What prevents a microwave oven from heating things outside the oven?

Question

Microwaves are pretty powerful, and it's intriguing that a plastic door and thin metal shell prevent those microwaves from escaping and cooking the cooker. Why don't they heat outside the box?

Answer 1

The chamber is technically called a Faraday Cage. Its made of any conducting material, any of which will block microwaves and other forms of EM radiation.

The walls of the microwave are made of this material, as well as the mesh screen in the microwave door.

If you've never noticed, look on the clear door of the microwave. See the little screen on the inside? It has lots of holes in it, BUT these holes are smaller than the wavelength of the microwaves, so the microwaves cant physically fit through those holes to come and cook you!

They simply cant fit through the holes, and microwaves arent energetic enough to penetrate the Faraday Cage.

hope this helps :)

edit: and just to note, the standards on microwave ovens actually allow them to leak quite a few milliwatts of microwave energy during operation. Its not enough to cook you of course, but i dont stand with my face pressed against the glass every time i use it just in case :)

Answer 2

Microwaves operate at a specific wavelength, and the screen, and the door seal, and also the fact that the oven will not operate with the door open, prevent you from getting heated up. Never run a microwave oven without the inner protective screen. It is what you look through when you see the oven light come on. The wavelength of microwaves is large enough that few of the microwaves can escape through the holes. They just bounce around inside the microwave oven. Never jury rig a microwave so it will operate with the door open. Bad stuff will happen. Also make sure the door seals tightly. Microwaves do not travel far, so any distance from the microwave, say 3 feet or so, effectively reduces the radiation to zero. Also, never let children or adults get close to the viewing window and be allowed to stare into the oven.

Answer 3

I love my convection oven for baking bread, cookies and pies. Those are things you wouldn't do in a microwave. Mostly I use my microwave for cooking some vegetables and reheating. You really need to do your cost comparisons at an appliance store. I don't think of a convection or microwave ovens as a substitute for one another. If I could only have one or the other, I'd have the microwave because I have an oven in my range.

Answer 4

The wavelength of microwaves is just about large enough not to fit through the grid on the door so it is reflected back into the oven. Kind of like a bouncy ball and fence where the holes are just too small to let the ball through when thrown at it. The reason why the light from the inside of the oven can get through it because the wavelength of light is much smaller than that of microwaves so can get through the grid. just as if a bb where shot at a chain link fence then it would be likely to get through.

Answer 5

As somebody else pointed out, microwaves at the frequency used in a microwave oven 'heat' up water.

But the human body is mostly water,so never use a damaged microwave as then the 'leakage' can be sufficient to cause damage (most commonly kidney, combination of relative height and being an organ with more relative water).

Microwaves will not heat up metal, plastic or anything else,and the hot plate or cup is from conduction from the food heating, not direct heating from the microwave.

Answer 6

Not to mention the obvious also. The magnatron will be of a specific size, and this size can only create a very specific amount of waves at any given time, so for more to be generated I am sure we would have to increase the size of the magnatron + Capacitator.
Meanwhile, the substance placed in the microwave oven will be absorbing a large percent of these microwaves generated, leaving very few to even attmept to exit the oven, those will be bounced around until they too are absorbed, I am sure.
It seems to me the greatest number to attempt to escape would be closest to the end of the cooking time, since fewer would be absorbed because of less liquid in the meat, for example.

Answer 7

The thin metal screen is designed to limit the escaping microwaves to a level not dangerous to humans at any interesting distance. I wouldn't keep my nose on the glass however. Trust that they do a pretty good job, but not perfect. They will interfere with 2.4 GHz phones. It seems thin and useless, but it is not.

By the way, microwaves don't primarily heat things, they mostly excite water molecules. Put some very dry sand in there (and a coffee cup with water to be safe) and see if you can "heat" up the sand. The water will boil, but the sand won't heat up much at all.

Answer 8

Micro waves are High Frequency magnetic waves,
The outside of the microwave oven is earthed. This acts as a shield and absorbs the waves (for want of a more technical and confusing wording) and conducts them to earth.

The shielding effect is the same that is used on co-axial TV antenna cable to shield the TV signal from other spurious waves.

Answer 9

Although Microwaves are powerful, they are reflected by the metal lining of the enterior of the oven,

Answer 10

as you said, it can not escape from the box. but be careful, some of the microwave may come out, and may cook your head if you feel hotter about your head during the running.