How does a microwave oven work?

Transcription

1 last lecture Electromagnetic waves oscillating electric and magnetic fields c = c = 3x10 8 m/s or 186,282 miles/sec Radios using the tank circuit to emit and receive electromagnetic waves of a specific frequency - AM and FM radios - Ionosphere reflection Cell phones using low power transmitters in cells, frequency can be reused. How does a microwave oven work? Microwaves are light, with wavelength in the range of 1 mm 1m. Which of the following can a microwave oven heat efficiently? A) Ice cube B) Dried fish C) Beef jerky D) All of the above E) None of the above 1

2 Physics-related Questions about Microwave Ovens - Can a microwave cook any type of food? - How does it cooks inside-out, not just on the surface? - Why do they have turntables? - Why do Warnings say Don t put metal objects inside? - When set to defrost, what does the microwave do? - Why does it have a screen mesh/glass window? - Can it heat ice or steam? The parts of a microwave oven Microwaves are sent into the cooking chamber 2

3 Some historical facts about microwave ovens In 1945, Percy Spencer, working for Raytheon to develop magnetrons for active radar signals, noticed a chocolate bar in pocket melted while standing in front of an operating magnetron. He then tested popcorn in front of the magnetron and it quickly popped all over the room. Jul Sep In 1947, Raytheon made the first microwave oven Radarange (6 ft tall, 3000 W power $3K) but did not sell well. In 1965, a countertop version was developed for $495. How does a microwave oven cook food? Electric field Magnetic field Microwave ovens selectively make liquid water hot. Most food, even "dry" foods, have water in them. Water (H 2 O) is a polar molecule with 2 hydrogen atoms being more positive than the single oxygen atom - + In liquid water, the molecules are in constant motion and are normally randomly oriented. 3

4 What happens if water molecules are in an electric field? E The molecules will experience a torque from the electric field, and become aligned with direction of the field. Therefore, water molecules are oriented by the electric field. What will happen to the molecules if the direction of the electric field is flipped rapidly (1 billion times/second)? What happens to water molecules in an oscillating electric field? - The polar water molecules follow the oscillation of the electric field. - They collide more frequently with the molecules (water and other) around them - The molecules move faster and faster, and the temperature increases (heating)! oscillating electric field This is how a microwave oven cooks food! time Question: Most microwave ovens use a wavelength of 12 cm. how many times does a water molecule flip in one second? 4

5 Physics Questions about Microwave Ovens Can a microwave cook any type of food? Microwave can only cook effectively foods with sufficiently amount of liquid water. How does it cook inside-out, not just on the surface? At 2.45 GHz, microwaves can penetrate the food and excite the water molecules inside. In contrast, traditional ovens heat food from the outside, relying entirely on thermal conduction/convection to carry heat in food. Why do they have turntables? Because light, and microwaves, reflect and interfere! Microwaves are reflected by the oven wall, forming standing waves in the cooking chamber. Standing waves waves that oscillate in place Nodes zero amplitude (no oscillation) at all times Anti-nodes locations with maximum amplitude In side a microwave oven, the electromagnetic waves also form standing waves from reflections at the walls. Without a turntable, the food will not be cooked uniformly. 5

6 Why do Warnings say Don t put metal objects inside? The rapidly changing electric and magnetic fields of the microwaves induce currents in the metal, generating a large amount of heating if the resistance is high. Never put aluminum foil or plates with metal lining inside a microwave oven. When set to defrost, what does the microwave do?? Defrosting is hard for the microwave to do since water molecules in the ice are not free to rotate. In defrosting, a few loose water molecules will be quickly heated, and they heat up their surroundings by conduction and convection, making liquid in little pockets. On defrost mode, the oven switches on/off the microwaves to allow heat conduction from liquid water to melt ice and subsequently heat the liquid water. Why does it have a screen mesh/glass window? The glass door with screen mesh prevents microwaves from leaking. The mesh size should be much smaller than the wavelength: d << λ. If d 5 inches, the microwaves will escape and hurt people in front of it. Can it heat ice or steam? In ice, molecules are locked into bonded arrangements, and can t break loose to flip-flop, therefore, cannot be heated by microwaves. In steam (water vapor), molecules are far apart. They flip back and forth, but don t collide with other water molecules frequently enough to increase their kinetic energy, therefore, no heating is generated. Of steam, water, and ice, only liquid water is heated by microwave Are microwaves harmful? The only thing microwaves can do is vibrate water molecules. - No adverse effects on the food cooked. - No harm to people when exposed to low dose exposure. 6