The electric field arrows in this
picture show the direction and give an idea of the strength that would
be exerted on a positive placed at any position in the figure.
You'll find the following on p. 60 in the photocopied ClassNotes.
We imagine turning on a source of EM radiation and then
a
short time
later we take a snapshot. The EM radiation is a wavy pattern of
electric and magnetic field arrows. We'll ignore the magnetic
field lines. The E field lines sometimes point up, sometimes
down. The pattern of electric field arrows repeats itself.
Note the + charge near the right
side of the picture. At the time
this
picture was taken the EM radiation exerts a fairly strong upward force
on
the
+
charge.
Textbooks often represent EM radiation with a wavy line like shown
above. But what does that represent?
The wavy line just connects the
tips of a bunch of electric
field
arrows.
This picture was taken a short time after the first snapshot when
the radiation
had
traveled a little further to the right. The EM radiation now
exerts a somewhat weaker downward force on the + charge.
The + charge is now being
pushed upward again. A
movie
of
the +
charge, rather than just a series of snapshots, would show the
charge
bobbing up and down much like a swimmer in the
ocean would do as waves passed by.
The wavy pattern used to
depict EM radiation can be described spatially in terms of its
wavelength,
the distance between identical points on the pattern. By
spatially we mean you look at different parts of the radiation at one
particular instant frozen in time.
Or you can
describe the radiation temporally
using the frequency of oscillation
(number of up and down cycles completed by an oscillating charge per
second). By temporally we mean you at one particular point for a
certain period of time.
EM radiation can be created when
you cause a charge to move up and
down.
If you move a charge up and down slowly (upper left in the
figure above) you would produce long wavelength radiation that would
propagate out to the right at the speed of light. If you move the
charge up and down more rapidly you produce short wavelength radiation
that propagates at the same speed.
Once the EM radiation encounters the charges at the right side of
the
figure above the EM radiation causes those charges to oscillate up and
down. In the case of the long wavelength radiation the charge at
right oscillates slowly. This is low frequency and low energy
motion. The short wavelength causes the charge at right to
oscillate more rapidly - high frequency and high energy.
These three characteristics: long wavelength / low frequency / low
energy go
together. So do short wavelength / high frequency / high energy.
Note that the two different types of radiation both propagate at the
same speed.
The
following
figure
(not shown in
class) illustrates how energy can be
transported from one
place to another (even through empty space) in the form of
electromagnetic (EM) radiation.
You add energy when you cause an
electrical charge to move up and down
and create the EM radiation (top left).
In the middle
figure, the EM
radiation then travels out
to the
right (it could be through empty space or through something like the
atmosphere).
Once
the EM radiation encounters an electrical charge at another location
(bottom right),
the energy reappears as the radiation causes the charge to move.
Energy
has been transported from left to right.
This is really just a partial list
of some of the different
types of EM
radiation. In the top list, shortwave length and high energy
forms of EM radiation are on the left (gamma rays and X-rays for
example). Microwaves and radiowaves are longer wavelength, lower
energy forms of EM radiation.
We will mostly be concerned with just ultraviolet light (UV),
visible
light (VIS), and infrared light (IR). Note the micrometer
(millionths of a meter) units used for wavelength for these kinds of
light. The visible
portion of the spectrum falls between 0.4 and 0.7 micrometers.
UV and
IR light are both invisible. All of the vivid colors
shown above
are just EM radiation with slightly different wavelengths. When
you see all of these colors mixed together, you see white light.
Here are some rules governing the emission of electromagnetic
radiation:
1.
Unless an object
is very cold (0
K) it will emit EM
radiation. Everything in the classroom: the people, the
furniture, the walls and the floor, even the air, are emitting EM
radiation. Often
this radiation
will be invisible so that we can't see it and weak enough that we can't
feel it (or perhaps because it is always there we've grown accustomed
to it and ignore it). Both the amount and kind (wavelength) of
the emitted
radiation depend on the object's temperature.
2.
The second rule
allows you to
determine the amount of EM radiation (radiant energy) an object will
emit. Don't worry about the units,
you can think of this as amount, or rate, or intensity.
Don't worry about σ either, it is just a
constant. The amount depends on temperature to
the fourth
power. If the temperature of an object doubles the amount of
energy emitted will increase by a factor of 2 to the 4th power
(that's 2 x 2 x 2 x 2 = 16). A hot object just doesn't emit a
little more energy than a
cold object it emits a lot more energy than a cold object. This
is illustrated in the following figure:
3.
The third rule
tells you something
about the kind of radiation emitted
by an object. We will see that objects usually emit radiation at
many different wavelengths but not in equal amounts. Objects emit
more of one particular wavelength than any of the others. This is
called lambda max (lambda is the greek character used to
represent wavelength) and is called the wavelength of maximum
emission. The third rule allows
you to calculate "lambda max." The tendency for warm objects to
emit radiation at shorter wavelengths is shown below.
The cool object is shown emitting
red light. It could be visible light or IR light. The
warmer object will also emit IR and red light but also shorter
wavelengths such as yellow, green, blue, and violet (maybe even some UV
if it's warm enough).
If the cool object is emitting visible light it would
appear red. Because the warmer object is emitting a mix of
visible colors, it would probably appear white.
The graphs at the bottom of p. 65 in the photocopied
ClassNotes also help to
illustrate the Stefan-Boltzmann law
and Wien's laws.
1.
Notice first
that both and warm
and
the cold objects emit radiation
over a range of wavelengths (the curves above are like quiz scores, not
everyone gets the same score, there is a distribution of grades)
2.
Lambda max has
shifted toward
shorter wavelengths for the warmer
object. This is Wien's law in action. The warmer object is
emitting lots of types of short wavelength radiation that the colder
object
doesn't emit.
3.
The area under
the warm object
curve is much bigger than the area under
the cold object curve. The area under the curve is a measure of
the total radiant energy emitted by the object. This illustrates
the fact that the warmer object emits a lot more radiant energy than
the colder object.
All four energy transport processes are illustrated in the figure
below. Can you name/identify them? Click here when
you think you have the answers.
