# How come a microwave oven can heat an empty plate?? **URL:** \ **Category:** Factual Questions\ **Created:** [March 6, 2013, 4:04pm UTC](https://boards.straightdope.com/t/how-come-a-microwave-oven-can-heat-an-empty-plate/652195 "2013-03-06T16:04:33Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Bones\_Daley](https://avatars.discourse-cdn.com/v4/letter/b/bc8723/32.png) [@Bones\_Daley](https://boards.straightdope.com/u/Bones_Daley)\ **Post date:** [March 6, 2013, 4:04pm UTC](https://boards.straightdope.com/t/how-come-a-microwave-oven-can-heat-an-empty-plate/652195/1 "2013-03-06T16:04:33Z")
I frequently use my microwave to get my serving plates scalding hot, particularly if if I am cooking on the hob, ( ie without using the stove oven). I am puzzled, however, by how this phenomenon occurs …my understanding is that the microwaves agitate water molecules, which causes food to become hot …but are there water molecules in ceramic plates? If there were, I would have thought that microwaving them would cause the plate to explode, but that has never happened to me. I await elucidation from the scientists on the forum … ---
**Author:** ![johnpost](https://avatars.discourse-cdn.com/v4/letter/j/f17d59/32.png) [@johnpost](https://boards.straightdope.com/u/johnpost)\ **Post date:** [March 6, 2013, 4:15pm UTC](https://boards.straightdope.com/t/how-come-a-microwave-oven-can-heat-an-empty-plate/652195/2 "2013-03-06T16:15:47Z")
water and fats are the intended targets in food. the microwaves will also heat some glass, ceramics and plastics; depends on what is in them. ---
**Author:** ![Baracus](https://avatars.discourse-cdn.com/v4/letter/b/2bfe46/32.png) [@Baracus](https://boards.straightdope.com/u/Baracus)\ **Post date:** [March 6, 2013, 4:17pm UTC](https://boards.straightdope.com/t/how-come-a-microwave-oven-can-heat-an-empty-plate/652195/3 "2013-03-06T16:17:53Z")
If your plates are porous at all, water can be absorbed during washing. Apparently some ceramics that are not fired to sufficiently high temperature can also have residual moisture locked in. ---
**Author:** ![voltaire](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/voltaire/32/313_2.png) [@voltaire](https://boards.straightdope.com/u/voltaire)\ **Post date:** [March 6, 2013, 4:23pm UTC](https://boards.straightdope.com/t/how-come-a-microwave-oven-can-heat-an-empty-plate/652195/4 "2013-03-06T16:23:55Z")
Plausible WAG: Humidity in the air is heated and conducts the heat to the plate? ---
**Author:** ![Keeve](https://avatars.discourse-cdn.com/v4/letter/k/f07891/32.png) [@Keeve](https://boards.straightdope.com/u/Keeve)\ **Post date:** [March 6, 2013, 4:24pm UTC](https://boards.straightdope.com/t/how-come-a-microwave-oven-can-heat-an-empty-plate/652195/5 "2013-03-06T16:24:56Z")
> [@johnpost](#): > > water and fats are the intended targets in food. the microwaves will also heat some glass, ceramics and plastics; depends on what is in them. Can someone expand on this? Of the materials which microwaves will/won’t heat up, what do they have in common? ---
**Author:** ![AndrewL](https://avatars.discourse-cdn.com/v4/letter/a/3d9bf3/32.png) [@AndrewL](https://boards.straightdope.com/u/AndrewL)\ **Post date:** [March 6, 2013, 4:27pm UTC](https://boards.straightdope.com/t/how-come-a-microwave-oven-can-heat-an-empty-plate/652195/6 "2013-03-06T16:27:57Z")
Microwaves will be absorbed by most polar molecules. Essentially any molecule which has an uneven charge distribution will be vibrated by the microwave radiation, resulting in heat. Water is strongly polar, and quite good at absorbing microwave radiation. Other molecules that are polar are also heated by microwaves. Metal is also good at absorbing microwaves, but that’s due to the free valance electrons which are easily moved around rather than by polar molecules. ---
**Author:** ![johnpost](https://avatars.discourse-cdn.com/v4/letter/j/f17d59/32.png) [@johnpost](https://boards.straightdope.com/u/johnpost)\ **Post date:** [March 6, 2013, 4:30pm UTC](https://boards.straightdope.com/t/how-come-a-microwave-oven-can-heat-an-empty-plate/652195/7 "2013-03-06T16:30:03Z")
the microwave oven has a changing electric field inside of it. dielectric molecules (positive and negative charges on different parts of the molecule) keep moving to align themselves with the field and cause friction. ---
**Author:** ![Nametag](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nametag/32/406_2.png) [@Nametag](https://boards.straightdope.com/u/Nametag)\ **Post date:** [March 6, 2013, 4:33pm UTC](https://boards.straightdope.com/t/how-come-a-microwave-oven-can-heat-an-empty-plate/652195/8 "2013-03-06T16:33:43Z")
Your understanding is incorrect. Microwave ovens produce and confine electromagnetic radiation of a particular frequency. This frequency is well-absorbed by most matter, including food components; water is particularly affected because of its large molecular dipole moment, not because the frequency is targeted toward it. “Microwave-safe” containers also absorb at this frequency; they are safe because they heat evenly, don’t melt or leach impurities, and don’t contain air bubbles that could lead to breakage. It’s true that some materials absorb poorly, usually plastics; this is predictable from their structure, though it will take a better chemist than I to explain how. ---
**Author:** ![AndrewL](https://avatars.discourse-cdn.com/v4/letter/a/3d9bf3/32.png) [@AndrewL](https://boards.straightdope.com/u/AndrewL)\ **Post date:** [March 6, 2013, 4:36pm UTC](https://boards.straightdope.com/t/how-come-a-microwave-oven-can-heat-an-empty-plate/652195/9 "2013-03-06T16:36:51Z")
> [@Nametag](#): > > It’s true that some materials absorb poorly, usually plastics; this is predictable from their structure, though it will take a better chemist than I to explain how. Hydrocarbon molecules are generally nonpolar. Because they don’t have a strong polar moment, the microwaves don’t make them vibrate, so they don’t get hot. Water and far molecules are polar, so they absorb microwaves, vibrate, and get hot. ---
**Author:** ![smiling\_bandit](https://avatars.discourse-cdn.com/v4/letter/s/e9a140/32.png) [@smiling\_bandit](https://boards.straightdope.com/u/smiling_bandit)\ **Post date:** [March 6, 2013, 4:40pm UTC](https://boards.straightdope.com/t/how-come-a-microwave-oven-can-heat-an-empty-plate/652195/10 "2013-03-06T16:40:14Z")
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. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [March 6, 2013, 4:40pm UTC](https://boards.straightdope.com/t/how-come-a-microwave-oven-can-heat-an-empty-plate/652195/11 "2013-03-06T16:40:28Z")
> [@AndrewL](#): > > Hydrocarbon molecules are generally nonpolar. Because they don’t have a strong polar moment, the microwaves don’t make them vibrate, so they don’t get hot. Water and far molecules are polar, so they absorb microwaves, vibrate, and get hot. This is why I avoid standing too close to the microwave. ---
**Author:** ![Ruken](https://avatars.discourse-cdn.com/v4/letter/r/f475e1/32.png) [@Ruken](https://boards.straightdope.com/u/Ruken)\ **Post date:** [March 6, 2013, 5:05pm UTC](https://boards.straightdope.com/t/how-come-a-microwave-oven-can-heat-an-empty-plate/652195/12 "2013-03-06T17:05:03Z")
Nonpolar molecules will often also have a weak rotational absorption spectrum if rotation causes distortion that results in a dipole. E.g. you’ll see a weak microwave absorption spectrum for methane due to rotation about a C3 axis. IIRC, microwave ovens don’t operate at a high enough energy to hit the resonance for water, but you’ll still get rotation (not vibration), which is disbursed as vibrational and kinetic energy. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [March 6, 2013, 5:20pm UTC](https://boards.straightdope.com/t/how-come-a-microwave-oven-can-heat-an-empty-plate/652195/13 "2013-03-06T17:20:03Z")
> [@Ruken](#): > > IIRC, microwave ovens don’t operate at a high enough energy to hit the resonance for water, but you’ll still get rotation (not vibration), which is disbursed as vibrational and kinetic energy. This is true. It should also be stated that, at 2.45 GHz, water absorbs the energy fairly well, but not \*too \*well. The choice of 2.45 GHz was sort of a balancing act. You don’t want to choose a frequency that is not readily absorbed by water, else the energy will travel through the water and not heat it up. On the other hand, you don’t want to choose a frequency that is very efficiently absorbed by water, else only the outside surface of the food will be cooked. (All of the energy will be absorbed by the outer surface, and little energy will penetrate the food.) ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [March 6, 2013, 5:40pm UTC](https://boards.straightdope.com/t/how-come-a-microwave-oven-can-heat-an-empty-plate/652195/14 "2013-03-06T17:40:36Z")
> [@Baracus](#): > > If your plates are porous at all, water can be absorbed during washing. Apparently some ceramics that are not fired to sufficiently high temperature can also have residual moisture locked in. It also happens to glass - where this cannot be the right answer. ---
**Author:** ![Michael63129](https://avatars.discourse-cdn.com/v4/letter/m/3da27b/32.png) [@Michael63129](https://boards.straightdope.com/u/Michael63129)\ **Post date:** [March 6, 2013, 6:42pm UTC](https://boards.straightdope.com/t/how-come-a-microwave-oven-can-heat-an-empty-plate/652195/15 "2013-03-06T18:42:59Z")
> [@AndrewL](#): > > Metal is also good at absorbing microwaves, but that’s due to the free valance electrons which are easily moved around rather than by polar molecules. Incorrect, or microwaves wouldn’t use metal for the oven cavity (often painted, but metal) - metal \*reflects \*microwaves. What actually happens when a piece of metal sparks in a microwave is that the [microwaves induce current in the metal](http://www.howitworksdaily.com/science/why-does-metal-react-so-violently-in-a-microwave-oven/), but otherwise reflects off of it, similar to how a magnet induces a current in a coil of wire (FWIW, the oven cavity itself doesn’t pose a problem because it is shaped to reflect the microwaves into the food, and doesn’t have any thin/sharp edges, this is also true for objects like spoons - leaving a spoon in a bowl of soup won’t cause a disaster, [as explained in this old thread](http://boards.straightdope.com/sdmb/showthread.php?t=419350)). ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [March 6, 2013, 6:50pm UTC](https://boards.straightdope.com/t/how-come-a-microwave-oven-can-heat-an-empty-plate/652195/16 "2013-03-06T18:50:06Z")
So if you turn on the microwave oven with nothing inside, where does the energy _go_? ---
**Author:** ![Michael63129](https://avatars.discourse-cdn.com/v4/letter/m/3da27b/32.png) [@Michael63129](https://boards.straightdope.com/u/Michael63129)\ **Post date:** [March 6, 2013, 7:06pm UTC](https://boards.straightdope.com/t/how-come-a-microwave-oven-can-heat-an-empty-plate/652195/17 "2013-03-06T19:06:07Z")
> [@Crafter\_Man](#): > > So if you turn on the microwave oven with nothing inside, where does the energy _go_? It [builds up](http://amasci.com/weird/microwave/voltage3.html) until something gives, whether it is an arc inside the oven cavity or the magnetron itself: > [@](#): > > On the other hand, if there’s no food in the oven, then the 1000W bounces back and forth, yet the magnetron still puts out more energy. This adds to the waves already there. It’s like wiggling your hand in a full bathtub: energy is stored as standing waves, and the waves build up higher and higher until frictional losses finally halt their growth. Inside an empty microwave oven you might have 50,000 watts in one direction and 49,000W in the other (with the magnetron supplying the 1000W difference.)… > > …With nothing in the oven chamber, either the metal walls and glass parts get very hot, or an electric arc bursts forth from a sharp metal point somewhere inside the oven cavity. Of note, if 50 kW was the breakdown point of an oven supplying 1 kW, then you’d have at least 50 seconds (50 seconds assumes no losses) for it to reach that point (of course, what would happen if you opened it after 49 seconds? I have always heard that microwaves instantly dissipate, thus opening a running microwave, which have an automatic shutoff, is safe). ETA: Or [this](http://youtu.be/4YzJ99CDPfk?t=2m58s) happens (just kidding) ---
**Author:** ![RedSwinglineOne](https://avatars.discourse-cdn.com/v4/letter/r/a9adbd/32.png) [@RedSwinglineOne](https://boards.straightdope.com/u/RedSwinglineOne)\ **Post date:** [March 6, 2013, 7:08pm UTC](https://boards.straightdope.com/t/how-come-a-microwave-oven-can-heat-an-empty-plate/652195/18 "2013-03-06T19:08:03Z")
> [@Michael63129](#): > > Incorrect, or microwaves wouldn’t use metal for the oven cavity (often painted, but metal) - metal \*reflects \*microwaves. 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. ---
**Author:** ![filmore](https://avatars.discourse-cdn.com/v4/letter/f/7993a0/32.png) [@filmore](https://boards.straightdope.com/u/filmore)\ **Post date:** [March 6, 2013, 7:10pm UTC](https://boards.straightdope.com/t/how-come-a-microwave-oven-can-heat-an-empty-plate/652195/19 "2013-03-06T19:10:47Z")
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? ---
**Author:** ![Michael63129](https://avatars.discourse-cdn.com/v4/letter/m/3da27b/32.png) [@Michael63129](https://boards.straightdope.com/u/Michael63129)\ **Post date:** [March 6, 2013, 7:22pm UTC](https://boards.straightdope.com/t/how-come-a-microwave-oven-can-heat-an-empty-plate/652195/20 "2013-03-06T19:22:21Z")
> [@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. As I said, metal does reflect microwaves - but they also induce a current in the metal, just like a magnet does (after all, they are electromagnetic waves). It is that current that makes thin metal objects, like the metal film used in crisping sleeves, get hot. Of course, if it gets hot, then some energy is effectively absorbed, just like losses in a transformer will “absorb” energy, otherwise there is no power lost. In other words, the metal walls of a microwave might reflect 99% of the microwaves that strike them, dissipating the rest as heat, but the much thinner metal film is a crisping sleeve reflects considerably less due to higher losses (obviously not all, say 90% reflected/goes through, or the hot pocket wouldn’t heat up internally). [Next page](https://boards.straightdope.com/t/how-come-a-microwave-oven-can-heat-an-empty-plate/652195.md?page=2)