The Secret Life of the Microwave Oven

An Article What I Wrote for the Wellcome Trust Guardian/Observer Science Writing Competition

Look at your microwave oven.  There’s a lot going on in that little box which you can’t see, and might not have thought about.  It’s a simple-to-use and indispensable feature of the contemporary kitchen.  But, beyond that, it’s also a reminder of how far we’ve developed our understanding of an invisible world – the world of electricity and magnetism.

Like many modern devices – phones and radios being just two examples – microwave ovens rely on electromagnetic waves.  Every single bit of space around you has a certain amount of electric and magnetic energy.  Waves carry this energy around, leading to constantly-moving peaks and troughs – like a water wave of energy.  We don’t feel this constant buffeting, but some objects are very sensitive to these changes in energy.  For these objects, such as bar magnets or electrically-charged particles, the space around us is like a sea in a storm.

One species of electromagnetic waves are microwaves.  When you switch on your microwave oven, the cavity fills with microwaves.  Microwaves are different from other electromagnetic waves in the influence they have on water molecules.  Water molecules have a peculiar atomic structure, which makes them behave like miniscule magnets.  Very weak magnets indeed, as you’ll see if you try lifting water with a bar magnet.  But they are strong enough to respond to the peaks and troughs of microwaves.  All that electric and magnetic energy causes the water molecules to spin rapidly around, bumping into all their neighbours.  We know this rolling and bumping of molecules by a particular name.  We call it heat.

Microwaves have a similar effect on some other molecules, though not nearly as strongly as for water.  Fortunately most foods have a high water content, and the water molecules collide with (and therefore heat up) the rest of the meal.  Heat also reaches the container, as numerous dropped dishes will testify.  However, this can be put to good use.  Some containers are manufactured with metallic or ceramic flakes, which absorb microwaves and convert them into infra-red waves.  These infra-red waves are absorbed by the surface of the food, as in a conventional oven.  With infra-red waves working alongside microwaves, the surface of the food gets extra cooking – which produces browning.

Although you can’t see microwaves, a visible sign of the oven in action is the rotating turntable.  The inside of the cavity is metal, which reflects microwaves.  So unlike mobile phones, which only send microwaves outwards, the cavity has microwaves bouncing around in all directions.  Colliding microwaves produce immobile hotspots of high energy at certain points, which would leave the food unevenly cooked.  Moving the dish around stops this occurring.  Some turntables reverse direction each time they are started.  This has nothing to do with cooking, but helps start rotation again more easily – the rotating turntable puts pressure on the turning mechanism, which gives a little extra kick when rotation starts in the opposite direction.

Other, less obvious, features of the microwave oven are also worth a look.  You’ll see that the front of your microwave, although transparent, has a metal grille.  Visible light is also a form of electromagnetic wave, and one way of distinguishing different kinds of electromagnetic waves is how they react to small gaps.  With the metal grille, microwaves will be reflected while visible light will pass through the holes.  This has the handy feature of allowing you to see your food without also cooking your face.  Even the electronic timer is an example of electrical ingenuity.  It contains a device called a capacitor, which can store electrical energy.  But energy leaks out of the capacitor, and how fast this leaking happens is entirely controlled by the design of the capacitor.  So if you fill your capacitor with a certain amount of energy, it will lose all that energy in a set amount of time – and that’s your countdown.

A couple of easy experiments can make some features of microwaves more visible.  For instance, if you remove the turntable and heat a thin piece of cheese you’ll see alternating spots of melting and not-melting.  These show you the hotspots caused by the peaks and troughs of microwaves.  Or you can (though probably shouldn’t) heat an egg in your microwave – the water in the liquid interior heats up and expands rapidly, while the solid shell doesn’t.  The result is, in technical terms, an eggsplosion.  Egg was actually the second footstuff to ever be heated in a microwave oven (the first was popcorn), which probably put the whole idea in peril.  Fortunately it went ahead, and now cooking a meal is as simple as pressing some buttons and waiting three minutes.  Which gives you just enough time to have a proper look at another of the everyday marvels in your kitchen.

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