Monday, 3 February 2014

The Microwave Myth

Microwaves cook from the inside out by making water molecules resonate. 

You don't need to know how a microwave works to use one, but I know it makes life a richer experience if you do. Unfortunately the explanation above, which can be found in numerous locations both digital and dead tree, is wrong. Well, mostly wrong, but the real explanation is far cooler. 

Microwaves are part of the electromagnetic spectrum, which is to say they are the same as light rays. What makes them different is the distance between peaks of the wave. In the case of the microwave oven waves this is 12.2 cm, while light is about 200 thousand times smaller. A crucial part of the electromagnetic microwave is the electro bit. Part of the wave is an electrical field that goes up and down with the wave, from positive to negative. 

Picture a molecule surrounded by microwaves. If this molecule has one side a bit more positively charged than the other, it will try to align itself to the electrical field of the microwave. Since this field goes up and down, the molecule tries to flip-flop with the wave. As it does, it bashes into other molecules around it and generously passes some of its newly found energy around. This type of energy transfer is called dielectric heating, as the name suggests, it’s just a way to make things hot.

The Microwave Flip-Flop Dance
Now imagine our theoretical molecule is a water molecule in a theoretical bowl of theoretical soup. Water molecules have an un-even spread of electrical charge so will happily do the microwave flip-flop dance. Before long the water molecules in our soup are jostling about, grabbing energy from the microwaves, passing it around and consequently heating the soup up. However, it’s not just water that jiggles, both fat and sugar can do this too. Anything with an uneven electrical charge will heat up, even pottery plates will warm in a microwave. What is more, this process has nothing to do with anything resonating, let alone water. Interestingly, water molecules in ice don’t do the flip-flop dance as they are not so free to move, which is why thawing frozen food seems to take forever in a microwave oven.

So, if that is what makes the microwave oven generate heat, how does it heat from the inside out? Well, it doesn’t, this too is a fallacy. Microwave ovens cook from the outside in, just like all ovens, but the microwaves do penetrate into your soup, or potato, or left-over curry a couple of centimetres. It is this that speeds up the cooking time. Rather than just heating the surface and the heat slowly being passed inwards, the heat is injected a little way into the food giving it a head start. The microwaves start heating instantly and you don’t loose any energy to the walls of your oven as these are metal and just bounce the microwaves away.

That’s why your microwave oven cooks so quickly.

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Science TV Presenter, live show performer, writer, strange prop builder and all round Science Bloke. All opinions expressed are mine alone. Not the BBC's, just mine.