|MadSci Network: Engineering|
Heat can be thought of as "thermal energy". For example, if you have a cool gas (room temperature air) and a hot item (heated pan), the air will pick up some of the energy from the pan and the pan will lose some energy to the air. This occurs until thermal equilibrium sets in, when the temperatures are the same. Because a pan is small, relative to the amount of air in the room (and the air's ability to absorb energy), the room doesn't get appreciably hotter. This isn't the case when you use an oven, however, where your heated item is large and generates lots of heat. Now, let's imagine this room temperature gas under pressure. You've already noted that things seem to cool down when gas under pressure is released. This is true. It can be shown that: (T2/T1)= (P2/P1)^ (gamma) Where T is temperature in kelvin, P is pressure (any units), gamma is the ratio Cp/Cv (= 5/3 for monoatomic gases and 7/5 for diatomic gases) (reference: Methods of Experimental Physics, Vol II Atoms and Molecules, F.B. Dunning and R. Hulet Eds.) Then, as the pressure drops, the temperature of a gas drops as well (because of a requirement for conservation of entropy). To generate heat, you'd have to reverse the process, adding energy. To generate heat, you could use a chemical process (disposable hand warmers like you find at the outdoor recreation stores or a combustion reaction: both liberate energy stroed as chemical bonds), or a physical process such as friction (bend a piece of plastic back and forth for a while and it will heat up). An electrical process is a physical method, as electrons dissipate power through a friction-like mechanism in a resistor. Hope this gets you started... My advice to you would be to start with a process you understand very well, and apply it to a problem or question. This is how many inventors find success. For example, you noticed that air is cooled upon expansion. Maybe think of a way to use that process, rather than try to make something happen which doesn't naturally. Good luck, Mike
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