Why don't microwaves spark off themselves?

07 January 2014

Question

The inner walls of a microwave oven are made of metal; so why does the oven explode when another piece of metal is placed inside it?

Answer

Dave - It's a very good question. So, the reason why metal sparks in a microwave is that the way a microwave cooks is it essentially applies a very rapidly changing electric field inside the box. It does that using things called microwaves which is a form of light and it causes electric currents to flow backwards and forwards in your food. If you put a piece of metal in there, it conducts much better than your food and you'll get currents flowing backwards and forwards in that metal. Now, if that metal is on its own and insulated from everything else, that will just cause a current to flow through it and it will get very, very hot and you'll have a hot piece of metal. You can actually melt aluminium like this. It's a form of aluminium casting which involves melting this in microwave. It's not something I have tried which is probably a good thing. But, if you have two pieces of metal next to each other and you've got electricity currents sloshing backwards and forwards in these two bits of metal then at some point, you might get a very high voltage at one and a very low voltage of the other and you'll get electricity jumping through the air gap in between and get a spark. But if it's all one piece of metal then you don't get any sparks. So, if you've got one solid case which is all one-piece of metal and it's all connected together and the door has got special connections so it's all connected together then it will tend to reflect the microwaves rather than absorbing them and you don't get any sparks.

Chris - But you nonetheless will get a current induced in the case of the microwave by the electromagnetic wave of the microwave inside the oven itself. Although you're saying it'll largely reflect so it will be a small one.

Dave - In fact, the current is what creating the electric fields and magnetic fields which cause it to reflect.

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