I’ve done a quick search on here and couldn’t find anything about this topic. So here we go.
Is it okay to run a microwave with nothing in it?
What difference should it make if there is nothing in it?
MtM
Running a microwave empty allows large amounts of microwave energy to be reflected back into the magnetron tube, which is the part that generates them. This overheats the tube and can cause it to fail eventually. It’s not an immediately catastrophic thing, so running it for a minute or so empty won’t generally cause any damage.
I don’t know why but NO . Apparently you can’t run a microwave with nothing in it. A quite intelligible friend of mine once was testing a microwave and he had a glass of water in it to absorb the excess energy. He was not prepared to run the microwave with nothing in it. But heres another question how does putting metal in a microwave cause such a pretty and dangerous show?
:dubious:
There you go Q.E.D beat me to it with a real answer as well.
Microwaves are radio waves and like all radio waves, they can induce a voltage across a conductor. The microwaves in an oven can induce VERY high voltages on the order of 1 to 2 kV, and these high votages drive large currents through metal objects. Arcing can occur across gaps in the metal obejcts or from the objects to the metal lining the oven cavity. These arcs can be hot enough to burn the metal and create glowing plasmas. Quite spectacular at times.
My microwave oven specifically warns against running it with nothing in it. I believe that Q.E.D. had the answer. Most microwave power output tubes have a maximum mismatched load impedance, which translates into standing-wave ratio, that they will tolerate without damage. If the energy isn’t absorbed by the contents then more of it gets reflected around and some of that will relect back into the output port of the power tube. The presence of this reflected wave adds to the tube output wave and since they are the same frequency this can easily create a large standing wave.
So is it safe to say the size of the object matters as well?
If I throw a pea in, is that enough? Or should the item be larger than say a golf ball?
I’d imagine this was not a good idea, but I put a small florescent light bulb in one once. It got very bright.
I think you just have to use ordinary care. It’s a matter of the intended use. The oven is intended to be used to cook meals for at least one person. Probably heating the water for a cup of coffee is OK but I don’t think I’d go much smaller than that.
I’ve read a couple of articles which say that older microwave ovens were prone to failure when running empty. The older magnetrons had a small glass cover over the antenna rod. This would have a glass/metal seal which could crack when hot, ruining the vacuum. The glass could even melt, producing a “suck-in” failure. Glass isn’t immune to dielectric losses, and being so close to the antenna (with its huge e-fields) would let the glass absorb a bit of EM and heat up.
Modern magnetron tubes use a ceramic “window” instead of glass, so they’re much more robust.
Once I was meaning to be using the microwave timer but instead turned it on while empty. the door blew open after a minute or so. This was about 1994.
Not to doubt your story, ZipperJJ, but there’s really nothing in an empty microwave oven cavity that would explode, causing the door to pop open. Was any other damage visible inside the oven?
So here I find this thread … it “sort of” answers my question but not exaclty soo …
I was taught in labor class to put some rice (uncooked, regular) in a sock, tie a knot in the sock, and nuke it for 2 min to get 20 min of heat.
However, when I relayed this to a friend, another one piped up that if I did that I’d ruin the microwave after a while, and that I should do it, but also have a glass of water in there because the microwave works by heating the water … and that there wasn’t enough water latent in the rice to keep from runing the magnetron.
Thoughts?
Its important to note that its not likely to be a one-time thing. I’ve done lots of MW experiments - you know, sticking things in like CDs, lightbulbs, the ol’ sparking grape trick - all without a glass of water and the MW still works fine.
Of course, YMMV.
Hey Trig, try some marshmallow peeps 
Trigonal Planar: Whats the old sparkling grape trick? What happens? I would have thought it would just explode. I’ve obviously got so much to learn…
Slice a grape almost in have crosswise, but leave a thin strip of the skin between them. Lay the joined halves face-down on a glass or ceramic plate and nuke. Observe.
Well, Sam Goldwasser thinks it’s a bad idea.
Or a fresh bar of Ivory® soap. 