So I know that a microwave oven heats things up by exciting the liquid molecules within the item (apologies if that is a simplistic explanation but it should suffice for now). If I put something in there with no liquid, or a tiny amount of liquid, like a rock or sand, does that mean it will either not heat up or heat up very slowly?
Also, can they make whatever it is the opposite of a microwave is to cool things down? Refrigeration simply pumps in cold air to lower something’s temperature, but is it possible to use a similar kind of wave to slow down the molecules in an object? Or some kind of energy sapper to take away all the energy of an excited molecule to cool it down, kind of like a subatomic handjob to release the energy?
Not really. Electromagnetic radiation (‘waves’) are energy, and imparting energy to something isn’t going to cool it down. There is, and can be no such thing as a freeze ray.
You’ve got it partly right. The frequencies that commercial microwave ovens used are tuned very specifically to match the properties of water molecules. All molecules obey the rules of quantum mechanics, and therefore have discrete states corresponding to discrete energy levels. Water molecules happen to have a vibrational energy level that corresponds to microwave frequencies. Hit the H20 with that frequency and the molecules shake - this un-directional motion is heat, and then the rest of the molecules absorb the heat by conduction.
Mangetout is correct that all EM radiation is energy, and in general you can’t cool something down by throwing energy at it. However, there are several laser cooling techniques that will gently coax atoms to emit radiation and slow their motion. The cooling effect comes not from the insertion of energy but from the interaction between the atom to be cooled and two or more laser beams that set up a more complicated “energy landscape” (jargon: potential) in the vicinity of the atom. Doing this requires incredible precision of the laboratory setup and is currently not feasible for broad applications.
Not really. Water, like any complicated molecule (and yes, by the standards of most molecules, water is pretty complicated) has a whole mess of absorption all over the place. The selection of frequency for a microwave oven has more to do with what’ll produce tolerable levels of radio interference, what tubes are easier to make, and the shape and size of the appliance. Almost any complicated molecule will react to microwaves in basically the same way.
This is not, technically, correct. Refrigeration pumps heat [FONT=Trebuchet MS]out of the enclosure, almost like a water pump moves fluid (in a very broad analogy). Cold, in this case, is better thought of as “lack of heat”.[/FONT]
A thought experiment regarding this is:
If you had a perfectly insulated room, with an electric outlet and a refrigerator plugged into it and running, and left the door to the refrigerator open—would the temperature in the room rise, fall or stay the same?
The answer, of course, is that the temperature would rise, since the compressor is consuming energy to cool the interior of the refrigerator, but the waste heat is being radiated by the coils in back, and all energy tends toward heat, eventually.
To expand this even further, you don’t want to have it at the “exact resonant frequency”, otherwise, the absorption of outer layers would attenuate the microwaves so much that the inner part would never receive any heating (your frozen hot-dog Popsicle as Baked Alaska). Instead, the frequency is skewed far enough from the resonant frequency that the microwaves attenuation (and therefore heating) occurs over a much longer distance–resulting in a delicous, tasty, and fully cooked hot dog.
You are right that it is not a true resonance (which is a common misunderstanding) but one of the reasons 2.54 GHz was chosen was because water does absorb it pretty well. It’s definitely not a resonance since water absorbs anything between about 1 and 10 GHz pretty well, though the absorption does tend to peak between 2 and 3 GHz. Because water absorbs these frequencies pretty well, they are some of the worst frequencies to transmit on, since water in the air is going to attenuate your signal. This is why these frequencies tended to be available, which the FCC likes since it means microwave ovens don’t tend to interfere with older communication equipment. You’ll also note that a lot of newer technology tends to use frequencies in this range, simply because it’s about the only part of the spectrum that hadn’t already been assigned to something.
A microwave oven operating anywhere from about 1.5 to 5 GHz would work pretty well.
As Chronos said, it’s not necessarily liquid that heats up. Sugars, fats, and other complex molecules heat up pretty well.
Microwaves will also induce eddy currents into metals. You can think of metals as acting like radio antennas and “receiving” part of the microwave radio signal and turning it into current. Those currents produce heat, especially if the metal is thin. Those gray pads in some microwave meals are a metallic film. A thin spoon will heat up more than a thick one, since the currents produced are forced through a thinner amount of metal. A thick spoon will take a while to heat up in a microwave. A thin decorative strip, like a gold strip around the top of a mug, will heat up very quickly. Steel wool, being thin metal, heats up very quickly and will actually start to burn, which makes a pretty cool light show. CDs make a cool light show too (note - this completely destroys the CD very quickly, so don’t try it with a CD or DVD that you ever want to use again).
Sand isn’t going to heat up much at all. Rocks are going to depend on their content. A rock containing iron ore will heat up quite a bit, for example, though if it’s a big lump of ore it’s going to have the same problem that a thick spoon has, namely that the currents have a lot of metal to disperse through and won’t cause as much heating.
One final note - microwaves generate radio energy, and if you don’t put anything inside the box that absorbs this radio energy, it ends up just bouncing around in there. Some of it ends up getting reflected back into the magnetron (the thing that generates the radio waves) which is a bit harsh on it. Early microwave ovens would often break if there wasn’t anything in the box when you turned them on. These days the magnetron is a bit more rugged, but it’s still possible to cause damage to one. Large pieces of metal can also generate reflections which go back into the magnetron. In other words (insert standard lawyer type disclaimer about breaking your microwave if you fiddle with it here).
One time we popped a CD in a microwave for a second. The patterns that resulted looked like fractal lighting bolts. So, what the hey, let’s plug it into the compact disc player on our IBM 286 computer with DOS v6.x. As expected, it locked up the computer. No matter, we’ll just removed the offending disc and hit the reboot button.
Damn–the machine remained locked up. Hit the reboot button, tried the keyboard… Had to physically switch power from the machine to get it to work again. We never were able figure out how that happened.
It was chosen because it doesn’t absorb very well. Same with the microwave ovens which use the 900MHz band, http://en.wikipedia.org/wiki/ISM_band Then there’s RFC corp’s “Macrowave” ovens which use 40MHz. Poor absorption means deeper heating as well as uniform heating from more passes through the object (more repeat reflections.)
The peak is above 10GHz, and rises higher as water heats up. It’s relatively smooth at 2-3GHz, see
Nonetheless, the problem still exists, right? That’s why they say that if you have, say, a bunch of chicken legs (which are gross microwaved, BTW), you should arrange them like a flower on the plate with thick parts toward the rim.
That’s not because of resonance. That’s because most microwave oven boxes are designed such that the incoming radio wave ends up producing a standing wave inside the box. This means that there will be hot and cold spots as you move around inside the box, which is no big deal if you are heating soup and you stop to stir it once or twice during heating, but is a big problem for chicken legs or other types of solid food. If the microwave oven has a rotating plate on the bottom (as most do these days) then you place your wholesome bits of chicken goodness out near the outside so that they will rotate through the hot spots, and leave the inedible bony parts at the center where the movement is very small. If your microwave oven doesn’t have a rotating plate, stop it part way through the cooking and turn the plate around so that you’ll even out the cooking a bit. Letting the food stand for a minute or so before eating it helps disperse the heat a bit more evenly through it too, since the hot parts of the food will transfer heat to the cooler parts.
I’ve put a few things into our microwave at work, but never liquid nitrogen. Then again, we don’t have that kind of lab at work either.
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Another fun thing to put in a microwave is a grape that has been sliced almost in two, but leave just a tiny bit of skin connecting the two halves. Makes a great light show, which is a bit surprising to some folks since it is after all just a grape.
A burned out incandescent light bulb will light up, which is kinda fun. The bottom of it tends to get a bit hot, so be careful when pulling it back out of the microwave oven. I’ve never tried a fluorescent bulb.
Cut carrots and other vegetables will sometimes arc. Having nice square edges helps to produce arcs. A lot depends on the mineral content of the soil that the carrot or veggies were grown in, which you won’t be able to tell if you just grab a package of diced veggies from the store. A higher power microwave oven makes your veggies more likely to arc as well.
CDs and steel wool have already been mentioned.
A bar of soap will become BIG UGLY MUTANT SOAP which is kinda fun. Marshmallows are supposed to do the same thing.
I’ve been told that peanut M&Ms will spark but I haven’t tried them personally.
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(time to repeat the disclaimer about possibly damaging your microwave doing stuff like this)
Which is really something of a cheat, since you need to know the frequency to do that, and the frequency printed on the back of the microwave was probably measured via a calculation that required the speed of light to begin with.
Well, too little absorption would be bad, as the food would simply not get hot. And too much absorption would be bad, as only the outer surface would get cooked.
Correct. It always bugged me that slight point was included. But of course the pedagogical reason was on wave lengths and frequencies in general.
It also always bugged me that it is deduction, not even counting the cheat. Although less tasty and fun to watch, the derivation of the speed of light through inductive reasoning is much more marvelous.
*ETA because of timeout:
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Question that just occurred to me: is inductively measuring the speed of light even possible any more (excluding the problem of trying to prove a negative)? There was a thread not so long ago on why c is c and nothing else but c; this is just a component of what I’m asking.
Then again, we don’t have that kind of lab at work either.
point was included. But of course the pedagogical reason was on wave lengths and frequencies in general.