Physics in the kitchen, microwave ovens.

Microwave ovens. They save us when we need to eat something quickly before leaving home, relieve us from the need to watch over heated pigeons so that they don’t burn out, allow us to quickly refresh the morning rolls for breakfast, or boil water for tea in no time. They are a miracle of the world for people who do not want to cook and buy ready-made dishes, but at the same time they can be the target of various, not necessarily real, accusations. In this article we will try to face the truths and myths that have grown up around them.

Where did this idea come from at all?

Microwave ovens are, from the point of view of their history, relatively very young devices that appeared in our kitchens. The beginning of their history dates back to the end of World War II and the period of intensive work on improving radar technology. Yes, microwave ovens have their origins in military technologies. According to anecdotal reports, one of the scientists investigating the effectiveness of different electromagnetic wavelengths used in radar systems discovered that a chocolate bar he was carrying in his pocket had melted as a result of these waves. How anecdotal and difficult to say how true this is, however, this observation drew the attention of scientists and caused the phenomenon initially treated as a curiosity, to gain practical significance. Initially, the effect of microwaves on various products was tested, including corn grains (which, it may be assumed, burst into popcorn) and eggs (which, according to military tradition, exploded).

The first ovens, or basically microwave ovens, were of considerable size and were intended to be used in large kitchens in hotels and restaurants. They were also very expensive. Over time, they began to penetrate into households and since the 1970’s they became more and more common – mainly due to the reduction in the size of the work piece and the stove itself, but also due to the lower and lower costs of their acquisition.

Around microwave ovens a lot of myths have grown up. Beginning with the fact that food heated with them loses its nutritional value, and ending with myths about the radiation they generate. So let’s try to get it right, but at the beginning we need to explain how the microwave oven works.

 

Cooking for physicists or physicists in the kitchen

Most of us associate heating food with providing heat, there is a source of heat (radiator, heating element, fire) and a food which we expose to this heat and which becomes hot itself. This scheme has been in operation since the time of baking over a bonfire of hunted game and is something obvious to us. But the microwave oven is almost magical in this respect: apparently there is no source of heat in it, the saucer on which we heated the food remains cold while the food itself is hot. Where does this heat come from?

The source of energy in the cooker is a special generator called magnetron. Its task is to generate electromagnetic waves of a certain frequency (electromagnetic waves are also radio waves or light). The frequency of generated waves is about 2.5 GHz – for illustration, the frequency of microwaves (because this is how we describe the frequency of waves contained in this band) is higher than radio waves, but lower than infrared waves and light. Microwaves penetrate into the food and stimulate the water molecules contained in them to vibrate. Why is this happening? The water molecule is a so-called dipole and in an alternating electromagnetic field it behaves like a magnet, i.e. it begins to react to changes in the strength and direction of waves. The frequency of microwaves emitted into the microwave oven is selected so that it is “compatible” with the reaction of water molecules, which is omnipresent in our food.

In short, microwaves charge water molecules with energy, thus raising their temperature. In order to prevent the working magnetron from heating everything around, the waves emitted by the magnetron are “closed” together with the food heated inside a special container, which prevents it from escaping outside (in physics, such an insulating container is called a Faraday cage). This is how the microwave oven works.

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