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1. Microwaves have a longer wavelength than
visible light, so doesn't that mean it has a
lower energy than visible light does? if that's
the case my students ask me why we can't pop a
bag of popcorn in a kitchen appliance that
bombards things with waves of visible light.
2. There is a name for every phase change and a
different name for each phase change in the
opposite direction (ie, vaporization and its
opposite, condensation) - what do you call it
when something changes from gas to plasma? is
there a name at all? is there a name for the
reverse process too?
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Question Date: 2004-12-16 | | Answer 1:
1. Good question about popcorn. While visible
light does have higher energy than microwaves, for
a material to absorb energy, a condition called
resonance must be met. A simple example of
resonance would be from recognizing that different
colors of light possess different energies. Red
light is absorbed by a green material because
there is a resonance condition between the color
of the material and the color (energy) of the
light. Green light has higher energy than red
light, but green light is not absorbed by a green
material (and thats why its green). Similarly,
while the water in the corn has a resonance
condition for absorbing the microwaves (which it
does and therefore heats up), there is no
resonance condition for the corn to absorb visible
light and turn it into heat.
2. When a gas
is tuned into a plasma, the change is more like a
temporary chemical change (the gas is ionized with
electrons being removed from atoms) rather than a
physical change. There is therefore no equivalent
of "melting" or "boiling" for the change between a
gas and a plasma. One could use the term
"ionization" but this should not be compared with
"evaporation". | | Answer 2:
1. The issue here is the distinction between
energy of a photon (indeed microwave photons have
lower energy) and intensity (i.e. number of
photons). A typical 100 Watt light bulb delivers
only a tiny portion of that energy as visible
light.. (Less than 1%). On the other hand,
atypical 900Watt microwave oven uses 1200W or
power and delivers about900W of microwaves. Some
years ago I used a CO2laser delivering about 150W
to cut sapphire substrates. It sliced the
material, about 2mm thick and nearly as hard as
diamond at about3 inches per second... Part of
the issue is that visible light sources are very
inefficient --even LED's are only in the 20%
range, at best. 2. The phase change from a gas
to a plasma is called ionization -- which refers
to the stripping of some electrons from the
neutral atoms. The term predates the concept of
thinking of plasma as a fourth phase of matter. I
don't know the term for gas condensation from a
plasma. | | Answer 3:
Turning a gas to a plasma is called ionization,
and the reverse process is called
recombination.
A gas can become a plasma
even if only a fraction of its atoms lose
electrons. The essential feature of a plasma is
that it has enough electrons and positive ions
flying around to be electrically conductive
(whereas gases are good insulators). Plasma's
conductivity makes it behave differently than gas
in other ways, too. For example, plasma is bound
to magnetic fields and can carry magnetic fields
with it, but ordinary gases have no interaction
with magnetic fields.
Plasma is actually by
far the most common phase in the universe. Stars
are made of plasma, and most space "gas" is
actually plasma. There are many other phases
besides the basic four: Neutron stars are made of
"degenerate matter", which is as dense as an
aircraft carrier in a cigarette lighter.
Super-cold helium can become "superfluid", which
means it can do a whole bunch neat tricks (like
escape containers by climbing the walls). Some
materials have a zoo of phases between solid and
liquid, collectively called "liquid crystals". | | Answer 4:
1. It's an absorption feature problem. The size of
the electric dipole of a water molecule is on the
same order of size as the wavelength of microwave
light,making water molecules perfect antennas at
that frequency. As a result, exposing water
molecules to microwave light will cause them to
spin, generating friction, and thus heat. Shorter
wavelength light cannot cause water molecules to
spin like that since they are too big, and longer
wavelength light cannot impart enough energy,
although they can on a longer antenna (e.g. radio
waves excite a radio antenna).
2. The
transition from gas to plasma is
called"ionization" (the particles that make up a
plasma are ions instead of molecules). I don't
know if there is a word for the reverse, but
"molecularization" is probably reasonable (gasses
being composed of molecules). | | Answer 5:
What, aren't you satisfied with how microwave
ovens work? Here are some problems with building
an optical wave oven to consider: 1)
Microwaves mostly interact only with the water in
food. An optical wave oven would interact with
just about everything in some way. So everything
gets really hot. Also, optical probably wouldn't
penetrate as easily into the food as the microwave
does. 2) I don"t think anyone has made a way
to generate optical power the way we can for
microwaves. It would have to be something like
being inside the tube of a high power laser. 3)
How do you build a box to contain all this
optical energy?
I think you would call it
"ionization" as the process of going from gas to
plasma involves ionizing the atoms or, in other
words, separating electrons from the atomic
nuclei. The name we use in my field
(astrophysics/cosmology) for going from ionized to
neutral gas is "recombination." A quick search on
the web tells me that plasma physicists probably
use these terms also. | | Answer 6:
Microwaves do have a longer wavelength than
visible light and that does mean that they have a
lower energy. Microwaves are able to pop the
popcorn by disrupting the rotational motion of the
water stored in the popcorn kernal. By disrupting
this motion, the water starts to move around
faster and eventually pressure build will pop the
kernal. I also believe that microwave light is
more intense than visible light. By this I mean
that if you were to increase the intensity of
visible light, such as having a certain amount of
visible light brought down to a point source like
a LASER, than you would be able to pop the popcorn. | | Answer 7:
You are absolutely right about the relationship
between wavelength and energy. If you want to
heat something up by bombarding it with radiation,
then ultraviolet light will work better than
visible light, and you'll be sitting around
forever waiting for your popcorn if you use
infrared light or radio waves. This kind of
heating works just by increasing the velocity of
the molecules in the object. Luckily for the
shareholders of General Electric, there's more to
it than that. What you have to know is that
different wavelengths of light have different
effects on individual molecules. UV light has
enough energy to break chemical bonds, which is
why you get skin cancer -- UV from the sun breaks
bonds in the DNA of skin cells. Visible light
can't break bonds, but it can knock electrons up
to a higher orbital state -- chlorophyll, retinal,
and other pigments work by changing their shape in
response to this kind of electron movement.
Microwave radiation is too weak to do any of those
things, but what it can do is cause polar
molecules to isolate. You probably know that
water is a dipole because one side of the molecule
has nonbonded electrons orbiting the oxygen atom
(and is therefore slightly negative), and the
other side has the two hydrogen atoms that are
somewhat positive because the electrons 'shared'
between the O and H atoms spend more time orbiting
the large O nucleus (which has lots of protons)
than the small H nuclei (which have only one
proton). So the water molecule is very much like
a small magnet with a + pole and a - pole. If you
put this dipole in a magnetic field it would
orient itself along the magnetic field, just like
a small magnet moves if you hold it near a large
magnet. If you put a dipole in an oscillating
magnetic field of the right frequency, the dipole
will oscillate, too. Well, waves of
electromagnetic radiation have that name because
they're made of an oscillating electric wave and
an oscillating magnetic wave. And it happens that
the microwave radiation we use in microwave ovens
has a magnetic component with precisely the right
frequency to cause water dipoles to oscillate. So
you put a glass of water in the microwave oven,
and the microwaves start flowing. The microwaves
cause the water molecules to start oscillating,
and this oscillation heats up the glass of water.
The microwaves don't make molecules move
around and heat up like UV radiation would, but it
does cause molecular motion by making the water
dipoles oscillate. As a result, microwaves only
heat up things with water in them, so you can't
heat up a brick in a microwave. Your kid sister's
Easy-Bake oven with a strong lightbulb in it will
heat up the brick, but it's going to take a lot
longer to boil a glass of water!
As for
your second question, I don't know of a name for
that phase change. A quick look on the internet
seems to indicate that there isn't a name for
plasma-related phase changes. On the one hand
this makes sense, because things don't change back
and forth from plasma very often, so we don't
really need a name for that. On the other hand,
scientists love naming things, and we have plenty
of fancy names that are perfectly useless, so it's
sort of surprising that nobody has invented a name
for plasma phase changes. So if you want a fun
project, you should just invent a name for a
gas-plasma phase change. Post it on the
internet, and the next time somebody is wondering
what the name for that is, they'll find your name.
Bam -- you've invented a scientific term!
Even better, you could find a physicist somewhere
who works on plasma. Email him or her and suggest
your term. If you get a physicist to start using
your term, then it will definitely take off.
Either that or you'll find out what the real
answer to your question is. Click Here to return to the search form.
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