the chance of developing skin
cancer.
The next higher frequency electromagnetic waves are x-rays. They extend from
about 1016 to 1020 hertz. An important feature of x-rays is that their range of
wavelengths (about 1028 to 10211 meters) includes the size of the spacing between
atoms in solids. X-rays are partially reflected by the regular array of atoms in a crystal
and so can be used to determine the arrangement of the atoms. X-rays also travel much
greater distances through most types of matter compared to UV, visible light, and other
lower-frequency EM waves. X-rays are produced by smashing high-speed electrons into
a “target” made of copper, tungsten or some other metal.
X-rays (and gamma rays) can be harmful because they are ionizing radiation—
radiation that produces ions as it passes through matter. Such radiation can “kick”
electrons out of atoms, leaving a trail of freed electrons and positive ions. This process
can break chemical bonds between atoms in molecules, thereby altering or destroying the
molecule. Living cells rely on very large, sophisticated molecules for their normal
functioning and reproduction. Disruption of such molecules by ionizing radiation can kill
the cell or cause it to mutate, perhaps into a cancer cell. The human body can (and does)
routinely replace dead cells, but massive doses of x-rays or other ionizing radiation can
overwhelm this process and cause illness, cancer, or death. Because medical x-rays are
the largest source of artificially produced radiation in the United States, comprising about
10 percent of the total annual radiation dose for the average resident, it is little wonder
that protecting the public from unnecessary exposure to damaging radiation in diagnostic
radiology is one of the greatest challenges to health and radiological physicists.
L. Blackbody Radiation
Every object emits electromagnetic radiation because of the thermal motion of its
atoms and molecules. We have already discussed how this radiation offers one method of
transferring heat (see Section 5.4). Without radiation from the Sun, Earth would be a
frozen rock. In this section, we take a closer look at heat radiation and consider some of
its uses. The nature of the radiation emitted by a given object—the range of frequencies
or wavelengths of EM waves present and their intensities—depends on the temperature of
the object and on the characteristics of its surface (for example, its color).
A hypothetical object that is perfectly black—one that absorbs all EM waves that
strike it—would actually be the best at emitting heat radiation. Referred to as a
blackbody, it would emit radiant energy at a higher rate than any other object at the same
temperature, and the intensities of all of the wavelengths of EM waves emitted could be
predicted quite accurately. The heat radiation emitted by such an object is referred to as
blackbody radiation (BBR). Blackbody radiation, an idealized representation of heat
radiation, has been analyzed thoroughly and is well understood. The actual radiation
emitted by real objects usually is not too much different from BBR, so we can use it as a
model of heat radiation.
The heat radiation emitted by any object (such as your own body, the Sun, a
blackbody) is a broad band of electromagnetic waves. Within this band, some
wavelengths are emitted more strongly than others: the intensity of the different
wavelengths, the amount of energy released per square meter of emitting surface per
second, varies with wavelength. For example, the heat radiation from the Sun contains
more energy in each wavelength of visible light than in each wavelength of IR radiation.
The intensity of the visible wavelengths is higher than that of the IR wavelengths. A
graph showing the intensity of each wavelength of radiation emitted by a blackbody is
called a blackbody radiation curve.
The temperature of an object can be determined by examining the radiation that it
emits. This is particularly useful when very high temperatures are involved, as in a
furnace, because nothing has to come into contact with hot matter. Special devices called
pyrometers measure the amount and types of radiation emitted and use the rules
mentioned above to determine the temperature. A similar process is used to measure the
temperature of the Sun and other stars. The electronic ear thermometer works in a similar
way: it determines the patient’s body temperature by measuring the intensity of infrared
radiation emitted by an eardrum.
Most things on Earth have temperatures that cause them mainly to emit infrared
light. Anything that can detect IR can use this fact to locate warmer-thanaverage objects,
because they will emit more IR. Rattlesnakes and certain other snakes use IR to hunt
mice and other warm-blooded animals at night. These snakes have sensitive organs that
detect the higher-intensity infrared emitted by objects warmer than their surroundings.
Infrared-sensitive photographic film, video cameras, and other detection devices have
many practical uses. For instance, IR photographs, called thermograms, can show where
heat is escaping from a poorly insulated house and can detect ohmic heating caused by a
short circuit in an electrical substation.
M. EM Waves and Earth’s Atmosphere
Many substances in the atmosphere surrounding Earth interact with EM waves in
important ways. Some of these interactions are crucial to the existence of life on this
planet, another helps us communicate, and some add to the beauty that characterizes life
on Earth. The visible phenomena—rainbows, for example— are described in the next
chapter dedicated to visible light. Some of the others are discussed here. The sunlight that
keeps Earth warm and provides the energy for plants to grow also contains ultraviolet
radiation that is harmful to living things. But life has evolved on this planet because the
atmosphere has protected it from much of this UV. In the region between about 20 and 40
kilometers above Earth known as the ozone layer, there is a comparatively high
concentration of ozone (O3 ), the form of oxygen with three atoms in each molecule. This
ozone absorbs most of the harmful UV in sunlight. The ozone layer has been a shield
protecting living things on Earth.
In 1974, it was reported that chlorofluorocarbons (CFCs), chemical compounds
such as Freon used in refrigerators, air conditioners, and as an aerosol propellant in spray
cans, could be depleting the ozone layer. The CFCs that are released into the air drift
upward to the ozone layer and chemically break up the ozone molecules with alarming
efficiency. (The 1995 Nobel Prize in chemistry was awarded to the discoverers of this
effect.) Because of this, CFCs were banned for use as aerosol propellants in the United
States. Then in 1985 it was discovered that a “hole” developed in the ozone layer over
Antarctica during the later part of each year.
The concentration of ozone in a huge section of the atmosphere was reduced by
about one-half. A review of old satellite measurements revealed that this hole had
developed in previous years as well and that the one in 1982 was twice the area of the
United States (Figure 8.43). Scientists now believe that the ozone hole is produced by a
complex set of processes involving chlorine that originates in CFCs. During winter in the
southern hemisphere, chlorine molecules (Cl2 ) are released by chemical reactions that
take place in extremely high clouds, called polar stratospheric clouds (PSCs), over the
sunless South Pole. The return of sunlight in spring then triggers the reactions that cause
chlorine to break up ozone molecules.
Global monitoring of the ozone layer revealed an overall decline during the latter
part of the 20th century and not just over the poles. Continued reduction in ozone levels
could have tragic consequences. Rates of occurrence of skin cancer could rise and crop
yields could decrease because increased UV adversely affects many plant species. But
unprecedented international cooperation has led to the banning of CFCs throughout the
developed countries. Levels of CFCs in the atmosphere seem to be declining, but the
extremely high chemical stability of these compounds means they will continue to do
damage for many years to come.
The greenhouse effect is so named because it is partly responsible for keeping
greenhouses warm in cold weather. Glass and certain other materials allow visible light to
pass through them while they absorb or reflect the longer-wavelength infrared radiation.
A glass wall or roof on a building allows visible light to enter and to warm the interior.
As the temperature of things inside increases, they emit more IR. Without the glass, this
IR would escape from the enclosure and carry away the added energy. But the glass
blocks the IR, so the added internal energy is trapped and the enclosure is warmed. (The
glass not only reduces heat loss by radiation but also eliminates convective losses: the
heated air that rises cannot leave and take internal energy with it.) A car parked in the
Sun with the windows up is much warmer than the outside air because of this heating.
Windows on the sunny side of a building help to keep it warm in the same way.
Evidence to sustain the view that global warming is occurring is highly
persuasive. For example, global sea levels rose approximately 17 cm (6.7 in.) in the last
century, with the rate of increase during the last decade being nearly double the average
over that period. Global surface temperature models show Earth has steadily warmed
since 1880, with the bulk of the effect occurring during the 1970s; all ten of the warmest
years have been measured in the last 12 years. This effect has also been seen in the
oceans, where the temperature of the upper 700 m (2300 ft) has increased by 0.1678C
(0.3028F) since 1969. One result of this rise in ocean temperature has been the rapid
shrinking of ice sheets in Greenland and Antarctica and the decline in sea ice in the
Arctic region.
The atmospheric greenhouse effect is such a complex phenomenon that it is
difficult to predict accurately what will happen. Different scenarios have been proposed:
as Earth heats up, huge amounts of CO2 dissolved in the oceans could be released and
could exacerbate the global warming. Increased evaporation of water would raise the
water-vapor content of the atmosphere and possibly cause more heating. Or the higher
humidity could lead to more cloud cover, which might cool Earth as less sunlight reaches
its surface. About all we can be sure of is that we are altering the life-sustaining blanket
in which we live even though we cannot predict precisely what the long-term effects may
be.
About 50–90 kilometers above Earth’s surface, in a region of the atmosphere
known as the ionosphere, there is a relatively high density of ions and free electrons.
Radio waves from surface transmitters travel upward into the ionosphere. Higher-
frequency radio waves, such as those used in FM radio and terrestrial television, pass
through the ionosphere and out into space (Figure 8.45a). Lower-frequency radio waves,
such as the 500–1,500 kilohertz waves used in AM radio, reflect off the ionosphere and
return to Earth (Figure 8.45b), allowing such radio stations to be picked up over distances
of several hundred kilometers. The range for high-frequency radio waves is limited to
“line-of-sight” reception with distances of about 80 kilometers or less. The curvature of
Earth eventually blocks the signal from the transmitting tower. Low-frequency radio
waves can skip off the ionosphere and travel farther around the planet, a fact that amateur
radio enthusiasts called DXer’s, after the telegraphic shorthand DX for “distant” or
“distance,” take full advantage of. The radio waves used to communicate with spacecraft
must have high frequency to pass through the ionosphere.
Stars, galaxies, and other objects in space emit all types of electromagnetic
radiation. Astronomers were originally limited to studying only visible light through
optical telescopes. Now they examine the entire spectrum of EM radiation to gain a more
complete understanding about the universe. The atmosphere is a hindrance to some of
these investigations. Ultraviolet light from stars and galaxies is absorbed by ozone and
other gases. Infrared light is also absorbed, mainly by water vapor. Lower-frequency
radio waves are absorbed in the ionosphere. Even visible light is affected by the random
swirling of the air, which causes stars to twinkle and degrades the images formed in large
telescopes. Microwaves and higher-frequency radio waves are about the only EM waves
from space that are not affected by the atmosphere.
Since the early 1960s, dozens of telescopes and other astronomical instruments
have been placed in orbit to overcome the deleterious effects of Earth’s atmosphere.
Among the most important and sophisticated of recent space missions are the four that
comprise NASA’s Great Observatories Program. Each was designed to examine different
parts of the EM spectrum from infrared light for studying cool stars and interstellar dust
to gamma radiation emitted in high-energy processes associated with supernova
explosions and neutronstar collisions. The most famous of these, the Hubble Space
Telescope (HST), was launched in 1990 and is equipped with instruments that analyze
visible, ultraviolet, and shorter-wavelength infrared light (Figure 8.46). After a shaky
start, HST has achieved enormous success: research astronomers, as well as millions of
people worldwide, have been captivated by the stunning images it has returned. (There is
more on HST in Section 9.2.) The other spacecraft in the program, along with the year in
which each was launched, are the Compton Gamma-Ray Observatory (1991), the
Chandra X-Ray Observatory (1999), and the Spitzer Space Telescope (infrared, 2003).
Astronomy textbooks for years to come are likely to contain illustrations and findings
supplied by these observatories. Space exploration continues to give astronomers access
to the entire electromagnetic spectrum.
