Transcript Ch 12: Electromagnetic Waves

12.1: What are electromagnetic waves?
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Electromagnetic waves:
 made by vibrating electric charges
 can travel through space (don’t
need matter)
 Travel by transferring energy
between vibrating electric and
magnetic fields.
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
A magnetic field
surrounds all
magnets.
An electric field
surrounds all charges.
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
Magnetic and electric fields:
 Exert forces without having to touch objects
 Enable magnets and charges to exert forces at
a distance.
 Exist even when there is no matter (in space)
Electric charges can also be surrounded by
magnetic fields.



All moving electric charges are surrounded by a
magnetic field.
The motion of electrons creates a magnetic field
around the object they flow through.
A change in one field causes a change in the other.
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
Recall: EM waves are made by a vibrating
charge.



This means that a vibrating charge has both an
electric field and a magnetic field.
As the charge vibrates, the electric and magnetic
fields change.
A vibrating electric charge creates an EM wave
that travels outward in all directions from the
charge.

EM waves are transverse waves because the electric
and magnetic fields vibrate at right angles to the
direction the wave travels.
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
All objects emit EM waves!
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The wavelengths of the EM waves emitted shorten
as the temperature of the material increases.
The energy carried by an EM wave is called
radiant energy.

EX: sunlight causes
the electrons in your
skin to vibrate and
gain energy.
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All EM waves
travel at
300,000,000 m/s in
a vacuum (space),
i.e. “the speed of
light.”


Nothing travels
faster!
EM wave speed
through matter
depends on the
material.
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
Frequency and wavelength are measured like
other waves.

As the frequency increases, the wavelength decreases.
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We define a wave as a disturbance that carries
energy and a particle as a piece of matter.
EM waves can behave as both a wave and a
particle (the particle-wave duality).
Phenomenon discovered in 1887 by Heinrich Hertz.
 Experiment known as the “photo-electric effect”:

 Shine a light on metal and the metal will eject electrons.
 Whether it happened depended on the frequency of the
light, not the amplitude.

Einstein later explained this: electromagnetic waves
can behave as a particle, called a photon, whose
energy depends on the frequency of the waves.
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
It is now known that all particles can behave
like waves.

The figures below demonstrate particles
experiencing diffraction when encountering an
obstacle (electrons on the left, water on the right)
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