Besides making food all bubbly and toasty good, the magnetron in a microwave oven can also be used to cause an entertaining and pretty discharge when a CD is cooked for about 2 seconds. Supposing that the CD is placed over the top of a coffee cup in such a way as to prevent arcing into the sides of the microwave, can a brief running damage the device?
The sides of the microwave are obviously virtually impervious to damage by microwaves; they are designed to reflect them, correct? Suggested by another party was the possibility that the magnetron itself would be damaged by the microwaves not being absorbed.
I find this suggestion not a little bit odd. Microwaves damaging a magnetron sounds like light damaging a light bulb to me! In any case, would filling the coffee cup with water help to absorb the excess microwaves and make the practice safe?
Yes, it can happen. Microwaves reabsorbed by the magnetron are converted to heat, and if the tube overheats sufficiently, it can eventually fail. And yes, a coffee cup full of water absorbs enough radiation to protect the magnetron.
The experiment as you describe it with the CD on top of the full coffee cup should be fine, however do note that a phenomenon called superheating can occur when heating water in a clean coffee cup. Without sufficient nucleation sites, boiling cannot start and the water can be heated to greater that the normal boiling point. Disturbing the cup can then cause the water to instatly boil and splash, presenting a scalding risk.
However, the repair literature for very old microwave ovens mentions that the glass vacuum envelope can overheat, melt, and suffer a “suck in” event. That, or the glass-to-metal seal can fracture. (If you let all the vacuum escape from the vacuum tube, it no workee anymore.) It appears that modern magnetrons use ceramic rather than glass, so I believe that they can grow far hotter without damage. When there’s no load on the magnetron the e-fields in the standing waves grow very large, and then the oven will heat up any substance which is the least bit absorbtive.
If you’re running your oven for many minutes with almost nothing inside (such as when frying a couple strips of bacon), then maybe you should worry about overheating of magnetrons in older ovens.
I’ve heard one eyewitness account from a guy who boiled water in a pyrex bowl for many hours (extra zeros accidentally punched into the keypad.) When he stopped the oven he found that the water was long gone, and there was a hole melted in the side of the bowl with bright red molten glass running down. This certainly can happen (after all, red-hot glass is a resistor, so when left in a microwave oven it just needs some sort of small trigger in order to suffer an outbreak of meltdown.)
The magnetron in a modern microwave is a little better designed than the one in an older microwave, so a modern oven is less likely to suffer damage from abuse like this. I know a lot of people who have put all sorts of things in their microwaves and don’t personally know anyone who has munged their microwave yet, but on the other hand I’ve seen several pictures from microwave oven repair sites showing burnt magnetrons so it definately can happen.
Steel wool also makes for a very interesting display.
A lightbulb radiates a lot of heat and relies on the heat being carried away. Wrap a lightbulb in insulation and see how long it lasts. Many electronic circuits are designed so that if they are not loaded they can be damaged. This is specially true of RF amplifiers and radio transmitters can easily be damaged if the load impedance is not well matched. Cut the antenna cable and you’ll probably damage the final amplifier (unless the system has some built in protection).
Yes, the water will absorb the microwaves and the oven will function as designed. But, as has been said, a modern microwave is unlikely to be damaged by a short period without load.