reid@Glacier.ARPA (02/01/85)
I assumed that the net would be flooded with the right answers about
microwave ovens, but in fact I have seen only two messages come across my
screen that contain truth. This message explains the situation in more
detail than you would ever want to know. I offer this up for those of you
who have an amateur interest in science and who like to know how the things
around you work. I don't think you need any training in physics or
electrical engineering to understand this explanation.
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A microwave generator is basically a little radio station. A transmitter. It
generates radio waves of a certain frequency. Radio waves are a kind of
electromagnetic radiation. Light, x-rays, infrared heat rays, and TV
station broadcasts are also electromagnetic radiation.
Electromagnetic radiation is absorbed by some substances, transmitted by
others, and reflected by yet others. Visible light is reflected by paper and
absorbed by charcoal and transmitted by glass, for example. X-rays are
absorbed by lead, reflected by certain kinds of steel, and transmitted by
wood and biomass. And so forth. The precise results depend on the frequency
of the electromagnetic radiation.
Whenever energy is absorbed, heat is produced in the thing that does the
absorbing. Whenever energy is transmitted or reflected, heat is not
produced. This is why cars that have black interiors get hotter in the
summer than cars that have white interiors. This is a fundamental law of
physics--that energy is conserved. If sunlight shines on black paper, the
energy is converted from electromagnetic (the light itself) to kinetic (the
vibration of the hot molecules of the paper) but no energy is actually lost.
Microwave ovens heat food by "broadcasting" electromagnetic energy at a very
carefully chosen frequency. That frequency lies between UHF television and
infrared light. The frequency is chosen so that it exactly matches one of
the natural vibration modes of the water molecule. A natural vibration mode
is a fancy way of saying that something wants to shake at its own speed.
Like a swing. If you put your baby in a swing and push her, you have to push
at the rate that the swing wants to be pushed, and not at the rate that you
want to push, because if you don't, then the swing isn't going to work, and
you will end up punching your baby in the kidneys instead. So we say that
the swing has a natural vibration mode.
When the microwaves hit the water molecules, they hit them at exactly the
right intervals, so that each successive microwave pushes the molecule to
shake a little faster, a little harder. This shaking, this vibration, is the
heat.
This is why you can't heat something that is completely dry. If you put a
cup of salt in your microwave and give it a minute at full power, you will
find that the salt gets barely warm (please do not try this on your
microwave oven, please just take my word for it. You could damage something).
This is because salt molecules have a different natural vibration frequency
than water molecules. They are like a swing with a much much shorter rope;
you have to push a lot more often and each swing is a lot faster.
So microwave ovens heat by glomming on to the water molecules and making
them shake, rattle, and roll. And this "glomming on" is really giving them a
series of pushes at exactly the right intervals, using a carefully chosen
radio frequency. The wattage of the microwave oven basically determines how
hard they push on each cycle. If you are pushing your baby in a swing you
can get her to swing very high by giving her 25 little pushes, timed
properly, or by giving her 2 or 3 big pushes, also timed properly. The more
watts your microwave has the bigger is each push that it gives the water
molecules.
Glass, pottery, clay, Corningware, and things like that are all quite
transparent to microwaves. The little buggers just zip right on through the
way that a flashlight shines through a window. A cup of salt is also pretty
transparent to microwaves. Putting a cup of salt in there for 60 seconds at
full power is just like running it on empty at full power, because the
microwaves go right through the salt without seeing it.
A flat sheet of metal reflects microwaves. If you stop for a moment to
consider how your microwave oven is constructed, you will realize that it
has metal walls. If it didn't have metal walls (or at least if it didn't
have walls made out of something that would block microwaves) then the thing
would radiate all over the room. This would be wasteful of power, and would
also subject you and your aforementioned baby to big doses of microwaves. Bad.
My own microwave oven has 6 metal walls; it does not have a glass door.
Yours probably has a glass door. If you look carefully at the glass door you
will note that it has a piece of metal attached to it or embedded in it, and
you will notice that the piece of metal has many holes drilled in it. It
turns out (quite a stroke of luck, actually) that the frequency that is
needed to excite water molecules is also a frequency that will be reflected
by a piece of metal with little holes on it. As long as the holes are little
enough the little buggers can't sneak through. The microwaves aren't micro
enough.
Now here comes some actual physics. Take a deep breath. Remember how I said
that all electromagnetic radiation is either absorbed, transmitted, or
reflected, and how I just said that water absorbes microwaves (turns 'em
into heat) and that flat sheets of metal reflect microwaves. Well, pieces of
metal that are not flat, that are shaped more like radio antennas, actually
turn out to absorb some microwaves while at the same time reflecting some.
You've all seen 2-way mirrors that absorb some light, reflect some light,
and also transmit some light. Well, a piece of metal that is not a big flat
sheet will absorb some waves and it will reflect some waves. The exact ratio
of how many get absorbed and how many get reflected depends on the exact
geometry of the piece of metal, and it takes a Ph.D. electrical engineer
quite a while to compute those numbers. As a cook you don't care, except to
know that some are reflected and some are absorbed.
Microwaves that are reflected will come back to haunt us in a later
paragraph. Let's look at the ones that are absorbed. Remember how I said
that when energy is absorbed it is in fact "conserved", that no energy
disappears, and that electromagnetic energy is converted into heat when it
is absorbed. Well, alas, when some of the microwave energy is absorbed by
odd-shaped metal objects, the energy is absorbed not as heat but as
electricity. This is just like a radio antenna (remember I said that a
microwave generator is just like a radio station). The metal object in the
microwave oven is acting like a radio antenna that is 12 inches away from
the radio transmitter, and it is getting a pretty stiff dose of electricity
generated in it.
If the electricity picked up by the metal object is energetic enough, it
will start sparking around to other (grounded) metal objects, such as the
walls of the microwave oven. This spectacular home-made lightning storm is
actually quite safe, though I am sure that none of you will have the nerve
to believe me and set up little lightning storms in your microwave ovens to
amuse yourselves late at night.
A little tiny bit of the electricity that is picked up by the metal object
will be converted into heat, just because the metal object is not a perfect
conductor of electricity. Sometimes if the voltages are excessive a spark
will jump. The rest of the time a spark does not jump. What happens to the
rest of the electricity? It is re-radiated as microwaves. The metal object,
acting like a radio antenna, also becomes a radio transmitter. This is
really quite intuitive--if you shine sunlight on a piece of black paper, it
will get very hot; soon it will get hot enough that it starts to emit
radiation in its own right. The radiation that it emits will be infrared
radiation. Well, the fork or TV-dinner lid or metal rack that you put in
your oven is just re-radiating microwaves back into the body of the oven.
The net result is almost exactly the same as if they had been reflected off
of it in the first place, so I could have just told you that they were
reflected, but then there would be no explanation for all the sparks.
Let me stop and summarize. I'm almost done with the explanation, but I want
to do a review before I do the conclusion. Microwaves heat by being
absorbed. Water absorbs. Many other objects do not absorb. Metal reflects,
except that sometimes it also sparks because of the bizzarre way in which
the reflection happens. Microwaves that are absorbed are turned into heat.
Microwaves that are not absorbed are not turned into anything.
But wait a minute. Let's say that you turn your microwave oven on for 60
seconds and start pumping the old 750 watts into the chamber, and that the
chamber is empty. Since the chamber is empty, nothing is absorbed. What
happens to those microwaves? Does the box slowly fill up with them? Do they
evaporate?
The answer is that I lied to you about the radio station. Well, not exactly
lied. It really is just like a radio station, as long as the radio station
has all of its listeners 12 inches away from the broadcasting antenna. The
situation is really a lot more complicated than an ordinary radio station
because there is a close coupling between the sender and receiver. What
happens to the microwaves mentioned in the previous paragraph is that they
are never gnerated, because there is an impedance mismatch between the
microwave generator and the microwave recipient.
I promised I wouldn't mention physics, and here I am talking about impedance
mismatches. Well, once again this is a simple concept. Whenever you are
trying to move energy from one place to another, it is important that the
sender and the receiver are in harmony about the details of this transfer.
Riding a bicycle is a good example. If you are trying to ride in too low a
gear, you can't go very fast because your feet spin around and around and
the bike barely moves. If you are trying to ride in too high a gear, you
also can't go very fast because your legs aren't strong enough to push the
pedals around against that very high gear. But if you are in just the right
gear, you can zoom, and all of that energy gets transferred between your
legs and the bicycle. If you are in "just the right gear", a physicist would
say that there is an impedance match between your legs and the bicycle. If
you are in a gear that is to high or too low, then there is an impedance
mismatch.
Electromagnetic energy transfer is kind of like the bicycle example. The
microwave generator is like your legs, and the food that you are trying to
heat is like the bicycle. Some guy at the microwave oven factory chose the
gear, and built it in. If the oven is empty (that is, if the oven contains
nothing that is absorbing the energy) then it is like pedaling in too low a
gear. If the oven is overly full, then it is like pedaling in too high a
gear. If the oven has just the right amount of water in it, then everything
is copacetic, and the water gets heated. The important concept here is that
unless the right amount of water is in the microwave oven, then there is no
energy being transferred out of the microwave generator into anything.
Unfortunately, there IS a steady supply of energy going in to the microwave
generator, even when nothing is going out. That steady supply is coming in
over the power cord, 750 watts' worth, and going on in to the magnetron tube
that does the actual microwave generation. If 750 watts are going in and
nothing is coming out, where are all those watts going? Remember
conservation of energy. The answer is that if those 750 watts don't get to
leave the magnetron in the form of microwaves heading for some water, then
they will stay behind and convert themselves into heat. The magnetron just
gets hotter and hotter. Pretty soon you get meltdown. Exactly how soon you
get meltdown depends on the cleverness of the engineers who built your oven.
If they installed lots of ways to get the heat out of your magnetron, then
you can run it empty all day without hurting anything but your electric
bill. If they didn't install very many ways to get the heat out of your
magnetron then you will burn it out.
When you turn your microwave oven on you always hear a fan. Pretty loud fan,
too. You never knew it was a fan, did you? You always thought it was just
the noise that microwave ovens make when you turn them on. Well, it's a fan,
and what it is doing is blowing hurricanes of air across the magnetron,
trying to cool it off, to make it harder for you to melt it down.
I'm almost done with my little essay. I'm actually done with the physics. I
just want to tie down a couple of loose ends. Let's think back to the
"impedance mismatch" paragraph, and the analogy of bicycles in the wrong
gear. Have you ever noticed that the label on TV dinners always says "when
heating two dinners, please heat one, then remove it and heat the other" ?
Have you ever noticed that if you are using the microwave at work to heat
your sandwich and the guy behind you asks if he can just stick his sandwich
in there too, that in fact neither your sandwich nor his sandwich gets hot?
Well, this is the other end of the impedance mismatch situation. If you put
too much into a microwave oven, then it is heated very inefficiently,
because there is an impedance mismatch. If you put a 22-pound turkey into an
ordinary microwave oven and try to heat it, you will find that it doesn't
get very hot. This is because the microwave oven is in the wrong gear.
Unfortunately, there is no way to shift gears on a microwave oven; they are
built with a fixed output impedance and therefore have a fixed gear.
For fastest service, heat small amounts of food at a time.
Oh yes, the word "impedance" is pronounced "imPEEdunce".
That's the end. Here is the summary:
* Microwave ovens work by heating water molecules. They are fundamentally
unable to heat anything but water molecules.
* Metal objects reflect microwaves. Some reflect more perfectly than
others. Imperfect reflection results in electricity being generated
in the metal object.
* Sometimes that generated electricity will spark over. This sparking
is quite harmless.
* If you run your oven totally empty or totally full you run the
risk of damaging it unless it is well made. The damage comes
from overheating the magnetron.
* The amount of heat that goes into your food, plus the amount of
heat that goes into your magnetron, is a constant. If the food is
not getting hot, then the magnetron is getting hot.
* Tell the guy behind you in the cafeteria line, the one who wants
to put his sandwich in with yours, that you can't let him do it because
it would cause a serious impedance mismatch. He will immediately
decide that you are a kook and he will leave to let you heat your
sandwich in peace.
--
Brian Reid decwrl!glacier!reid
Stanford reid@SU-Glacier.ARPA