Microwaves 9/12/21

Science with Richard Bleil

In his show “Home Improvement”, Tim Allen suggested that fire good, microwave magic.  Well, it may not be magic, but microwave ovens are fascinating.

I had a student who explained that his mom’s microwave worked even when the door wasn’t closed.  This is excessively dangerous, and I convinced him to buy her a new one.  He brought the old one to me.  I have no idea why.  Maybe as a sacrifice?

Of course, I felt obliged to destroy it lest it should fall into somebody else’s hands.  Well, that was my excuse.  What I wanted to see was the “microwave gun”, the generator that creates the microwaves.  I was rather shocked to discover that it was basically a piece of molded aluminum (unless you’re British, in which case it’s molded aluminium).  Truly fascinating.

Microwaves were created at MIT at the start of WWII.  Doing an experiment across a deep channel, the scientists had built a microwave emitter on one side of the channel, and a receiver on the other.  They were mightily annoyed whenever they would lose the signal as a ship passed, until somebody had the idea that this could be used as a ship detection device.  Thus started the very first and very shaky collaboration between the military and the eggheads, er, I mean the academic physicists.  The military built a physics building for them that was still there when I visited my friend and played racquetball with him.  It looked like a barracks (the best thing the military is at building), with the safety showers in the hallway outside of the labs so as not to risk getting the notes wet because, you know, the notes are more important than the people.  The earliest radar nets were limited because they relied on something coming between the transmitter and receiver, but it didn’t take long for the physicists to discover these objects also reflected the radar, meaning the emitter and detector could be part of the same device.  The jarheads were thrilled with this development.

There.  Now I’ve insulted both of them.  Happy?

One of the physicists accidentally had a lab coat with a candy bar in its pocket get in the way of a microwave emitter, and it melted the candy bar, making it far hotter than could be explained in conventional “fire good” methods.  He started bringing in various foods and would heat them for lunch every day.  Before long, the other eggheads noticed that he always had hot lunches and started bringing things for him to magically heat up as for them.  GE heard of this and sent a representative to check out the “magic bad” device, but unfortunately, the physicist was experimenting on an egg that day, and, yes, it exploded.  GE decided there was no practical use for home use.

Microwaves are part of the electromagnetic spectrum, the same spectrum that includes visible light, x-rays, television and radio signals, ultraviolet and even infra-red radiation.  It is truly a form of radiation.  What differentiates these types of electromagnetic radiation is the wavelength of the waves.  Microwaves just happen to have a wavelength that specifically excites the rotational energy of water.

And, yes, I did the experiment.  I heated a container of glycerin (the active ingredient in lip gloss and other items), and it didn’t warm even a single degree.  With no water, the microwaves had no effect.  This is why Styrofoam and ceramics do not heat in a microwave oven unless they are in touch with food which does heat.

Heat is known to be a form of translational energy.  The faster molecules travel (on average), the hotter the temperature.  So, with microwaves, water (which is a major component of most forms of food) “excite” to a higher rotational energy.  That is, they begin to spin faster.  Like billiard balls, as they spin faster, if they run into other molecules in the food, that faster spin pushes the molecules, making them move faster.  Hence, hotter food.

This is why that microwave oven was so dangerous.  We, you and I, are mostly water.  We are easily affected by microwaves, and a strong dose can do quite a bit of harm (smaller amounts, like in cell phone signals, are less harmful as this excess energy is not significant enough for too much damage).  Interesting, every microwave I’ve ever seen has a metal screen built into the door window.  This screen has little holes in it.  Just like that physics experiment, metal reflects microwaves, but if we just put a sheet of metal there, well, it would be about as effective of a window as I was effective as a husband.  As it turns out, those little holes in the metal are actually smaller than the wavelength of the microwave radiation.  As it turns out, the microwaves are too large to pass through that screen, so any damage to that screen is also very dangerous. 


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