Microwaves -- Congrats, QED

Today’s Staff Report “How are the microwaves in ovens different from those in cell phones?” is written by our own Q.E.D. … who is now a full-fledged member of the Straight Dope Science Advisory Board. We’re delighted and look forward to many more Staff Reports from him in future [/hint]

Yeah yeah…the check’s in the mail, too, right? :wink:

Allow me to begin the new member initiation hazing:

“So, if you know the frequency in hertz, you can find the wavelength in meters by dividing by 299 million.”

Should be “by dividing into”.

Welcome aboard.

Kap

You are correct. I blame Ed. Yes, I wrote it that way, but he’s the editor, so i’m blaming him.

There is one crucial difference the report misses between the waves used by Microwave Ovens and the rest of the spectrum: Resonance. The waves in the microwave ovens are tuned to the exact frequency of vibration of the H-O-H bonds in water molecules, so they vibrate like crazy and the soup heats up as a consecuence.

If microwaves where higher or lower in frequency, the soup would not heat as much or as fast. Besides, if what you are trying to heat does not have a significant water content (i.e. the ceramic bowl), it won’t get any hotter.

Ah, but I didn’t miss this. And I didn’t include it, because it is not correct. Microwave ovens are specifically not tuned to the maximum absorption frequency of water (about 10 GHz or so) because otherwise foods would not be cooked in the middle much, if at all, since all the energy would be absorbed near the surface and little, if any, would penetrate deeper. There are also industrial microwave ovens which operate at 915 MHz, rather than 2450 MHz. I’ve addressed this in a future Staff Report. Although microwaves do excite natural resonances in water molecules, this is not the main vibrational mode, since these resonances only occur in water vapor, not in liquid water or water ice (which microwaves have some difficulty melting until liquid water forms on the ice surface).

Great article, Q.E.D. I’ve never had so much fun reading about microwaves before … :wink:

I’m not sure I’ve ever read so much about microwaves before! Bravo, Q.E.D.! And congratulations on your astounding appointment. :slight_smile:

While we’re at it, “299 million” should read “300 million”. In addition to being a nice round number, it’s more correct. To four significant figures, c = 2.998 * 10[sup]8[/sup] m/s, so if you’re rounding to three sigfigs (or two, or one), you should round up to 3.00 (or 3.0, or 3) * 10[sup]8[/sup].

I’m not a native English speaker, but doesn’t
“so, if you know the frequency in hertz, you can find the wavelength in meters by dividing by 299 million”
mean that you divide the frequency by 299 million, when you should do exactly the opposite? Divide 299 million by, say 3000 for a 3kHz wave’s wavelength?

Am I wrong, or am I the only one to notice this (and therefore brilliant)?

-M-

I am sorry to insist on my point, but I still think QED was wrong when he says that

I admit I didn’t know there were higher frequencies with better (higher) absortion, but I knew there were several.

In a quick search in the Internet, I found (How Stuff Works) this:

It also seems that other food ingredients absorb energy at that frequency, which I also ignored.

From here:

No, you are correct. But you aren’t the first person to notice: see the third post in this thread. In any case, the error has been addressed and fixed.

Congrats, Q.E.D.. I envy you your energy in responding to threads and doing reports. Impressive. Also, how well you come up with informative/scholarly sites.

If I haven’t missed it, would I be correct in saying that the frequency range that microwave ovens use today was a choice made to allow us to cook the food somewhat evenly WITHOUT interferring with communications due to leakage?

Did I also understand that there is a frequency which COULD be used, if leakage/communications were NOT a factor, which would allow one to cook the food, and not heat a ceramic dish?

That would be truly wonderful.

So I’m correct, but sloppy and easily excited. And also probably quite stupid (for no particular reason, it’s just a fair assumption for most people.)

Thanks, and sorry,
-M

You are corect that the choice of 2450 MHz was made so as not to interfere with communications equipment that existed at the time the microwave oven was invented.

The second question is a bit trickier. As a general rule, lower frequencies tend to penetrate deeper into moisture-laden foods, but the ceramic dish thing is a different story. Different ceramics react to different wavelengths, well, differently. Try putting various ceramic items in a microwave oven for a minute or so. Put a glass of water in there with them to absorb some of the energy and reducing the strain on the magnetron. Notice how some can get quite hot, while others stay comparatively cool.

An old friend of mine’s stepdad told him to always put a cup of water in the microwave oven when there was no food in it, so that he wouldn’t accidentally run it empty. It was never explained exactly what would happen if the microwave were operated without anything in it, but the clear implication was that it would be a catastrophe of Biblical proportions.

Said old friend later used that microwave-running-empty scenario as the origin story for a super hero.

Older microwave ovens had a magnetron tube with an envelope made of glass and were subject to failure due to the glass melting when the oven was run empty. Modern magnetrons are made with a ceramic envelope and are much less likely to fail this way. I’ve run ovens empty for several minutes without failure, but I always advise the inclusion of a glass of water in empty-oven experiments as a safety factor. I don’t needt someone screaming at me that I killed their favorite microwave oven.

Cordless phones operate at 2.4 Ghz and 5.8 GHz, not 2.8 and 5.6.

– Kirk

Whoops! You’re right. :smack: