Download Electromagnetic Radiation

Chapter 15
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Waves in Space
One thing that all the forms of electromagnetic
radiation have in common is that they can
travel through empty space. This is not true of
other kinds of waves; sound waves, for
example, need some kind of material, like air or
water, in which to move.
Magnetic Fields
A magnetic field exerts a force
on other magnets, causing
them to line up along the
direction of the magnetic field.
Electric Fields
The electric field surrounding
a charged particle exerts a
force on all other charged
particles in the field.
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A moving charged
particle is surrounded
by an electric field
and a magnetic field.
As a charged particle
moves up and down,
it
produces
an
electromagnetic
wave.
Electromagnetic
waves are produced
by charged particles,
such as electrons,
that move back and
forth or vibrate.
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As
a
charged
particle
vibrates by moving up and
down or back and forth, it
produces changing electric
and magnetic fields that
move
away
from
the
vibrating charge in many
directions.
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Electromagnetic waves have a wavelength and
a frequency.
1. One complete vibration of the charged
particle up and down creates one wavelength
in an electromagnet.
2. The number of wavelengths that pass by a
point in 1 second is the frequency of the
electromagnetic wave.
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Radiant energy- energy
electromagnetic wave.
carried
by
an
The amt. of energy carried by an
electromagnetic wave is determined by its
frequency.
The higher the frequency, the more energy
the EM wave has.
Electromagnetic waves travel at the speed of
light, 300,000 km/s, in space
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The difference between a wave and a particle might
seem obvious  a wave is a disturbance that carries
energy, and a particle is a piece of matter.
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However, in reality the difference is not so clear.
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In 1887, Heinrich Hertz found that by shining light
on a metal, electrons were ejected from the metal.
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Hertz found that whether or not electrons were
ejected depended on the frequency of the light and
not the amplitude.
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Because the energy carried by a wave depends on its
amplitude and not its frequency, this result was
mysterious.
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Years later, Albert Einstein provided an explanation
 electromagnetic waves can behave as a particle,
called a photon, whose energy depends on the
frequency of the waves.
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Because electromagnetic waves could behave as a
particle, others wondered whether matter could
behave as a wave.
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If a beam of electrons were sprayed at two tiny slits,
you might expect that the electrons would strike only
the area behind the slits, like the spray paint.
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Instead, it was found that the electrons formed an
interference pattern.
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This type of pattern is produced by waves when they
pass through two slits and interfere with each other.
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The electromagnetic spectrum is the
complete range of electromagnetic waves
placed in order of decreasing frequency.
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Radio Waves have the lowest frequency and
carry the least energy. (wavelength longer
than 0.3 m)
AM and FM radio signals and T.V. signals are
types of radio waves; they can be sent with
radio waves using a transmitting and
receiving antenna.
a. One way to make radio waves is to make electrons
vibrate up and down in a piece of metal called an
antenna.
a. The radio waves from a transmitting
antenna can cause electrons in another
piece of metal, such as a receiving
antenna, to move up and down, creating
an alternating current.
b. . The current can be used to produce
sound from a loudspeaker, allowing for
the transmission of music, television
shows, and telephone signals.
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Microwaves have a higher frequency and
shorter wavelength than radio waves; they are
used for some phone calls and to heat food.
(wavelengths b/w about 0.3 m and 0.001 m)
 Radar uses electromagnetic
waves to locate objects by
measuring the time it takes for
the waves to reach the object, be
reflected, and return.
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Infrared Waves have wavelengths between
one thousandth and 0.7 millionths of a meter
and feel warm or hot.
1. Infrared detectors sense objects that are warmer
or colder than their environment; TV and VCR
remotes also use I.R. waves.
2. Some animals, such as piranhas and rattlesnakes,
can detect I.R. waves, which helps them find prey
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Visible Light has wavelengths between 0.7
and 0.4 millionths of a meter.
1. what you see as different colors are EM waves of
different wave lengths.
2. Red light has the longest wavelength (low freq.);
blue light has the shortest wavelength (high freq.).
ROYGBIV
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Ultraviolet Radiation is higher frequency and has
shorter wavelength than visible light.
1. Too much exposure to ultraviolet radiation from the Sun
can cause sunburn and other health problems.
2. Since U.V. radiation can kill cells, it is sometimes used to
sterilize equipment.
3. The Ozone Layer in Earth’s upper atmosphere helps
protect the surface by absorbing much of the Sun’s U.V.
radiation.
Ultraviolet Light Box
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Much of the ultraviolet radiation arriving at Earth is
absorbed in the upper atmosphere by ozone.
Ozone is a molecule that has three oxygen atoms
and is formed high in the Earth’s atmosphere
Chemical compounds called CFCs, which are used
in air conditioners and refrigerators, can react with
ozone molecules and break them apart
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X Rays and Gamma Rays, with even higher
frequencies than U.V., can go right through skin
and muscles.
1. Too much exposure to X-rays or gamma rays can
damage or kill cells.
2. X-rays are useful in Medical Diagnosis if used with
appropriate precautions.
3. Gamma rays, which have the highest frequency, can be
used to treat Cancerous Tumors and to kill bacteria in
food.
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Radio Transmission:
◦ Radio stations change
sound to EM waves & then
your
radio
receiver
changes the EM waves
back to sound waves
again.
How does a radio receive
different stations?
◦ Each station broadcasts at
a certain frequency which
you tune in by choosing
their frequency.
◦ Dividing the Radio
spectrum
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◦ Carrier wave- the frequency of the EM wave that a station
uses
◦ Microphones convert sound waves to a changing electric
current or electronic signal containing the words & music.
◦ The modified carrier wave vibrates electrons in the station’s
antennae creating a radio wave that travels out in all
directions at the speed of light to your radio antennae.
What is AM radio?
◦ In AM amplitude changes but frequency does not. AM
frequencies range from 540,000 Hz to 1,6000,000 Hz
usually listed in kHz.
What is FM radio?
◦ In FM radio stations transmit broadcast information by
changing the frequency of the carrier wave. The strength
of FM waves is always the same and is in megahertz.
Mega=million
pressure
modulation
(sound)
electromagnetic
wave
modulation
Amplitude Modulation (AM)
uses changes in the signal
strength to convey
information
Frequency Modulation
(FM) uses changes in the
wave’s frequency to
convey information
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Television:
◦ Uses radio waves to send electronic signals in a
carrier wave.
◦ Sound is sent by FM; color and brightness is sent at
the same time by AM signals
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What is a cathode-ray tube?
◦ There are 3 types of spots, red, green and blue. The
electron beams move back and forth across the
screen.
◦ The signal from the TV station controls how bright
each spot is. Three spots together can form any
color.
◦ You see a full color image on the TV.
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Telephones
◦ Sound waves microphone electric signal radio
waves transmitted to and from microwave tower 
receiver electric signal  speaker sound wave
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How do cordless phones work?
◦ Cell
and cordless telephones are
transceivers, device that transmits one signal &
receives another radio signal from a base unit.
◦ You can talk and listen at the same time because
the two signals are at different frequencies.
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phones
How do pagers work?
◦ A pager is a small radio receiver with a phone
number. A caller leaves a message at a terminal
with a call-back number.
◦ At the terminal, the message is turned into an
electronic signal transmitted by radio waves.
◦ Newer pagers can send and receive messages.
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Communications Satellites
◦ Thousands of satellites orbit Earth. A radio or TV station
sends microwave signals to the satellite which amplifies the
signal and sends it back to a different place on Earth.
Satellite uses dif freq to send & receive.
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Global Positioning System
 GPS is a system of 24 satellites, ground monitoring stations and
portable receivers that determine your exact location on Earth.
GPS receiver measures the time it takes for radio waves to travel
from 4 different satellites to the receiver. The system is owned
and operated by the US Dept of Defense, but the microwaves
can be used by anyone.