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Suppose I have plenty of food I want to heat (which will provide load) in the microwave, and one item I don't want to heat. What properties would make a material a a good shield, to reduce or prevent heating of that item? I know metal can work as a barrier in the form of a Faraday cage, but that there are also potential issues with arcing. Presumably some kind of smoothly flexible metal mesh would be a good candidate - what properties would it need to be effective? (For example, we might consider mesh spacing, wire diameter, and the choice of metal.)

(Note: this is based on this question, which was recently migrated over to Seasoned Advice (food and cooking), but I think that it could really use physics expertise, so this is a rephrased version, addressing the complaints I saw leading to that question's migration.)

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I think it should be a well conducting material. Screens are usually made from metals. –  freude May 3 '13 at 18:04
    
@freude Sure, that's why I mentioned metal. But how much does conductivity matter - could it be something cheaper than copper? What about mesh spacing and wire diameter? And how does arcing fit in? –  Jefromi May 3 '13 at 18:48
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Excellent question, by the way - this is exactly how I think the original question should have been asked. –  David Z May 3 '13 at 19:01

1 Answer 1

The answer is this: You will need to cover the item you don't want to heat with a conducting material, creating a so called Faraday's cage, which is connected to an earth potential equal to the potential of the metal casing of the microwave oven.

The reason for this is threefold: The covering material needs to be a conductor, because conductors have the property that free charges can move free without significant resistance inside it. Because of this, the electric field inside the conductor is zero, for elsewise charges would move due to the electric field. But they move themselves in such a position that they cancel out the fiel again. Now, because the electric field inside the conductor material is zero, is also has to be zero in cavities of the conductor. For if it would not be zero inside the conductor, one could move electrons through a loop partially through the cavity, and partially through the conductor. The path integral through this closed loop would then be non zero. In onther words, one could generate energy without costs, obviously in contradiction to the law of conservation of energy. Therefore, wrapping an item in conducting material creates a so called Faraday's cage, whereby the electric field through the wrapped item is zero.

Now this is what we want, because the microwave oven heats items with the use of electric field waves. So, shielding the item off from electric fields, also shields it from heating.

However, charges inside the conductor start moving because of the electric field waves. This can create a potential difference with the shielding material of the casing of the microwave oven. It is because of this difference that arcing could occur. That is why you will also need te connect the conductive wrapping to the earth potential of the metal casing.

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Thanks for all the explanation! I'm really trying to ask about what specific properties are necessary here, like material, mesh spacing and wire diameter. The most useful solution would be something light and flexible, that can easily be shaped, but if there's too little metal, presumably you'd get undesirable resistive heating, and maybe some microwaves passing through? –  Jefromi May 3 '13 at 20:30

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