Why do metal objects behave violently in microwave radiation?

[misskitty]

I'm not really sure where this question goes. I was wondering why metal behaves so violently when exposed to microwave radiation. For example if you put a metal object into a microwave, it sparks and if there are things attached such as plastic tops will explode off of what ever is in it. What is it about the metal which causes this to happen?

~Kitty

 

[shyboy]

It is because metals are good conductors. The dissipated heat is roughly proportional to the substance conductivity, so metals can be easily heated to very high temperatures. Sparks are actually the discharges from the metal edges, because the electric field tends to concentrate in such places.

 

[Gokul43201]

Specifically, metals have a large number of "free" electrons which repsond to the fields of the microwave radiation. It is the collective agitation of this large number of free electrons that makes metals do interesting things in microwave ovens.

 

[Danger]

I was very surprised to see that the instruction manual for my brand-new oven says that you can put metal in it as long as it's kept away from the edges! There must be some modification from the original designs to allow that.

 

[Antiphon]

I'm not really sure where this question goes. I was wondering why metal behaves so violently when exposed to microwave radiation. For example if you put a metal object into a microwave, it sparks and if there are things attached such as plastic tops will explode off of what ever is in it. What is it about the metal which causes this to happen?

~Kitty

Microwaves are just powerful radio waves. When there is metal around,
it acts like an antenna. The antennas in your radio generate a few
milivolts from the radio stations nearby.

But when the "antenna" (a spoon) is in a microwave, it's like being right up
against a powerful transmitting tower. The voltages become high enough to
cause sparks in the air depending on the shape and position of the metal in
the oven.

 

[BobG]

On an only slightly related topic, CD's don't do very well in the microwave, either. It does turn them into interesting desk coasters, though. The theme even matches.

 

[Locrian]

I was very surprised to see that the instruction manual for my brand-new oven says that you can put metal in it as long as it's kept away from the edges! There must be some modification from the original designs to allow that.

That depends. Metal was never bad for the microwave itself so long as the arcs didn't damage the walls or door. You don't get any more reflected power with metals in the machine than you do running it empty, and reflected power is really about the only thing that damages magnatrons other than physically abusing them.

Maybe you are dealing with something new? To tell, I think it best to see what happens when you put metal objects inside.

As an interesting side note, I had the pleasure of having dinner with a couple of gentlemen from Lambda Technologies recently. They do most of their business curing glue on semiconductors. They use a special technique involving quickly altering the frequency of microwaves that prevents arcing entirely.