Metal in Microwave Ovens: A Scientific Explanation

[kashiark]

I recently purchased a new microwave oven, and there's a metal rack in it. The salesmen informed me that it shouldn't cause any problems as long as it didn't touch the sides; however, he was unable to tell me why this is. I have a basic understanding of how microwaves(as in the radiation not the ovens) interact with polar molecules causing heat and how they interact with delocalized electrons, and I can't think of an explanation as to why not touching the sides would prevent this interaction from occurring. Well, that's a lie; a more accurate statement would be that I cannot think of a satisfactory explanation. Any thoughts?

 

[Born2bwire]

I cannot think of any problems if it touches the sides of the chamber, I would think that the shelf spans nearly the width of the chamber already and is just separated by a few centimeters of holding brackets (if even).

A microwave is a resonant chamber, the chamber is enclosed by good conductors like metal. The main problem with metals in a microwave is that the electromagnetic waves in the oven induce currents and charge localizations in (and mostly on) conductive objects. Objects with sharp points create an effect known as field enhancement because the density of currents and charges at a point is very high despite the actual magnitude being low. The field enhancement can cause the air to breakdown allowing sparks to occur. But a rounded metal object is usually ok which is why you can place the rack in the oven and I cannot imagine any real problems of the rack coming into contact with the walls that wouldn't occur already by just having it in close proximity to the walls.

 

[Phrak]

Are you going to read the users manual or take the word of an appliance clerk?

 

[rcgldr]

I don't know about the issue of touching the sides, but the effect of a microwave on metal, depends on what the metal is. Some metals are greatly heated by microwaves, depeding on the "harmonics" of that metal. Other metals give off sparks, which probably interferes with the operation of the microwave. Note that microwave popcorn uses a small amount (thin strip(s)) of metal that is heated up by a microwave.

It isn't just metals, I seem to recall an some type of dinnerware (an early version of pyrex?) that would shatter into shards or dust when microwaved.

 

[kanato]

It might be that if the rack gets really close to the sides but not quite touches, then the E field between the rack and the metal side will be large enough to cause breakdown so a spark can jump between them.

 

[kashiark]

There was no user's manual. I guess going cheap isn't always what it's cut out to be. Why do only sharp points cause these field enhancements?

 

[Meir Achuz]

It might be that the standing EM wave in the cavity has a nodal plane at the location of the rack.

 

[Born2bwire]

It might be that the standing EM wave in the cavity has a nodal plane at the location of the rack.

Not likely, the wavelength is small compared to the size of the cavity so there will be a very large number of modes that should permeate the cavity. In addition, the stirrer also makes it a mixed mode cavity and it further helps in making more even coverage.

 

[Born2bwire]

There was no user's manual. I guess going cheap isn't always what it's cut out to be. Why do only sharp points cause these field enhancements?

It has to do with the cancellation of the fields inside of a conductor. Charges and currents accumulate on the surface to cancel out incident fields and waves. However, this cancellation is only guaranteed on the interior of the conductor. On the exterior we see a reinforcement of the fields and waves due to the reflection of the waves and bending of the field lines. If we have a conductor that has sharp points, this causes a high density of currents and charges to accumulate at the points. There is not necessarily an increase in the amount of charges that accumulate, but due to the small surface area the density that need to accumulate becomes high. This causes a strong reinforcement of the fields in the immediate volume around the sharp point. Air, and other dielectrics, will eventually breakdown into a plasma if we apply a strong enough electric field. The field enhancement due to the sharp points, like the tines of a fork, greatly facillitate the breakdown of the air which causes all the sparking.

Lightning rods are an application of field enhancement, they provide a thin conducting rod that will, due to being grounded, induce a large amount of charge density compared to the rest of the building or structure. This makes the lightning rod the most likely path for lightning since the fields around the rod are the strongest.

 

[kashiark]

Ah, ok I get it. Thanks!