Microwaves interact with metal in interesting ways. It will likely induce high voltages in the computer's wiring, causing the chips to release all of their magic smoke.
I grew up in a time when microwave ovens were first released. As such the very first thing you learned is no metal in the microwave oven.
I'm wondering if people who are younger somehow think of this differently or aren't automatically taught the same thing. I mean it seems so obvious (to me) that I wouldn't even think to point it out to someone actually. It's like saying "don't let the car run over you" or "don't play catch with the laptop".
This is entirely not true.
When microwaves were first released, they were so low powered that microwave cookbooks gave advice like "wrap the edges of the chickin in foil".
I don't think "most" microwaves have a metal rack, although they're not uncommon.
It's also not uncommon to see microwave-safe food containers that contain metal. I've seen grocery store deli soups, for example, that end up with a big ring of metal around the top when you open them, but can still be microwaved.
Still, "no metal" is a good approximation. "Unless it says you can use it" is probably OK to leave implied.
I've seen that as well. I'm wondering (from my own experiment) if it has something to do with angles vs. no angles [1] in terms of the metal in addition to the blocking by metal of the actual microwaves.
[1] Same as with stealth airplanes avoiding radar, right?
You're on the right track. The strength of the electric field around a charged, curved metal surface is inversely proportional to the radius of the curve. Sharp points have a very small radius, so they generate a large electric field, which can easily ionize air and thus cause arcing.
So spoons are okay, as they normally have no sharp points. Forks can cause problems, though.
Actually there's another issue here. If you stick an unopened can of food in the microwave, the microwaves can't reach the food; you get the same effect as if you ran the oven empty, which is that the field becomes very intense and risks burning out the magnetron or other components. The oven is designed to have its output absorbed by something, not just reflected back.
microwaves induce a current in metal. if there's a gap for that current to jump, you'll see it jump, and it'll probably cause problems including fire, but if there's no gap to jump, all that voltage potential just stays in the metal.
put a table spoon in the microwave and turn it on - nothing happens. i do this all the time when reheating soup.
i've won money on small bets like this too. non-technical people have no idea WHY metal poses a problem in the microwave, so i just bet them $10 that i can put a spoon in there without problems and they never believe me. they just think i have special spoons.
"Came out" refers to when they became in widespread use. That was in the 70's.
As far as low power I can find nothing that indicates that it was ever ok to put tin foil in however I do know that in some cases you might put metal to specifically block cooking. So perhaps with a low power microwave there was no sparking etc. (I can't find anything on that and I don't see the link you sent showing that (which page is it on in the cookbook?).
A regular oven provides heat from a heating element (which may be gas, electric, wood fuel etc), which heats the air in the oven. A microwave provides heat by using radiowaves to agitate responsive molecules (water is one) in the food - any warmth you feel when opening a microwave has come from heating the food, not the microwave itself.
When metal is in a microwave, the radiowaves hitting the metal create sparks, and sparks kill electronics.