|MadSci Network: Engineering|
Spencer, The answer to your question has been asked by a lot of different people with advanced degrees in science and engineering. There are several different answers to your question. I will try to answer what I BELIEVE you want to know. First, you have seen that a microwave oven heats objects without any apparent application of heat. There are no flames or glowing cooking elements, as you would see on a gas or electric stove. The result of something heating in a microwave oven is caused by forcing microwaves, a form of energy found in sunlight and many other sources, into a box that contains something that can absorb these microwaves. Materials such as water, meat (protein), fats, carbohydrates, granular metal, carbon, house bricks, and such absorn the energy very well. The energy penetrates millimeters to several centimeters into the material, depending upon its properties (dielectric behavior). Once inside the material, the energy converts to molecular motion and some chemical bond distortion. This increase in motion and disorder creates heat. To reverse the effect of a microwave, we would have to be able to REDUCE molecular motion by application of a form of energy. If we cool the air around an object, its molecular motion is reduced, but only because the average heat in the objects surroundings is lower, allowing the object to lose heat. If we apply a vacuum to a wet object, the object cools due to the liquids instability at that pressure, and its resulting evaporation, which is a cooling process. Still, we have not done the opposite of a microwave oven, although we have cooled instead of heated. There is a special case where we can cause the opposite of microwave heating, although it is only useful on a few different materials. Let me explain. Hydrogen is normally a gas. It is so hard to liquify that it becomes a liquid at just a few degrees above absolute zero. Water liquifies at 373 degrees (Centigrade type) above abolute zero, and freezes at 273 degrees above absolute zero, so you can imagine how cold hydrogen must get to become a liquid. A few degrees cooler, I THINK (I did not look this up), at about 4 degrees Kelvin, hydrogen can become a solid. How do we cool the solid hydrogen down any further? What refrigeration process will work? Many years ago, some bright scientists decided that since heat is the measure of disorder, that anything that forces order on a system will cool it down. Hydrogen (see your periodic chart) is a metal. Because it is a metal, it can react to magnetic fields. They imposed a powerful magnetic field on a small sample of very cold hydrogen and discovered that the temperature of the hydrogen dropped to a few tenths, then a few hundredths, then a few thousandths of a degree above absolute zero! The ordering of the hydrogen molecules, which were already highly ordered, was improved in a volumetric manner. Since microwave heat materials by imposing disorder in a volumetric manner, these act in an opposite manner. Unfortunately, a huge amount of energy was required to cool a very small sample of hydrogen just a few degrees. As a practical source of volumetric cooling, magnetic fields do not qualify, but from a scientific standpoint, they are very interesting. If you would like to discuss this further, email me at firstname.lastname@example.org Edward Peterson, PhD, P.E.
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