10_1425_web_Lec_33_HeatandEnergy

10_1425_web_Lec_33_HeatandEnergy - Heat and Energy...

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Heat and Energy Conservation Physics 1425 Lecture 33 Michael Fowler, UVa
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Heat Flow If something warm is in contact with something cooler, the warm thing cools down as the cool thing gets a little warmer. This flow of energy, called heat , was until the 1800’s envisioned as an invisible fluid, called caloric. This wasn’t such a crazy idea—using fluid flow equations, Fourier correctly analyzed heat flow in the Earth, inventing Fourier series to do it. And, there existed at least one invisible fluid: electricity. The idea was that temperature acted like a potential for the caloric fluid, and it flowed downhill to lower T . q
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What’s Wrong with a Fluid Heat Theory? If you believe that things only rise in temperature when something fluid flows in, it’s hard to account for the sudden rise in temperature of gas in an insulated (no heat flow possible) container when it is suddenly compressed by pushing down the piston. Basically, the caloric theory does fine in describing heat flow, even in complicated shapes, but breaks down when work is done. a
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The Right Approach: Energy Transfer ! We’ve discussed work as energy transfer : the spring does work on the mass, the spring loses potential energy as the mass gains that amount of kinetic energy. Heat flow is another form of energy transfer —we’ve seen that a hotter gas has more kinetic energy per molecule. a Extension x F kx = − Spring’s force m
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Units for Heat Since heat flowing from a hot body to a cooler one is just a transfer of energy, the natural unit is the same as for energy: the joule (J). The kiloJoule, kJ , is often a convenient unit. An old unit, still in wide use, is the calorie , the heat needed to raise the temperature of 1 gram of water by 1 ° C. The calories used in food labeling are actually kilocalories.
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This note was uploaded on 12/07/2011 for the course PHYSICS 1425 taught by Professor Michaelfowler during the Spring '10 term at UVA.

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10_1425_web_Lec_33_HeatandEnergy - Heat and Energy...

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