How come a microwave oven can heat an empty plate??

[naita]

@smiling_bandit

I may be misunderstanding things, but if memory serves the basic issue is this:

(1) Often, substances that we don’t think of as “having water in” still do have water molecules, though it might be a very small amount.

(2) Microwaves are intended only to heat water, but it’s not 100% perfect and as a practical matter can’t be: there will be stray waves of different lengths which can and will heat other things. And as a basic matter the box is throwing around energy, which has to be absorbed by something eventually.

Point two is wrong, microwaves are absolutely not intended to heat only water. See Crafter_Mans earlier post on the choice of 2.45GHz.

[iamthewalrus_3]

@Crafter_Man

So if you turn on the microwave oven with nothing inside, where does the energy go?

There’s probably something inside the field.

If you have a turntable on little rollers, it’ll go into that. You’ll hear a really loud bang after 10-20 minutes of heating that’s the turntable glass breaking apart from heat pressure (source: accidentally destroyed a microwave by using the cook function instead of the timer function).

[Learjeff]

It’s a bad idea to use the microwave to heat serving plates, especially if they don’t heat up very quickly. If the plates aren’t absorbing the energy, then it’s building up or (worse yet) leaking out and damaging the microwave oven itself.

Instead, put a cup of water on top of the plates. It probably won’t work as well for heating the plates evenly, but it sure is safer.

[engineer_comp_geek]

@RedSwinglineOne

If what you say is true, then why do so many microwave foods use metal in their packaging, particularly foods that don’t cook well with microwaves alone? I cook my Hot Pockets in a metal sleeve and the whole thing cooks, not just the ends.

If you put metal in a microwave, depending on its shape and orientation it can reflect a lot of the microwave radio waves. Also, though, the radio waves will induce eddy currents into the metal. You can kinda think of this as the metal acting like an antenna and it’s picking up some of the energy from the radio wave. Since the metal isn’t a superconductor, these eddy currents cause the metal to heat up. If you have a thin bit of metal, like the metal film on hot pocket sleeves and microwave pizza boxes, the metal gets hot fairly quickly, which then cooks the outside of the hot pocket or the bottom of the pizza so that it ends up nice and crisp instead of soft and doughy.

If you put something like a metal spoon in the microwave, it will also get hot, though not quite as quickly since the metal is thicker.

If you have a piece of metal that has sharp edges on it, electrical charge will build up on the pointy bits and may arc to other pointy bits if they are close by. Usually the tines of a fork are a bit too far apart and you won’t get any arcing, but there are other metal things that will arc quite well in a microwave. Cut vegetables will also arc if they were grown in soil with a high enough mineral content, and cut grapes will usually arc as well, which surprises a lot of people.

@filmore

As long as we’re talking about microwave ovens… Is there an optimum place I should put the food for optimum heating results? My microwave has a turntable. Should I put the food in the middle so it rotates in place? Or farther out towards the edge so it moves in a big circle. Or does it matter at all?

Microwave ovens are usually designed so that they produce a standing wave inside the box, so you’ll have hot spots and cold spots all over the box at locations that are related to multiples of the microwave wavelength. If you are heating something like water or soup, the uneven heating will set up small convection currents in the liquid so it will move around and disperse the heat evenly all on its own and doesn’t matter much. If you are heating something solid then you are probably better off placing it towards the outside of the turntable so that it isn’t close to stationary in the middle of a cold spot.

[TSBG]

@TriPolar

This is why I avoid standing too close to the microwave.

I believe that newer (post-1970s) microwaves are well shielded, but you can do this experiment to test it:

Heat a cup of water in the microwave. Drape a cool damp towel over the front. If after a few minutes any part of the towel is hot, there’s a leak.

[dstarfire]

Back to the OP’s question. Several other molecules are polar, besides water. Odds are the plate contains at least one of these (especially since a few are commonly used as solvents).
Here’s a list of some of the others (sorry for the chemspeak, I just found the list and I’m too lazy to look up their common names):
HF, NH3, HCl, HBr, HI, OF2, SeCl2, SCl2, PCl3, SO2 Ch3Cl, CH3Br, SeCl, CHCl3, CO(CH3)2, H2S, CH3Cl, KBr & H20(ion-dipole forces), H2O2, CH3OH(methanol), CH3COOH(acetic acid), CH3NH2 methy amine, C2H5Oh ethyl alcohol, (C6H12O6 glucose), CH3CH2OH ethanol, 1- propanol CH3Ch2CH2OH, 2-propanol CH2CH2OHCH3, 1-butanol CH3CH2CH2CH2OH, acetone (CH3)2CO, H3O+, H202, and CH20.

[TriPolar]

@TSBG

I believe that newer (post-1970s) microwaves are well shielded, but you can do this experiment to test it:

Heat a cup of water in the microwave. Drape a cool damp towel over the front. If after a few minutes any part of the towel is hot, there’s a leak.

You have to read the post I was responding to and my username again.

[Leo_Bloom]

@TriPolar

This is why I avoid standing too close to the microwave.

@Michael63129

…

ETA: Or this happens (just kidding)

I’m trying to work some thermite-testicle joke here but can’t quite get to it.

(NB: it’s not really thermite in the vid. But my balls are. See? That wasn’t so difficult.)

[TSBG]

@TriPolar

You have to read the post I was responding to and my username again.

Yoink.

[Tim_T-Bonham.net]

@engineer_comp_geek

Microwave ovens are usually designed so that they produce a standing wave inside the box, so you’ll have hot spots and cold spots all over the box at locations that are related to multiples of the microwave wavelength. If you are heating something like water or soup, the uneven heating will set up small convection currents in the liquid so it will move around and disperse the heat evenly all on its own and doesn’t matter much. If you are heating something solid then you are probably better off placing it towards the outside of the turntable so that it isn’t close to stationary in the middle of a cold spot.

An easy way of seeing where the hot & cold spots in your microwave are is to cover the bottom of the microwave with close-together slices of bread with thin slices of cheese on them, or even just buttered bread, and run the microwave for a minute or so. You can easily see where it is melted and where it is not. (A turntable often makes this irrelevant anyway.)