How do microwaves move through space

How does a microwave oven heat up?

Microwave ovens work without contact with a heat source. Because here water molecules are turned back and forth by microwave radiation and generate heat through friction.

The first microwave oven came on the market in 1947 and was gigantic: it weighed 350 kilograms, was 1.70 m high and sold for $ 5,000. Since then, microwave ovens have become much lighter, smaller and cheaper. The principle with which they heat food is still the same. Microwaves set water molecules in motion and thus create frictional heat.

Electrically imbalanced

Microwave oven

The water molecules can only be rotated by the microwaves because the particles are electrically unbalanced. They have a positive and a negative side and thus form a so-called electric dipole. In an electric field - like that of microwaves - different forces act on these two sides and the molecule tries to align itself like a compass needle. In a constantly changing field, the particles then rotate back and forth.

When the molecules are mobile but still tightly packed, they rub against each other and heat is generated. Therefore, microwaves work best with liquid water. The molecules whiz around in water vapor so freely that neither friction nor heat can arise. In ice, on the other hand, the particles are bound too rigidly and cannot rotate. In the defrosting stage of microwave ovens, liquid water is therefore only briefly heated and then it is waited until this warm liquid water has melted ice around it. Then comes the next batch of microwaves.

Danger from the kitchen?

Since microwaves can bring water to a boil, there is in principle a danger to biological cells, such as those of humans. It is therefore important that the microwave oven is well shielded. This is done via a metal cage and a metal grille behind the glass pane. In addition, it must be ensured that the door is firmly locked during operation.

Microwave whisk

A problem with heating with microwaves is that the food is not always heated evenly. This is due to the fact that places in the cooking space can form where a lot of energy is available for heating, while other places can go empty. The so-called stirrer - a rotating metal loop that, like a microwave whisk, disrupts the electromagnetic field - provides a remedy here. On the other hand, the food is usually on a turntable, which also ensures more uniformity.

Microwaves in Research

Microwaves are also used in particle physics - but not to heat electrons and protons, but to accelerate particles. For this purpose, microwave radiation is fed into a particle accelerator, which supplies the particles with additional energy. While so-called magnetrons are used in microwave ovens to generate microwaves (see the box below), so-called klystrons are used in particle acceleration. These generate microwaves in a narrower frequency range than is the case with magnetrons. In this way, energy can be supplied to the particles in a controlled manner.