Sunday , December 15th 2019
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How a Microwave Oven Works


Video Transcript

[Music] this microwave oven is a truly remarkable feat of Engineering the rapid heating that makes microwaves popular is made possible by power provided from this vacuum tube now if you picture a vacuum tube at all it’s likely in a radio like this inevitably tiny transistors and microchips replace clunky vacuum tubes but it’s too soon to relegate them to the museum microchips can’t easily replace tubes for producing power for example and heating food now a microwave contains three main components a vacuum tube called a magnetron it generates the energy that heats food a waveguide hidden in the wall to direct that energy to the food and a chamber to hold the food and safely contain the microwave radiation [Music] in principle a microwave oven heats no differently than any other type of heat transfer at a molecular level heat is a transfer of energy that results in increased motion of the molecules of the substance since we aren’t quantum sized we observe this increase in motion as a rise in temperature in a traditional oven or stove we heat food by placing a pan on a burner or in the oven where the walls radiate heat which cooks the outside of the food the insides cook wood heat transfers from the surface of the food to its interior in contrast energy from the magnetron penetrates into the food which means the whole mass of a food can be cooked simultaneously how does it do this well our food is filled with water which is positively charged at 1:00 in and negative at the other to give these molecules more energy we expose it to electromagnetic waves that emanate from the tube by definition the waves have the electrical and magnetic fields that change direction rapidly for this oven the direction of the fields change 2.45 billion times per second water will try to align with the radiations electric field the changing field rocks the water molecules back and forth rapidly and molecular friction from this creates heat as the motion disrupts the hydrogen bonds between neighboring water molecules now you can get an idea of the wavelength of the energy emitted from the magnetron using cheese [Music] now if you could see on here sections where the cheese is completely melted in other sections where it’s completely unheated the ovens metal walls only reflect waves of a link that fits inside the oven this standing wave causes hot and cold spots inside the oven the three-dimensional pattern of ways it’s difficult to predict but the principle can be seen by looking at the waves in a single dimension the peaks and valleys in the wave represent the greatest energy of the wave while the nodes here correspond to the cold spots inside the chamber if I measure the distance between melted cheese spots I find about two and a half inches that would be half the wavelength the distance between nodes and is pretty close to the actual wavelength of microwave radiation used using that wavelength I can estimate the microwave radiations frequency the frequency is related to the wavelength by the speed of light I get an answer that only has a four or five percent error not bad for this primitive measurement now the real engineering in the microwave oven lies in creating the magnetron that generates high-powered radio waves it’s truly an amazing and revolutionary device the vacuum tube is inside here these are cooling fins thin pieces of metal that dissipate the heat as the magnetron operates the key parts are these two magnets and the vacuum tube now I have another one so you can see the inside you apply a large voltage across both the inner filament and the circular copper outside this voltage boils electrons off the center filament and they fly toward the circular copper section the filament is made from tungsten and thorium tungsten because it can withstand high temperatures and thorium because it’s a good source of electrons the magnets Bend these electrons that they swing back toward the center filament we adjust the magnetic strength so that the now orbiting electrons just rush past the opening of these cavities like blowing over a half-filled pop bottle to make it whistle this creates an oscillating wave the microwave radiation that he food it’s simply astonishing that these cavities can be made with high-precision low-cost and incredibly high reliability i’m bill hammack the engineerguy this video is based on a chapter in the book eight amazing engineering stories the chapter features more information about this subject learn more about the book at the address below [Music]

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