1 00:00:02,000 --> 00:00:06,000 How a Microwave Oven Works EngineerGuy Series #4 2 00:00:07,000 --> 00:00:10,000 This microwave oven is a truly remarkable feat of engineering. 3 00:00:10,000 --> 00:00:13,000 The rapid heating that makes microwaves popular 4 00:00:13,000 --> 00:00:15,000 is made possible by power provided from 5 00:00:19,000 --> 00:00:20,000 this vacuum tube 6 00:00:20,000 --> 00:00:23,000 Now, if you picture a vacuum tube at all it’s likely in 7 00:00:25,000 --> 00:00:27,000 a radio like this. 8 00:00:27,000 --> 00:00:30,000 Inevitably, tiny transistors and microchips replaced 9 00:00:30,000 --> 00:00:33,000 clunky vacuum tubes, but it's too soon to 10 00:00:33,000 --> 00:00:34,000 relegate them to the museum. 11 00:00:34,000 --> 00:00:37,000 Microchips can’t easily replace tubes for producing power. 12 00:00:37,000 --> 00:00:38,000 For example, in heating food. 13 00:00:42,000 --> 00:00:45,000 Now, a microwave contains three main components 14 00:00:45,000 --> 00:00:48,000 A vacuum tube called a magnetron 15 00:00:48,000 --> 00:00:50,000 it generates the energy that heats food. 16 00:00:50,000 --> 00:00:54,000 A waveguide hidden in the wall to direct that energy to the food 17 00:00:54,000 --> 00:00:57,000 and a chamber to hold the food and safely contain the microwave radiation. 18 00:01:08,000 --> 00:01:10,000 In principle, a microwave oven heats no differently 19 00:01:10,000 --> 00:01:12,000 than any other type of heat transfer. 20 00:01:12,000 --> 00:01:15,000 At a molecular level heat is a transfer of energy 21 00:01:15,000 --> 00:01:18,000 that results in increased motion of the molecules in a substance. 22 00:01:18,000 --> 00:01:19,000 Since we aren't quantum-sized, 23 00:01:19,000 --> 00:01:21,000 we observe this increase in motion 24 00:01:21,000 --> 00:01:22,000 as a rise in temperature. 25 00:01:22,000 --> 00:01:25,000 In a traditional oven or stove we heat food 26 00:01:25,000 --> 00:01:26,000 by placing a pan on a burner 27 00:01:26,000 --> 00:01:29,000 or in the oven where the walls radiate heat, 28 00:01:29,000 --> 00:01:31,000 which cooks the outside of the food. 29 00:01:31,000 --> 00:01:34,000 The insides cook when heat transfers from the surface 30 00:01:34,000 --> 00:01:36,000 of the food to its interior. 31 00:01:36,000 --> 00:01:38,000 In contrast, energy from the magnetron penetrates 32 00:01:38,000 --> 00:01:41,000 into the food, which means the whole mass of the food 33 00:01:41,000 --> 00:01:42,000 can be cooked simultaneously. 34 00:01:42,000 --> 00:01:43,000 How does it do this? 35 00:01:43,000 --> 00:01:45,000 Well, our food is filled with water, 36 00:01:45,000 --> 00:01:47,000 which is positively charged at one end, 37 00:01:47,000 --> 00:01:48,000 and negative at the other. 38 00:01:48,000 --> 00:01:50,000 To give these molecules more energy, 39 00:01:50,000 --> 00:01:53,000 we expose it to electromagnetic waves that emanate from the tube. 40 00:01:53,000 --> 00:01:57,000 By definition, the waves have electrical and magnetic fields 41 00:01:57,000 --> 00:01:58,000 that change direction rapidly. 42 00:01:58,000 --> 00:02:01,000 For this oven, the direction of the fields change 43 00:02:01,000 --> 00:02:04,000 two point four five billion times per second. 44 00:02:04,000 --> 00:02:07,000 Water will try to align with the radiation’s electric field. 45 00:02:07,000 --> 00:02:09,000 The changing field rocks the water molecules 46 00:02:09,000 --> 00:02:12,000 back and forth rapidly and molecular friction 47 00:02:12,000 --> 00:02:14,000 from this creates heat as the motion disrupts 48 00:02:14,000 --> 00:02:17,000 the hydrogen bonds between neighboring water molecules. 49 00:02:17,000 --> 00:02:20,000 Now, you can get an idea of the wavelength of the energy 50 00:02:20,000 --> 00:02:22,000 emitted from the magnetron using cheese. 51 00:02:41,000 --> 00:02:43,000 Now, you can see on here sections where the cheese 52 00:02:43,000 --> 00:02:46,000 has completely melted, and other sections where it’s completely unheated. 53 00:02:46,000 --> 00:02:49,000 The oven’s metal walls only reflect waves of 54 00:02:49,000 --> 00:02:50,000 a length that fits inside the oven. 55 00:02:50,000 --> 00:02:53,000 This standing wave causes hot and cold spots inside the oven. 56 00:02:53,000 --> 00:02:56,000 The three-dimensional pattern of waves is difficult to predict, 57 00:02:56,000 --> 00:02:59,000 but the principle can be seen by looking at 58 00:02:59,000 --> 00:03:00,000 the waves in a single dimension. 59 00:03:00,000 --> 00:03:03,000 The peaks and valleys in the wave represent 60 00:03:03,000 --> 00:03:04,000 the greatest energy of the wave, 61 00:03:04,000 --> 00:03:07,000 while the nodes here correspond to the "cold" spots inside the chamber. 62 00:03:07,000 --> 00:03:10,000 If I measure the distance between melted cheese spots 63 00:03:10,000 --> 00:03:12,000 I find about 2 1/2 inches. 64 00:03:12,000 --> 00:03:13,000 That would be half the wavelength 65 00:03:13,000 --> 00:03:16,000 the distance between nodes 66 00:03:16,000 --> 00:03:19,000 and is pretty close to the actual wavelength of microwave radiation used. 67 00:03:19,000 --> 00:03:24,000 Using that wavelength I can estimate the microwave radiation's frequency. 68 00:03:24,000 --> 00:03:27,000 The frequency is related to the wavelength by the speed of light. 69 00:03:27,000 --> 00:03:30,000 I get an answer that only has a 4 or 5 percent error. 70 00:03:30,000 --> 00:03:33,000 Not bad for this primitive measurement. 71 00:03:33,000 --> 00:03:35,000 Now, the real engineering in the microwave oven 72 00:03:35,000 --> 00:03:37,000 lies in creating the magnetron that 73 00:03:37,000 --> 00:03:39,000 generates high powered radio waves. 74 00:03:39,000 --> 00:03:42,000 It's truly an amazing and revolutionary device. 75 00:03:46,000 --> 00:03:47,000 The vacuum tube is inside here. 76 00:03:47,000 --> 00:03:48,000 These are cooling fins 77 00:03:48,000 --> 00:03:52,000 thin pieces of metal that dissipate the heat as the magnetron operates. 78 00:03:54,000 --> 00:03:57,000 The key parts are these two magnets and 79 00:03:59,000 --> 00:04:01,000 the vacuum tube. 80 00:04:01,000 --> 00:04:03,000 Now I have another one so you can see the inside. 81 00:04:05,000 --> 00:04:07,000 You apply a large voltage across both 82 00:04:07,000 --> 00:04:10,000 the inner filament and the circular cooper outside. 83 00:04:10,000 --> 00:04:13,000 This voltage “boils” electrons off the center filament 84 00:04:13,000 --> 00:04:15,000 and they fly toward the circular copper section. 85 00:04:15,000 --> 00:04:18,000 The filament is made from tungsten and thorium. 86 00:04:18,000 --> 00:04:20,000 Tungsten because it can withstand high temperatures 87 00:04:20,000 --> 00:04:22,000 and thorium because it’s a good source of electrons. 88 00:04:22,000 --> 00:04:26,000 The magnets bend these electrons so they swing 89 00:04:26,000 --> 00:04:27,000 back toward the center filament. 90 00:04:27,000 --> 00:04:29,000 We adjust the magnetic strength so that the now 91 00:04:29,000 --> 00:04:33,000 orbiting electrons just brush past the opening of these cavities. 92 00:04:33,000 --> 00:04:36,000 Like blowing over a half filled pop bottle to make it whistle, 93 00:04:36,000 --> 00:04:41,000 this creates an oscillating wave - the microwave radiation that heats food. 94 00:04:41,000 --> 00:04:44,000 It’s simply astonishing that these cavities can be made 95 00:04:44,000 --> 00:04:47,000 with high precision, low cost, and incredibly high reliability. 96 00:04:47,000 --> 00:04:49,000 I’m Bill Hammack, the EngineerGuy. 97 00:04:49,000 --> 00:04:51,000 This video is based on a chapter in the book 98 00:04:51,000 --> 00:04:54,000 Eight Amazing Engineering Stories. 99 00:04:54,000 --> 00:04:57,000 The chapter features more information about this subject. 100 00:04:57,000 --> 00:05:00,000 Learn more about the book at the address below.