Tag: microwave ovens

How can we polarize a molecule? — AD, Manaus City, Amazonia, Brazil

Some molecules, including water, are naturally polarized. This means that they have a positively charged end and a negatively charged end. But even normally non-polar molecules such as carbon dioxide can be polarized by exposing them to strong electric fields. Electric fields exert forces on electric charges and cause the electric charges in a molecule to rearrange—the positive charges in the molecule shift in one direction and the negative charges in that molecule shift in the other. As a result of this applied electric field, the molecule acquires a polar character—a negatively charged end and positively charge end. However, this polar character disappears as soon as the electric field is removed.

Who invented the microwave oven and how did he think of it?

In 1945, American engineer Percy Le Baron Spencer was working with radar equipment at Raytheon and noticed that some candy he had in his pocket had melted. Radar equipment detects objects by bouncing microwaves from them and Spencer realized that it was these microwaves that had heated the candy (as well as his body…oops!). Raytheon soon realized the potential of Spencer’s discovery and began to produce the first microwave ovens: Radaranges. These early devices were large and expensive and it wasn’t until 1967, when Amana, a subsidiary of Raytheon, produced the first household microwave oven, that microwave ovens became widely available.

If microwaves “bounce” or reflect inside the cooking chamber, is it important for all of the surfaces (walls) of the oven to be flat? What would happen if the cooking chamber were cylindrical or circular? Would the microwaves bounce off the walls and then cancel each other out?

A microwave oven with a cylindrical or spherical cooking chamber would have a problem with non-uniform cooking. But before I look at why, I should note that even a microwave oven with a box-like cooking chamber exhibits non-uniform cooking. That’s because the microwaves that are bouncing around inside the cooking chamber are all coherent—they are parts of a single, giant wave—and they can interfere strongly with one another. That means that several reflected microwaves can cancel or enhance one another as they cross, leading to regions inside the cooking chamber that cook quickly and other regions that cook slowly. That’s why it’s important to move the food around the cooking chamber during cooking—so that the food cooks evenly.

If the oven’s cooking chamber weren’t box-like, there would be a new problem to contend with: a tendency for the microwaves to be concentrated or focused in a particular region. Just as a cylindrical or spherical mirror bends the light rays it reflects, so the curved walls of a non-boxlike cooking chamber would bend the microwaves it reflects. It would tend to focus those microwaves in particular regions (such as the center of the cylinder or sphere) so that there would certain regions inside the chamber where the microwaves would be particularly intense and cooking would proceed very quickly.

Does microwave cooking break molecular chains? Does any recombination of ions take place in the food and, if so, is there a possibility of eating some type of toxin formed during cooking?

The answers to all of these questions are no. Microwave cooking merely heats the water molecules, which in turn heat the food. The only molecular rearrangements that occur are those that are caused by warming the food toward the boiling temperature of water. In fact, there is less chemistry done during microwave cooking than is done in a normal oven. For example, one of the problems with microwave cooking is that food doesn’t brown because the high temperatures needed to chemically modify the food molecules (and cause browning) aren’t reached in microwave cooking. So you shouldn’t have any fear of food cooked in a microwave oven. The microwaves don’t damage it any more than heating it in boiling water would.

How does a microwave oven heat food?

A microwave oven uses a vacuum tube called a magnetron to create intense microwaves inside the cooking chamber. These microwaves are electromagnetic waves with a frequency of 2.45 gigahertz or 2,450,000,000 cycles per second. They are similar to normal radio waves, except that they have a higher frequency. Because of these microwaves, the electric field at any point inside the cooking chamber fluctuates back and forth 2.45 billion times each second. That means that an electrically charged particle at any point in the cooking chamber will be pulled first one way and then the other, back and forth 2.45 billion times each second. While water molecules aren’t electrically charged overall, they do have electrically charged ends—one end is positively charge and the other is negatively charged. In the presence of the microwave radiation, these water molecules find themselves twisted back and forth very rapidly. As they twist, they rub against one another and friction heats them up. The water becomes hot and this hot water, in turn, cooks the food. Food that doesn’t contain water (like salt or oil) won’t get hot. Neither will food in which the water molecules can’t turn (like ice or frozen food). That’s why it’s hard to defrost frozen food in a microwave.

Why are you required to have an item in the microwave oven while it is operating?

When a microwave oven is cooking food, electrons move rhythmically back and forth inside the magnetron tube and create the microwaves. These microwaves flow through a metal pipe and into the cooking chamber, where they are absorbed by the water in the food and thus heat the food (the twisting back and forth of the water molecules, described elsewhere on this page, not only heats the food—it also absorbs the microwaves). If there is no food in the cooking chamber, the microwaves build up in the cooking chamber until they are so intense that large numbers of them flow backward through the pipe to the magnetron. These microwaves reenter the magnetron and disrupt the motion of electrons inside it. The magnetron begins to misbehave and can be damaged as a result. To avoid such damage, you want to be sure that there is something in the cooking chamber to absorb the microwaves before they return to the magnetron and cause trouble. In short, don’t run the microwave empty for any long periods of time.

There is a story circulating by email about a 26 year old man who heated a cup of water in a microwave oven and had it “explode in his face” when he took it out. He suffered serious burns as a result. Is this possible and, if so, how did it happen? — JJ, Kirksville, Missouri

Yes, this sort of accident can and does happen. The water superheated and then boiled violently when disturbed. Here’s how it works:

Water can always evaporate into dry air, but it normally only does so at its surface. When water molecules leave the surface faster than they return, the quantity of liquid water gradually diminishes. That’s ordinary evaporation. However, when water is heated to its boiling temperature, it can begin to evaporate not only from its surface, but also from within. If a steam bubble forms inside the hot water, water molecules can evaporate into that steam bubble and make it grow larger and larger. The high temperature is necessary because the pressure inside the bubble depends on the temperature. At low temperature, the bubble pressure is too low and the surrounding atmospheric pressure smashes it. That’s why boiling only occurs at or above water’s boiling temperature. Since pressure is involved, boiling temperature depends on air pressure. At high altitude, boiling occurs at lower temperature than at sea level.

But pay attention to the phrase “If a steam bubble forms” in the previous paragraph. That’s easier said than done. Forming the initial steam bubble into which water molecules can evaporate is a process known as “nucleation.” It requires a good number of water molecules to spontaneously and simultaneously break apart from one another to form a gas. That’s an extraordinarily rare event. Even in a cup of water many degrees above the boiling temperature, it might never happen. In reality, nucleation usually occurs at a defect in the cup or an impurity in the water—anything that can help those first few water molecules form the seed bubble. When you heat water on the stove, the hot spots at the bottom of the pot or defects in the pot bottom usually assist nucleation so that boiling occurs soon after the boiling temperature is reached. But when you heat pure water in a smooth cup using a microwave oven, there may be nothing present to help nucleation occur. The water can heat right past its boiling temperature without boiling. The water then superheats—its temperature rising above its boiling temperature. When you shake the cup or sprinkle something like sugar or salt into it, you initiate nucleation and the water then boils violently.

Fortunately, serious microwave superheating accidents are fairly unusual. However, they occur regularly and some of the worst victims require hospital treatment. I have heard of extreme cases in which people received serious eye injuries and third degree burns that required skin grafts and plastic surgery.

You can minimize the chance of this sort of problem by not overcooking water or any other liquid in the microwave oven, by waiting about 1 minute per cup for that liquid to cool before removing it from the microwave if there is any possibility that you have superheated it, and by being cautious when you first introduce utensils, powders, teabags, or otherwise disturb very hot liquid that has been cooked in a microwave oven. Keep the water away from your face and body until you’re sure it’s safe and don’t ever hover over the top of the container. Finally, it’s better to have the liquid boil violently while it’s inside the microwave oven than when it’s outside on your counter and can splatter all over you. Once you’re pretty certain that the water is no longer superheated, you can ensure that it’s safe by deliberately nucleating boiling before removing the cup from the microwave. Inserting a metal spoon or almost any food into the water should trigger boiling in superheated water. A pinch of sugar will do the trick, something I’ve often noticed when I heat tea in the microwave. However, don’t mess around with large quantities of superheated water. If you have more than 1 cup of potentially superheated water, don’t try to nucleate boiling until you’ve waited quite a while for it to cool down. I’ve been scalded by the stuff several times even when I was prepared for an explosion. It’s really dangerous.

For a reader’s story about a burn he received from superheated water in a microwave, touch here.

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.

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.

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.