Are microwaves distributed unevenly in the oven? Why do manufacturers claim that…

Are microwaves distributed unevenly in the oven? Why do manufacturers claim that microwaves with turntables are more effective than microwaves without turntables?

As the microwaves bounce around the inside of the cooking chamber, they tend to interfere with one another. There are usually regions in which the waves that follow various paths almost cancel one another and regions in which the waves reinforce one another. These regions don’t cook food equally well. If the microwaves are canceled in one region, cooking will be slow there. If the microwaves reinforce one another in another region, cooking will be fast there. If you simply leave food in one place and try to cook it in the microwaves, the cooking will be uneven. However, if the food is rotated continuously, these good and bad cooking regions will be blurred away so that the food will all cook at about the same speed.

In microwaves – you heat up food really fast. Is it true that microwaved food wi…

In microwaves – you heat up food really fast. Is it true that microwaved food will cool down faster than oven heated food? Someone told me “if it heats fast, it will then cool fast.”

No. Microwaves cook the food in a very different manner than normal thermal heating, but microwaved food has the same thermal energy that it would have if it had been warmed by more traditional methods. Microwaves heat food by exerting torques on the individual water molecules in the food. These molecules jiggle back and forth and sliding friction between them heats the food. This peculiar route to energy addition explains why frozen portions of the food don’t heat well: the water molecules are rigidly oriented and can’t jiggle back and forth in order to become hot. But despite the fancy heating scheme, the food retains no memory of how it was heated. Once it is uniformly hot, it cools at a rate that depends only on how heat is transported out of it. Microwaved food cools just as slowly as normally cooked food.

Are microwaves harmful to you? Is eating microwaved food harmful?

Are microwaves harmful to you? Is eating microwaved food harmful?

Microwaves can heat your body by adding thermal energy to the water molecules in you. This heating can be damaging if it’s not controlled. Most of your body is protected from slow heating because your blood carries heat away from any local hot spots so that you warm evenly. However there are a few places that aren’t cooled by your circulation and can heat up locally enough to denature the protein molecules and cause biological injury. The cornea of your eye is a good example. It can be heated and damaged because it’s not cooled well. That’s why you must be careful not to look into a strong beam of microwaves. As for microwaved food, the only effect of cooking with microwaves is hot food. There is no “radiation damage” or “radioactivity,” as there might be with x-ray or gamma radiation. Some foods should not be cooked in a microwave only because the uneven heating may allow certain parts to become too hot. Those parts may burn you when you eat them or they may suffer thermal damage that diminishes their nutritional value.

Inside the microwave oven, what is it that heats the food? How does the heat com…

Inside the microwave oven, what is it that heats the food? How does the heat come out; where did it come from?

The food is heated by the microwaves themselves and these microwaves are piped into the cooking chamber from the magnetron. The magnetron has electric charge sloshing back and forth in its tines. A small antenna uses that sloshing charge to emit microwave radiation. The water molecules in the food absorb this microwave radiation and turn its energy into heat. The usual rules of heat transfer don’t apply in the heating process—the energy arrives at the food as microwaves, not heat.

Can microwaves be emitted to travel in one direction?

Can microwaves be emitted to travel in one direction?

Yes. Like all electromagnetic waves, microwaves can be focused and concentrated in a particular direction. That is exactly what microwave dish antennas (e.g., satellite dishes) do. At the transmitter, they focus the microwaves emitted by a smaller antenna so that those microwaves travel as a parallel beam. At the receiver, they focus the parallel beam of microwaves onto a smaller antenna. You can think of the microwaves as very long wavelength light waves, so that anything you can do with light (e.g., focus it, form images with it, or bend it with optical devices), you can also do with microwaves. The only problem is that the optical elements you use for microwaves must be larger, because the microwaves have longer wavelengths.

On the subject of defrosting frozen food in a microwave oven, you must refer to …

On the subject of defrosting frozen food in a microwave oven, you must refer to the old BTU formula which states “It takes one BTU to raise the temperature of 1 pound of water 1° (Fahrenheit), but when water is changing state from a solid (ice) to a liquid (water), it must absorb 144 BTUs (per pound).” – George R.

This observation accounts for much of difficulty with defrosting food in general and defrosting food in a microwave oven in particular. It often takes more heat to melt ice in the food than it does to actually cook the food once the ice has melted. Since ice doesn’t absorb microwaves well, heating frozen foods in a microwave oven is a tricky business. Any region of food that melts early will absorb microwaves strongly and overheat while any region of food that remains frozen won’t absorb microwaves well and won’t receive the enormous amounts of heat it needs just to melt. The result is typically a food item with some frozen parts and some boiling hot parts. To avoid this problem, microwave oven defrost cycles let the food sit in between bursts of microwave heating. That way, there is time for heat to flow through the food and keep the internal temperatures relatively uniform. Parts of the food that heat well have time to transfer heat to parts that don’t heat well and the whole item thaws and heats together.

How can microwaves heat something? Radio waves don’t warm things very much.

How can microwaves heat something? Radio waves don’t warm things very much.

The electric field of a microwave flips back and forth at just about the right frequency to have the largest effect on water molecules. The water molecules try to follow the reversing electric field and, in doing so, become hotter and hotter. Radio waves flip too slowly to have very much effect on water. Furthermore, the microwaves in an oven are far more intense than the radio waves that we’re used to have around us so that common radio waves just don’t do very much cooking.

Why does water react in a violent and dangerous way when overheated in a microwa…

Why does water react in a violent and dangerous way when overheated in a microwave oven? CA

Water doesn’t always boil when it is heated above its normal boiling temperature (100 °C or 212 °F). The only thing that is certain is that above that temperature, a steam bubble that forms inside the body of the liquid will be able to withstand the crushing effects of atmospheric pressure. If no bubbles form, then boiling will simply remain a possibility, not a reality. Something has to trigger the formation of steam bubbles, a process known as “nucleation.” If there is no nucleation of steam bubbles, there will be no boiling and therefore no effective limit to how hot the water can become.

Nucleation usually occurs at hot spots during stovetop cooking or at defects in the surfaces of cooking vessels. Glass containers have few or no such defects. When you cook water in a smooth glass container, using a microwave oven, it is quite possible that there will be no nucleation on the walls of the container and the water will superheat. This situation becomes even worse if the top surface of the water is “sealed” by a thin layer of oil or fat so that evaporation can’t occur, either. Superheated water is extremely dangerous and people have been severely injured by such water. All it takes is some trigger to create the first bubble-a fork or spoon opening up the inner surface of the water or striking the bottom of the container-and an explosion follows. I recently filmed such explosions in my own microwave (low-quality movie (749KB), medium-quality movie (5.5MB)), or high-quality movie (16.2MB)). As you’ll hear in my flustered remarks after “Experiment 13,” I was a bit shaken up by the ferocity of the explosion I had triggered, despite every expectation that it would occur. After that surprise, you’ll notice that I became much more concerned about yanking my hand out of the oven before the fork reached the water. I recommend against trying this dangerous experiment, but if you must, be extremely careful and don’t superheat more than a few ounces of water. You can easily get burned or worse. For a reader’s story about a burn he received from superheated water in a microwave, touch here.

Here is a sequence of images from the movie of my experiment, taken 1/30th of a second apart: