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I have a fuzzy understanding that microwaves heat food because the waves interact with polar molecules in the food, causing them to vibrate, and the vibrations are heat? Correct me if I'm wrong.

To this end, I placed an empty plate (ceramic presumably) for 2 minutes, and it was hot to the touch... so that was a fail.

Is there some common item/material that can be placed in the microwave oven that won't heat up proving this property of microwave oven heating?

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    $\begingroup$ Look for "low loss microwave dielectric materials". Polyethylene and polystyrene may be good starters. I just microwaved a 1/2" polyethylene block for about a minute and it didn't get any warmer to the touch, so I would say that you can probably get heating of <5K/minute or so out of ordinary low cost polyethylene stock. At tens, if not hundreds of times the cost you can get specialty teflon based materials that are probably another order of magnitude better while also being temperature resistant. $\endgroup$ – CuriousOne Mar 20 '16 at 4:57
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    $\begingroup$ I suspect it can be rather unhealthy for some microwaves to be turned on with nothing to heat. $\endgroup$ – user10851 Mar 20 '16 at 5:05
  • $\begingroup$ @ChrisWhite: Hence my low hanging experimental limit for the OP. Nuking the microwave to answer a physics question is not considered acceptable adult behavior in many households. ;-) (As for the physics of it... I suspect that having a large piece of low-loss dielectric could be worse for the oven than not having anything in there, at all. It does alter the resonance of the empty volume and it might actually lead to a higher reflected power at the magnetron. It may not... but I wouldn't take the chance without having it on an analyzer.) $\endgroup$ – CuriousOne Mar 20 '16 at 5:09
  • $\begingroup$ Thanks for the suggestions! I didn't know it was a bad idea to run the oven without something to absorb the waves! Thanks for the heads up, considering it was recently bought. $\endgroup$ – Jet Blue Mar 20 '16 at 5:34
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    $\begingroup$ always add a glass of water. The EM wave have to get absorbed somewhere ! It might even be that it this condition your ceramic plate keep cold. (still, some ceramics contain a bit of water or metal). $\endgroup$ – Fabrice NEYRET Mar 20 '16 at 10:27
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As a rough approximation, the energy pumped out by a microwave will bounce around until it gets absorbed by something. If there is something in the oven cavity which readily absorbs energy, the energy won't bounce around very long before it gets absorbed. If some objects in the cavity absorb energy, but not as readily, the signals will bounce around longer but still end up getting absorbed. The trays that are included with many microwave ovens are often designed to absorb energy, but not too readily. If nothing in the cavity absorbs energy, it may eventually get absorbed by parts of the oven itself, in ways that may damage it.

If you want to operate a microwave oven without heating a particular object inside it, and without damaging the oven, you should place something in the oven which absorbs energy more readily than the other object. That won't shield the object from all of the effects of the microwaves, but it will be heated far less than if it were the only thing in the cavity.

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