Transcript chapter8-Section1

Vern J. Ostdiek
Donald J. Bord
Chapter 8
Electromagnetism and EM Waves
(Section 1)
Metal Detectors
• Metal detectors are the first line of defense against
persons trying to smuggle weapons onto
passenger planes or into schools, government
buildings, and many other places.
Metal Detectors
• Metal detectors probe your clothing and body
without physically touching you, looking for metal
that could be part of a gun, a knife, or other
dangerous object.
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A device that can find hidden items on a person
walking through an arch seems like something from
science fiction.
But, in today’s world, it is routine.
Metal Detectors
• How do these devices work their magic?
• Although metal detectors operate on electricity, it
is magnetism that probes you.
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Brief magnetic pulses are sent around and through
you, typically at a rate of about 100 times a second.
The device carefully monitors how swiftly each
magnetic pulse dies out.
Metal Detectors
• Any metal object encountered by a pulse is
induced to produce its own magnetic pulse, which
affects how rapidly the total pulse dies out.
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Sophisticated electronics in the metal detectors
sense this change and signal that metal is present.
They detect iron and other metals that ordinary
magnets attract as well as metals such as
aluminum and gold that do not respond to magnets.
Metal Detectors
• This explanation might raise some questions in
your mind.
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How are the magnetic pulses produced?
How does the metal detector monitor how the
pulses die out?
Why do they cause nonmagnetic metals such as
aluminum to produce magnetic pulses?
• The answers lie in the key concepts presented in
this chapter, the fundamental ways in which
electricity and magnetism interact with each other.
Metal Detectors
• Magnetism and its useful interrelationship with
electricity are the subjects of this chapter.
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First, the properties of permanent magnets and
Earth’s magnetic field are described.
Next, we demonstrate how electric fields and
magnetic fields intertwine whenever motion or
change is involved.
Metal Detectors
• These concepts are used to explain how many
common electrical devices operate.
•
They also suggest the existence of electromagnetic
(EM) waves.
• The properties and uses of the different types of
EM waves are the main topics of the latter half of
this chapter.
8.1 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.
8.1 Magnetism
• 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.
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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.
8.1 Magnetism
• Now magnets are made into a variety of sizes and
shapes out of special alloys that exhibit much
stronger magnetism than lodestone.
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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.
8.1 Magnetism
• All magnets have both poles.
• If a magnet is broken into pieces, each part will
have its own north and south poles.
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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.
• Simply put: like poles repel,
unlike poles attract (just as
with electric charges).
8.1 Magnetism
• 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.
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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.
8.1 Magnetism
• 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.
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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.
8.1 Magnetism
• 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:
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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.
8.1 Magnetism
• A compass can be thought of as a “magnetic field
detector” because its needle will always try to align
itself with a magnetic field.
8.1 Magnetism
• 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.
• 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.
8.1 Magnetism
• 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.
8.1 Magnetism
• Earth’s magnetic field has the same general shape
as the field around a bar magnet, with its poles
tilted about 11 with respect to the axis of rotation.
8.1 Magnetism
• The direction of “true north” shown on maps is
determined by the orientation of Earth’s axis of
rotation.
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The axis is aligned closely with Polaris, the North
Star.
• Because of the tilt of Earth’s “magnetic axis,” at
most places on Earth compasses do not point to
true north.
8.1 Magnetism
• For example, in the western two-thirds 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.
8.1 Magnetism
• The difference, in degrees, between the direction
of a compass and the direction of true north varies
from place to place and is referred to as the
magnetic declination.
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In parts of Alaska, the magnetic declination is as
high as 25 east.
• This must be taken into account when navigating
with a compass.
8.1 Magnetism
• Earth’s field is responsible for the magnetism in
lodestone.
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This naturally occurring ferromagnetic ore is weakly
magnetized by Earth’s magnetic field.
• Another thing to note about Earth’s magnetic field:
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Earth’s north magnetic pole is at (near) its south
geographic pole, and vice versa. Why?
8.1 Magnetism
1. The north pole of a magnet is attracted to the
south pole of a second magnet.
2. The north pole of a compass needle points to the
north.
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Therefore, a compass’s north pole points at the
Earth’s south magnetic pole.
• This is not a physical contradiction:
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It is a result of naming the poles of a magnet after
directions instead of, say, + and –, or A and B.
8.1 Magnetism
• Some organisms use Earth’s magnetic field to aid
navigation.
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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).
8.1 Magnetism
• The strength of the field 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.
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Some migratory species travel thousands of miles
before returning home, guided—at least in part—by
sensing Earth’s magnetic field.
8.1 Magnetism
• Superconductors, so named because of their
ability to carry electric current with zero resistance,
react to magnetic fields in a rather startling
fashion.
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In the superconducting state, the material will expel
any magnetic field from its interior.
This phenomenon, known as the
Meissner effect, is why strong
magnets are levitated when placed
over a superconductor.
8.1 Magnetism
• When trying to determine whether a material is in
the superconducting state, it is easier to test for
the presence of the Meissner effect than it is to
see if the resistance is exactly zero.
• You have probably noticed that magnetism and
electrostatics are very similar:
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There are two kinds of poles and two kinds of
charges.
Like poles repel, as do like charges.
• There are magnetic fields and electric fields.
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However, there are some important differences.
8.1 Magnetism
• Each kind of charge can exist separately, whereas
magnetic poles always come in pairs.
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Modern theory indicates the possible existence of a
particular type of subatomic “elementary particle”
that has a single magnetic pole, which has not been
found.
• Furthermore, all conventional matter contains
positive and negative charges (protons and
electrons) and can exhibit electrostatic effects by
being “charged.”
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But, with the exception of ferromagnetic materials,
most matter shows very little response to magnetic
fields.
8.1 Magnetism
• We should also point out that the electrostatic and
magnetic effects described so far are completely
independent.
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Magnets have no effect on pieces of charged
plastic, for instance, and vice versa.
• This is the case as long as there is no motion of
the objects or changes in the strengths of the
electric and magnetic fields.
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A number of fascinating and useful interactions
between electricity and magnetism take place when
motion or change in field strength occurs.
Concept Map 8.1