Hot stuff: the physics of microwave ovens


Do you salivate like one of Pavlov's dogs when you hear the 'ding!' of this common household item? Jake Port explains how the device warms your dinner in his latest tech explainer.


A thermal image of a bowl of food in a microwave. How does it warm up without a flame or heating element?
Science Photo Library / Getty Images

It’s a lazy night; you don’t feel like making dinner or going out but don’t want to wait for home delivery. So you rummage through the fridge and find last night’s leftover sausages and mashed potato. Dinner is sorted, but there’s just one problem – it’s cold.

Luckily, you can pop it in the microwave and a minute or two later, hot food comes out.

So what’s taking place inside the microwave that almost magically heats food and drinks without a flame or heating element?

The principle behind the microwave is very simple – it’s all about the atom. When you add energy to an atom or molecule, it vibrates. More energy equals more vibrations, and this generates heat.

While heat energy can make molecules vibrate, there is another way to achieve the same result – taking advantage of electrical charges.

Water molecules are what are called “dipoles”. Like a magnet, they each have a positive charge at one end and a negative at the other. This means they can be jostled around with a magnetic field.

In a microwave oven, this is achieved by generating and passing short radio waves, known as microwaves, within the unit.

The device that converts electrical energy to microwaves – called the magnetron – sends microwaves into the oven cavity, where they bounce off the reflective inner surface.

Microwaves, like other waves on the electromagnetic spectrum, are wave-shaped patterns of electricity and magnetism that move together at the speed of light.

When microwaves pass through food, their magnetic component agitates water molecules within to flip back and forth billions of times a second, generating energy and building up heat in the process.

This also means microwave ovens are very energy-efficient – as the waves pass harmlessly through microwave-safe plastics, glass and ceramics, the device only expends energy heating the food.

So the next time you heat up leftovers or warm a mug of chocolate milk, spare a thought for these unassuming pieces of engineering that can transform unappetisingly cold food to hot, tasty morsels in the time it has taken to read this article.

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