Metal inside microwave oven

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Fire inside microwave

How Microwave works

Popular microwave generator MAGNETRON . Initially that technology designed for defence. After that a revolutionary use of microwaves which results :- Microwave Oven which use microwaves to cook or heat your food. These are electromagnetic waves ,  in case of microwave oven frequency used is 2.5 gigahertz . At this frequency water ,sugar and fats absorb this waves ( waves are nothing but actually the energy distribution in space ). The absorb energy results into atomic motion which increase the temperature of the stuff . But other material like glass, plastic and ceramic do not absorb them.

Microwave generation

A simple diagram showing generation of the microwave

MAGNETRON ( from wikipedia )

Have you ever wondered why their is metal walls inside the oven?

Because thick piece of metal in wall plays a major role in reflecting the microwave .This is because is there is no metal walls then microwave will spread beyond the microwave boundaries which increase the risk of exposure to radiation. If you were to put your food in a thick metal container in the microwave, it wouldn't cook. Here shielding effect works, due to this effect microwave not able to penetrate into the food instead reflect back. But in other case food in thin metal container would prevent your delicious food from overcooking.

Metal walls inside oven

Microwave source at the top and metal walls

Precaution

Do not use foil container inside microwave

I hope you definitely observe this precaution if you have a microwave in your home. So why microwave manufacturers always write this precaution on their appliance. Reason is the generation of sparks. If you never seen this kind of situation so still you have to read it for future protection. Sometimes the microwave's field mixed up which results in spark ( also called electrical discharge). this mixing is because some items like carrots ( if grown in huge mineral contained soil) etc ( to know more how leave your comment). But major cause is curved or pointed aluminium foil. The major problem arises when the the foil is small, thin and pointed because that foil cannot withstand the flow of microwaves over them and heat until foil ignite. Any thin , or folded ( crinkled)  edges allow current to flow along them which generate a huge potential difference between pointed edge and the microwave walls it's reason is scientific... let me explain:- 

when a charge is supplied on a metal piece which is pointed so here surface charge density works ( surface charge density= charge / surface area ) as area of pointed surface is less so this surface charge density is more which results in generation of huge potential at the sharp point ( this process aso called corona discharge). That potential ionize the air and making path from it's point to wall conducting which generate sparks, setting fire to your food.  

If caught immediately, you can avoid seriously damaging your microwave by simply removing the offending bit of metal. However prolonged microwaving of metal (or wadding up an entire roll ) will likely cause irreparable harm to the appliance and probably set other parts your kitchen on fire.

That would be the result if you do not follow the instruction.

Be safe.................