Tag: microwave ovens

I heard recently of someone with a pacemaker who went near a microwave oven and his pacemaker faulted, with him needing urgent medical attention. How did this happen? I also know of someone currently undergoing chemotherapy, who was told by his doctor not to eat food from a microwave oven. Why?

A pacemaker contains electronic circuits and wires that can act as antennas for microwaves. If a pacemaker is exposed to sufficiently intense microwaves, currents will begin to flow in those wires and circuits, and these currents may cause computational errors to occur or they may cause the circuitry to overheat. But while a pacemaker is far more sensitive to microwave radiation than say your hand is, I’m still surprised that enough microwave radiation leaked out of the oven to cause trouble. I’d suspect a real problem with that oven.

As for the chemotherapy question, I can’t think of any reason why the doctor would suggest avoiding cooking food in a microwave oven. Unless I hear otherwise, I would suspect ignorance on the part of the doctor. The doctor may not understand the difference between “microwave radiation” and “gamma radiation”.

Why does microwave radiation affect plant seeds differently? If you microwave sunflower seeds 30 seconds, they germinate faster than if you did not microwave them at all, and yet if you microwave them for 60 seconds, the seeds do not germinate at all. If you do this same experiment with carrot seeds, the non-radiated seeds, the 30 second and 60 second seeds all germinate within 14 days. Why? Is it because the sunflower seeds are larger and absorb more radiation than the smaller carrot seeds? — ST, Mobile, AL

When you expose the seeds to microwave radiation, you are selectively heating portions of the insides of the seeds. Fats and oils don’t absorb microwaves well but water does, so the parts of the seeds that become hottest are those that contain the most water molecules. Evidently, heating the water-containing portion of a sunflower seeds slightly cause that seed to germinate faster, but heating that same portion too much sterilizes the seed. That observation indicates that a moderate temperature rise causes the chemical reactions of germination to occur more rapidly while a more severe temperature rise denatures some of the critical biological molecules and kills the seed. The absence of any effect in carrot seeds may indicate that they don’t have enough water in them to absorb the microwaves. It may also indicate that they can tolerate higher temperatures without undergoing the chemical reactions of germination and without experiencing damage to their critical molecules.

Is there an inexpensive device for detecting leaks from a microwave oven?

Yes. You can get one from a hardware or appliance store for about $5 to $30. ComfortHouse.com sells one on-line at www.comforthouse.com. While I have tended to downplay the leakage issue in the past, I bought a tester and found that the microwave oven in my laboratory actually leaked significantly. I had used it in many class demonstrations, so it had been abused and the door wasn’t properly aligned any more. I retired it. Incidentally, the tester contains only two components: a fast diode and a current meter. It detects microwave in the same way that a crystal radio detects an AM radio broadcast. However, I should note that both the International Microwave Power Institute (IMPI) and the FDA caution against trusting those simple and not particularly accurate meters, and recommend that you take your microwave oven to a service shop for inspection with an FDA certified meter.

Can one’s health be adversely affected by the use of certain wraps, films, or containers, when heating food in the microwave?

When various plastics become hot, their molecules become more mobile. The most obvious such case is when a plastic actually melts. But even before it melts, a plastic can begin to lose molecules to objects that are touching it. However, the plastics used in cooking are pretty non-toxic, so that even eating pieces of those plastic won’t cause you any significant trouble. On the other hand, I would be careful with plastics that weren’t intended for cooking. Some non-food related plastics are mixed with additives called “plasticizers” that keep them softer than they would be if they were pure. These plasticizers have a tendency to migrate out of the plastics, giving such things as “vinyl” their characteristic odors. Heating a plastic containing a plasticizer can drive this plasticizer out of the plastic and into something else. I don’t think that it’s a good idea to eat plasticizers so I would suggest not cooking with plastics that weren’t intended for use with food. Still, not all plasticizers are bad—water is an excellent plasticizer for such common plastics as hair and cotton.

Do microwaves have no effect on gas?

While water vapor can absorb microwaves at certain frequencies, the absorption mechanism is very different from the one that causes liquid water to become hotter. Steam isn’t affected very much by a microwave oven.

What are the key components of a microwave oven?

In addition to the digital controller that runs the microwave, it contains (1) a power relay that allows the controller to turn on and off the microwave source, (2) a power transformer that produces the high voltage electricity needed by the magnetron, (3) a power rectifier that converts the alternating current from the transformer into the direct current needed by the magnetron, (4) a capacitor that smoothes out ripples in the direct current leaving the rectifier, (5) a magnetron that uses the high voltage direct current to produce an intense beam of microwaves, (6) a wave guide that transports the microwaves from the magnetron to the cooking chamber, and (7) a cooking chamber in which the food absorbs the microwaves and becomes hotter.

In cooking, what are some examples of absorbing microwaves, transferring microwaves, and reflecting microwaves? – K

In a microwave oven, water-containing foods absorbs microwaves. The microwaves disappear as they pass through the food and the food becomes hotter. Microwaves are transferred from the small antenna near the magnetron to the cooking chamber by sending those microwaves through a metal pipe. This rectangular pipe is typically a few inches wide and an inch or so tall, and is called a “wave guide.” Finally, the walls of the cooking chamber reflect the microwaves. When a microwave encounters a metal surface, it pushes electric charges back and forth in the metal and this moving charge causes the microwave to reflect.

In your explanation of why microwaves don’t penetrate the oven door, you said it is because the holes in the screen are smaller than the wavelength of a microwave. Wouldn’t it be the amplitude of the wave and not its wavelength? – P

When a microwave tries to pass through the holes in the metal screen, electric charges in that screen begin to move. The microwave’s electric field fluctuates back and forth rapidly and the charges reverse directions rapidly as a result. If the electric current made up of these charges has enough time to travel all the way around each hole before it reverses directions, it will be as though the screen were made of solid metal and the screen will be able to completely reflect the microwave.

Like any electromagnetic wave, a microwave has a wavelength (the spatial distance between adjacent wave crests) and a period (the temporal spacing between adjacent wave crests). The electric current that a microwave propels through a metal travels about one microwave wavelength during one microwave period. Therefore, the current can work its way around a hole in the metal only if the hole is significantly smaller than the microwave wavelength. The amplitude of the microwave doesn’t matter—increasing the amplitude of the microwave just makes more current flow.

Where is the best place to put a microwave oven? Is it dangerous to place it on the refrigerator? – PTW

You can put a microwave oven anywhere that it’s stable and where it has adequate ventilation. A microwave oven has a fan and vents through which it gets rid of its excess heat. You mustn’t block the vents or the oven will overheat.

Don’t microwaves penetrate metal at all? — DR, Tampa, FL

If the metal is a good conductor, then the microwaves don’t penetrate more than a fraction of a millimeter. That’s because the microwave electric fields push on the metal’s mobile electrons and those electrons immediately rearrange in such a way that they cancel the microwave fields inside the metal. Only the skin of the metal responds to the fields and it shields the rest of the metal from the microwaves.