HOW THINGS WORK
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Microwave Ovens

Louis A. Bloomfield, Professor of Physics, The University of Virginia


Think of this site as a radio call-in program that's being held on the WWW instead of the radio. If you ask how something works, using the button below, I'll try to provide an explanation. You'll find a more comprehensive discussion of many common objects in my book: How Things Work: the Physics of Everyday Life. Feel free to create links to this site; I'm trying to encourage everyone to learn about the physics and science of the world around them. Please let me know what you think about this site. -- Lou Bloomfield

If a microwave oven door were to open while it was still on, what would happen? Could it hurt you? - JP

Are microwaves attenuated in air?

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

If a microwave oven with painted inside walls has some of the paint removed due to a very small fire caused by arcing, is it still safe to use?

My science book said that a microwave oven uses a laser resonating at the natural frequency of water. Does such a laser exist or was that a major typo?

Would it be possible to put a thermometer inside a microwave oven? Would the microwaves have an effect on an electronic thermometer? Would they have an effect on a mercury thermometer? -- R

You said that microwaves heat food by twisting water molecules back and forth and having those water molecules rub against one another to experience a molecular form of "friction." Since vibrating molecules are the fundamental manifestation of heat, why is the friction necessary at all? -- GS, Kanata, Canada

You said that some rooms in the physics building are made with metal to specifically keep electromagnetic waves out. How does that work?

Can microwave ovens leak microwaves? Is my mother's warning not to stand in front of the microwave while it's on valid?

The frequency at which microwave ovens operate is about 2.45 GHz, which is about the resonant frequency of the free water molecule. Can you calculate this resonant frequency or was it determined experimentally? -- GW

When I warm more than one cup of coffee or milk together in a microwave oven, some of them warm more than others. Why does this happen? Is there something wrong with our microwave oven? -- ON, Istanbul, Turkey

You claim that the metal walls of the cooking chamber in a microwave oven protect us from the microwaves. How can they protect us from microwaves when they aren't even able to keep sound contained? You can hear popcorn popping through the walls. -- RB, Beltsville, MD

Could microwave heating be used to treat sewage to wipe out disease organisms in it? -- KO

What about the effects of microwaves on the cellular structure of the item in the oven? I've heard that cells are ruptured violently by microwave radiation and that the ingestion of such materials affects the immune system. - AB

Is there a standard time that one should wait before eating food that has been heated in a microwave oven? - M

Does microwave cooking affect the nutritional value of food?

I recently place a green tomato in the microwave oven. I forgot to turn on the microwave and in the morning the tomato was ripe. Can you explain this? -- KH

When two identical items are cooked, one with a microwave oven and the other on the stove, which will cool faster? -- CR

I know that microwaves only heat polar molecules but what about aluminum foil and graphitic carbon, which are both heated by microwaves even though they have no dipole moments? -- EB

Assuming microwave ovens cook on the principle of "moist" heat cookery, what are the general effects of microwave cooking on various foods, including effects on chemical structure? -- EJ, Sydney, Australia

I know that an electromagnetic wave cannot pass through the holes in a metal cage (a Faraday cage) if those holes are significantly smaller than the wavelength of the wave. But what if it is just a constant electric field? What determines the hole size now? -- KBH, Logan, Utah

How do microwave ovens affect people fitted with pacemakers? -- W

Do hand carried microwave heaters exist or must the microwaves always be enclosed, as they are in a microwave oven? -- AL, Umea, Sweden

Why does food become soggy after heating in the microwave oven; particularly pastry?

Why can you put a can of frozen concentrate juice in the microwave? The metal doesn't spark or burn.

If you stand near a microwave oven, looking at your food, is it dangerous--tissue damage or make you blind?

Is it possible isolate a room or part of it totally from microwaves? -- DMJ

Have you made RF leakage measurements on a sample of microwave ovens? I understand that the FDA requires that if measured 5 cm away from any of the oven's surfaces, the RF leakage must be less than 1 mW/cm2 for new ovens and less than 5 mW/cm2 over the oven's life time. I'm just curious what actual measurements reveal about a "typically used" oven. -- S

Is it true that microwaves cause cancer?

Is it harmful for children to sit too close to microwave ovens? Is it possible to get "burned" opening the microwave oven during a cycle or too soon after a cycle? I realize the oven shuts off, but is there residual radiation? -- C

What metals and other substances are used in microwave ovens? Specifically, what is the substance on the inside of the microwave that absorbs all the microwaves? -- AD, San Anselmo, CA

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

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?

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

What are the key components of a microwave oven?

Do microwaves have no effect on gas?

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

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

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

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

Is it possible to eat a microwave while you eat food that was cooked in the microwave oven? - PTW

If the condenser in a microwave is bad, what is the most likely reaction the microwave generator will exhibit? -- IF, Bakersfield, CA

How can we clean the microwave oven? - PTW

Don't microwaves penetrate metal at all? -- DR, Tampa, FL

How can I check the magnetron in a home microwave oven? I have checked the HV (high voltage) transformer, the rectifier, and capacitor and all are OK. Does the magnetron output decrease with age? The oven has a hum which is much louder than normal. -- AA, Ontario, CA

Is heating milk by microwave advisable? - I

Why do metal objects spark/arc in the microwave? Why don't the metal walls of the microwave spark? - JR

Why is it bad to put metal in a microwave oven? - OR

What is the black holey stuff on the doors of microwave ovens? Is it for looks, protection, or what? - K

Why don't microwaves get stuck in the food we put in the microwave oven?

There is an experiment involving grapes and microwaves that we found on the internet. If a grape is cut in half--with a piece of skin attached between the two halves--and it is then microwaves, sparks are produced. What is happening? -- GB, Antioch, CA

Don't microwaves change the molecular structure and composition of food, by ejecting some electrons from atoms and forming cancer-causing free radicals? If I should stand away from a microwave to avoid possible leakage, why would I eat microwaved food?

How does radar absorbent materials work. How effective is stealth technology? -- DP, Scottsbluff, NE

In a microwave oven, does food cook from the inside out or outside in? -- KS, Essex, England

I recently acquired a microwave that "doesn't cook as fast as it used to." Does this sound right? What type of service might need to be performed? - W

If microwave cookers are so energy-efficient, why can't similar machines be used as hot water heaters or in central heating systems? - GB

What does the inside of a microwave oven look like? Please show illustrations. -- Dade County, FL

How does a magnetron work? -- MM, Czech Republic

I have heard that microwaving can destroy certain nutrient molecules in food, such as vitamins. Is this true? -- D, Boulder, CO

Can plastic melt in a microwave oven? How does this process work? Can plastic burn in a microwave oven? - HD

I've heard from many people that you should not stand directly in front of and no closer than 30 feet while a microwave oven is on? Why? If this is a myth how did it get started?

Does cooking in a microwave oven destroy the nutritional value of foods? Are microwaves radioactive? Does radiation "leak" from the oven? - DL

Why does a microwave oven heat organic material and not inorganic material? -- JM, Columbus, OH

Is 2.45 gigahertz the best frequency for a microwave oven? Is that frequency at or near a water molecule resonant frequency? Do water molecules have a resonant frequency?

My mother owns a microwave oven that is about 20 years old. It looks like new and has always been well taken care of. However, I was wondering whether it is still safe to use. Should I have it tested for leakage? -- KE, Milwaukee, WI

If you microwaved bean plant seeds over a period of weeks while they were growing, would they grow faster or longer, and if they would, would that be due to the heat or some effect of the microwave radiation? - DS

Were microwaves invented for the microwave oven?

What is the purpose of the grid on the glass door of the microwave oven?

Can we add a section to a microwave oven that gets the food or drinks cold? - MH

Does the volume in the cooking chamber of a microwave oven affect the rate at which it cooks the food? In other words, which cooks faster, a small microwave oven or a large one? - RP

What is a microwave and what does it do? -- AH, Rochester, MN

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

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

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?

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

How does a microwave oven heat food?

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?

You said an ice cube will not get hot in the microwave because the molecules won't "flip". If this is so, then why do frozen foods cook in the microwave?

Why do some microwave ovens not seem to have a metal surface in the cooking area?

Why do microwave ovens cook so rapidly?

What happens if you start the microwave oven with nothing inside?

What exactly goes on when you're cooking a potato in the microwave and it explodes?

What containers are not safe to use in a microwave? I am particularly concerned about Styrofoam containers as I use them to make TV dinners for my family. Is it OK to heat directly in these containers?

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.

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

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."

If a radio station operated at 2.45 gigahertz, could you pick it up when your microwave was turned on and attached speakers?

If a microwave does not melt ice, how does the "Defrost" setting on the microwave work?

I'd heard that if I cook in the microwave oven, there will be a possible formation of free radicals. Is it true? If yes, how? -- Angela I.

I have a friend who refuses to stand in front of the microwave oven in his kitchen, because he feels the "nuclear waves" leak and will cause his sperm to deform (and he doesn't want ugly kids). Is this true? What about car phones? He heard they were bad, too!

How does the resonant cavity in the magnetron work?

How does a microwave oven defrost foods? Doesn't it only work with water, not ice?

How do metal rods short out the microwaves?

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

Can microwaves be emitted to travel in one direction?

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

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


Last Updated Monday, August 14, 2000 at 14:57:23
Copyright 1997-2000 © Louis A. Bloomfield, All Rights Reserved