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Old 03-06-2013, 11:04 AM
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How come a microwave oven can heat an empty plate??

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 ...
Old 03-06-2013, 11:15 AM
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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.
Old 03-06-2013, 11:17 AM
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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.
Old 03-06-2013, 11:23 AM
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Plausible WAG: Humidity in the air is heated and conducts the heat to the plate?
Old 03-06-2013, 11:24 AM
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Originally Posted by johnpost View Post
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?
Old 03-06-2013, 11:27 AM
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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.
Old 03-06-2013, 11:30 AM
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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.
Old 03-06-2013, 11:33 AM
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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.
Old 03-06-2013, 11:36 AM
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Originally Posted by Nametag View Post
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.
Old 03-06-2013, 11:40 AM
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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.
Old 03-06-2013, 11:40 AM
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Originally Posted by AndrewL View Post
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.
Old 03-06-2013, 12:05 PM
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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.
Old 03-06-2013, 12:20 PM
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Originally Posted by Ruken View Post
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.)
Old 03-06-2013, 12:40 PM
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Originally Posted by Baracus View Post
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.
Old 03-06-2013, 01:42 PM
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Originally Posted by AndrewL View Post
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, 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).
Old 03-06-2013, 01:50 PM
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So if you turn on the microwave oven with nothing inside, where does the energy go?
Old 03-06-2013, 02:06 PM
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Originally Posted by Crafter_Man View Post
So if you turn on the microwave oven with nothing inside, where does the energy go?
It builds up until something gives, whether it is an arc inside the oven cavity or the magnetron itself:

Quote:
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 happens (just kidding)

Last edited by Michael63129; 03-06-2013 at 02:09 PM.
Old 03-06-2013, 02:08 PM
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Originally Posted by Michael63129 View Post
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.
Old 03-06-2013, 02:10 PM
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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?
Old 03-06-2013, 02:22 PM
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Originally Posted by RedSwinglineOne View Post
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).
Old 03-06-2013, 02:54 PM
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Quote:
Originally Posted by smiling bandit View Post
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.
Old 03-06-2013, 03:09 PM
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Quote:
Originally Posted by Crafter_Man View Post
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).
Old 03-06-2013, 03:22 PM
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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.
Old 03-06-2013, 04:48 PM
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Quote:
Originally Posted by RedSwinglineOne View Post
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.

Quote:
Originally Posted by filmore View Post
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.
Old 03-06-2013, 05:23 PM
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Originally Posted by TriPolar View Post
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.
Old 03-06-2013, 05:41 PM
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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.
Old 03-06-2013, 06:05 PM
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Originally Posted by TSBG View Post
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.
Old 03-06-2013, 06:38 PM
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Originally Posted by TriPolar View Post
This is why I avoid standing too close to the microwave.
Quote:
Originally Posted by Michael63129 View Post
....

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.)

Last edited by Leo Bloom; 03-06-2013 at 06:39 PM.
Old 03-06-2013, 07:33 PM
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You have to read the post I was responding to and my username again.
Yoink.
Old 03-07-2013, 01:27 AM
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Originally Posted by engineer_comp_geek View Post
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.)
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