The Big Question – 01 – Foil in the Microwave

In this new series, I will be examining the day-to-day questions in which the answer lies in science. You know, those questions that you get when something happens and you find yourself wondering why?

Today’s question is based on a little incident that happened to me recently. And, if I’m being honest, it’s not the first time.

The Big Question is…

Why does metal spark in the microwave?

white microwave with door open
it’s getting hot in here

I am sure most of us have been there. You forget yourself and the next thing you know, your microwave is giving you a mini fireworks display. I have decided to look into why this happens to ensure that it’s on my mind the next time I feel like warming up cold hotdogs wrapped up in foil.

As per How Stuff Works, it turns out its to do with how the microwave heats up the food. A microwave emits actual radio waves at a frequency of 2.4Ghz – and these waves are absorbed by water, fats and sugars and release heat, allowing the food to be warmed up. The waves are not absorbed by plastic, glass or ceramics – hence why you can heat food in these dishes. If you have ever heated something out of a packet, you’ll be familiar with instructions that say to use ‘non-metallic’ containers/bowls/plates.

This is because the radio waves emitted by the microwave are not absorbed by metal.

When they come into contact with small pieces of metal, like the edge of a piece of foil, an electrical current runs through and this is what ignites sparks. If the affected foil is attached to any form of paper, this can be what leads to fires.

Why does this happen?

Well, metals like aluminium foil tend to be flat and thin. When the food inside is heated, the water molecules evaporate into steam – releasing bursts of energy. With the food covered, there is nowhere for this energy to go and the foil itself becomes very hot. This can be very dangerous if the microwave is left unsupervised.

So, the lesson here is to always take precautions and ensure that foil is not used inside the microwave!

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