Why do metal objects spark in the microwave oven?

12 February 2006



Why is it that when you put something metal in the microwave, you get a firework display, and yet the inside of the microwave is made out of metal?


Microwaves are essentially the same as radio waves, or in fact Light waves. They are a form of light and they establish what's called a standing wave. In other words, if you wiggle a piece of rope which is fixed at one end and in your hand at the other end, you'll see that it forms a loop or a sine wave. If you wiggle it fast enough you get and almost static picture on the rope. The microwave creates something like that in the microwave. The peak to peak length of the wave is about 12 centimetres, which is why you need a turntable. The energy is greatest at the tips of the waves and lowest in the middle. If you have a turntable, it moves the food through the hot spots on the wave and makes sure you get even heating. So that'' bouncing backwards and forwards between the two sides of the microwave. If you put something metallic in, you sometimes get a beautiful fireworks display. I wouldn't advise you to do it, but if you put a normal light bulb into the microwave, it will light up. The microwaves are essentially an electromagnetic wave, so if you put something metallic in the microwave, the electromagnetic wave tries to make electricity in the metal. That's also why a light bulb will light up. I wouldn't advise you to do it, but it will work! If the metal you put in the microwave is a good conductor, then all you do is make electricity in the metal, which flows round and creates a short circuit and gets hot. This wouldn't spark. The reason you get a spark is that if you put something very thin or a bad conductor in the microwave, it creates lots of charge in one place and a lot of charge in another place. As it's not a good conductor, to equalise out the two charge differences, the air carries the electricity as a visible spark between the two high charge areas. So if you put a spoon in a cup in the microwave, it will not cause a problem. Neither will putting your Chinese takeaway foil dish in the microwave, as it's made of the good conductor aluminium. However, if you put your mum's best tea set with gold leaf round the edge in the microwave, that will create the world's biggest firework display. The resistance of the metal is quite high and you get areas of high charge.


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