Chapter 8 Electromagnetism and EM Waves (Section 1)

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Chapter 8Electromagnetism and EM Waves(Section
1)
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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.

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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.
• A device that can find hidden items on a person
  walking through an arch seems like something from
  science fiction.
• But, in todays world, it is routine.

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Metal Detectors

• How do these devices work their magic?
• Although metal detectors operate on electricity,
  it is magnetism that probes you.
• 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.

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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.
• 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.

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Metal Detectors

• This explanation might raise some questions in
  your mind.
• 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.

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Metal Detectors

• Magnetism and its useful interrelationship with
  electricity are the subjects of this chapter.
• First, the properties of permanent magnets and
  Earths magnetic field are described.
• Next, we demonstrate how electric fields and
  magnetic fields intertwine whenever motion or
  change is involved.

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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.

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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.

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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.
• 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.

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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.
• All simple magnets exhibit the same compass
  effectone 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.

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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.
• 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).

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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.
• 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.

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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 themthey 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.

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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
• 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.

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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.

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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.

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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 Earths field has been mapped
  carefully over the course of many centuries
  because of the importance of compasses in
  navigation.

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8.1 Magnetism

• Earths 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.

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8.1 Magnetism

• The direction of true north shown on maps is
  determined by the orientation of Earths axis of
  rotation.
• The axis is aligned closely with Polaris, the
  North Star.
• Because of the tilt of Earths magnetic axis,
  at most places on Earth compasses do not point to
  true north.

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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.

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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.
• In parts of Alaska, the magnetic declination is
  as high as 25? east.
• This must be taken into account when navigating
  with a compass.

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8.1 Magnetism

• Earths field is responsible for the magnetism in
  lodestone.
• This naturally occurring ferromagnetic ore is
  weakly magnetized by Earths magnetic field.
• Another thing to note about Earths magnetic
  field
• Earths north magnetic pole is at (near) its
  south geographic pole, and vice versa. Why?

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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.
• Therefore, a compasss north pole points at the
  Earths 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, and , or A
  and B.

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8.1 Magnetism

• Some organisms use Earths magnetic field to aid
  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 Earths field or its direction (or both).

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8.1 Magnetism

• The strength of the field allows the animal to
  determine its approximate latitude (how far north
  or south it is) because Earths magnetic field is
  stronger near the magnetic poles.
• Some migratory species travel thousands of miles
  before returning home, guidedat least in partby
  sensing Earths magnetic field.

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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.
• 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.

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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
• 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.
• However, there are some important differences.

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8.1 Magnetism

• Each kind of charge can exist separately, whereas
  magnetic poles always come in pairs.
• 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.
• But, with the exception of ferromagnetic
  materials, most matter shows very little response
  to magnetic fields.

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8.1 Magnetism

• We should also point out that the electrostatic
  and magnetic effects described so far are
  completely independent.
• 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.
• A number of fascinating and useful interactions
  between electricity and magnetism take place when
  motion or change in field strength occurs.

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Concept Map 8.1