Thread: microwave

there are many small tiny holes on the front door of the microwave owen what are those for? and suppose if i keep a solid object of uniform shape (say a iron bar) in the owen will there be uniform temperature in the object after the process?

Originally Posted by sabarinath

there are many small tiny holes on the front door of the microwave owen what are those for? and suppose if i keep a solid object of uniform shape (say a iron bar) in the owen will there be uniform temperature in the object after the process?

DON'T DO THAT!

The metal mesh at the front door prevents the microwave radiation from penetrating the door. The wavelength of the microwave radiation is in the range of centimetres, while the holes are only millimetre sized. In this way, the radiation cannot pass and is reflected.

The microwave oven heats material that has a resonance oscillation frequency similar to the microwaves generated inside. This is water. Water molecules are excited and start to move quickly as if they were heated. As a result, everything that contains water is heated up to the boiling point, but not higher. This is why you cannot fry with a microwave oven.

Metal objects put into a microwave oven produce sparks than can potentially damage the oven or even possibly cause a fire.

You may want to check this out.

http://www.phys.unsw.edu.au/~jw/superheating.html

Originally Posted by sabarinath

there are many small tiny holes on the front door of the microwave owen what are those for? and suppose if i keep a solid object of uniform shape (say a iron bar) in the owen will there be uniform temperature in the object after the process?

As noted above the holes are quite a bit smaller than the wavelength of the microwaves so the mesh is reflective, and the microwaves stay inside the oven -- that way they cook the food instead of you.

The holes are there so that you can see inside. Visible light has a wavelength quite a bit smaller than the holes so it can go in and out of the oven through the mesh easily.

If you put metals in the microwave then the microwaves will cause the electrons in the conduction band of the metal to flow in eddy currents in the metal, and may result in high local fields and sparking. As recommended DON'T DO THAT. Microwave ovens are not particularly good at heating conductive materials. They are really geared to heating polar molecules, particularly water.

Microwave ovens are quite safe if used in the ordinary manner. But if you mess around not knowing what you are doing you can hurt yourself, burn down the house, and generally create havoc.

One reason that I could think of why distilled water can be overheated is that the electrostatic bonds that are always there between the hydrogen and the free electrons of the oxygen should be much more abundant without a contaminant. So the water molecules are connected stronger so more energy is needed to break these bonds. This can be administered either by overheating or perturbation. Therefore, the increased electrostatic attraction has a similar effect like higher ambient air pressure.

Posts in this thread of a questionable nature and responding posts have been moved to:
http://www.thescienceforum.com/microwave(pseudo)-15650t.php
Some posts may have been edited to remove references to the moved posts.

Any posts attempting to continue the discussions of the moved posts in this thread shall be deleted.
Janus

Originally Posted by sabarinath

if i keep a solid object of uniform shape (say a iron bar) in the owen will there be uniform temperature in the object after the process?

Say a block of cheese. My understanding and kitchen observation is that microwaves don't penetrate much through water. So the cheese with high water content does not really cook from "inside out". However a burrito filling will be heated directly by microwaves that pass through the dry tortilla. Bread will heat uniformly as it's nearly transparent to microwaves.


I'd never heard of superheated water before. Thanks (In)Sanity. Is this related to the Mpemba effect? There we have currents promoting phase change, and here with microwaved water we have no currents.

Originally Posted by Pong

Originally Posted by sabarinath

if i keep a solid object of uniform shape (say a iron bar) in the owen will there be uniform temperature in the object after the process?

Say a block of cheese. My understanding and kitchen observation is that microwaves don't penetrate much through water. So the cheese with high water content does not really cook from "inside out". However a burrito filling will be heated directly by microwaves that pass through the dry tortilla. Bread will heat uniformly as it's nearly transparent to microwaves.


I'd never heard of superheated water before. Thanks (In)Sanity. Is this related to the Mpemba effect? There we have currents promoting phase change, and here with microwaved water we have no currents.

There are several posts regarding superheating that were moved to pseudoscience. You might want to look there. The posts in response to William McCormick are accurate. You can safely ignore Williams posts though.

Superheating does not require microwaves ,but microwaves do provide relatively heating which does help to reach a superheated condition. Also the vessels used in microwave ovens tend to have smooth interiors with do not provide nucleation sites for the formation of bubbles.

What you observe with cheese is accurate. There is a distinct attenuation that takes place when an electromagnetic wave passes through water. That effect can be calculated. High frequency waves are attentuated more severely than low frequency waves.

That is why submarines cannot receive ordinary radio signals, and in the past received very slow communications from the ELF (Extremely Low Frequency) system, which of course had very low banndwidth. So your cheese heats on the outside and only much more slowly on the inside. On the other hand your burrito is not a homogeneous material, and microwaves penetrate the interstices more readily to heat the burrito more fully, also there is some convection of the fluids in the burrito to aid in heat transfer.