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 …
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.
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.
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.
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.
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.
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.
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.
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.)
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).
It builds up until something gives, whether it is an arc inside the oven cavity or the magnetron itself:
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).
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 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?
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).