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At 04:49 PM 4/20/99 -0400, Michael Cunningham wrote: >> While it is true that increasing temperature causes the >> viscosity of fluids to decrease, increasing temperature >> causes the viscosity of gases to *increase*. Counter-intuitive, >> but true. > >When you apply heat to gas molecules they begin to move faster >dont they? This would indicate that the viscosity decreases as >a result of the increased movement. What heating a gas does is decrease its *density* not its viscosity which is a different animal altogether. >Cooling a gas or any molecule >for that matter will slow the molecule movements down. This is >the point of absolute zero basically.. a temperate where no >reactions occur. You're confusing density with viscosity. >If you are correct, then how does heating a >gas decrease the movement of the gas or energy basically, when >you are removing energy from the gas by cooling it per your >hypothesis. It's not a hypothesis. This property can be demonstrated with a viscometer in a high-school chemistry lab. I'll try to explain why this happens. Note that the math behind the explanation is outrageously complicated and could easily fill a book, so I'll leave it out here. If you're interested in the math, write me privately so we don't bore the list. Viscous drag in liquids is caused by short-range molecular cohesive forces, but in gases it's caused by collisions between fast-moving molecules. Although it's easy to intuitively accept that the viscosity of liquids drops with temperature, it's not easy to explain and science has yet to conclusively explain it. The consensus is that it probably has to do with the fact that at higher temperatures, the molecules of a liquid are farther apart so the cohesive forces of internal molecular friction are reduced. The increase in viscosity of gases with increasing temperature is easier to explain. Viscosity in gases is caused by molecular collisions. At higher temperatures, the molecules move more quickly and collide more often causing the rise in internal friction and hence viscosity. As an aside, it's interesting to note that helium II (He at < 2.2K) has practically no viscosity at all. This, despite it being a liquid. It flows more easily than a gas and can flow through apertures that would stop a gas. If left in a beaker, it will creep up the inside surface of the beaker, over the edge, and flow out of the beaker on its own. If you carefully stood a pencil on its eraser, then dumped a bucket of He II onto it, the pencil would not fall; the He II would just flow around the pencil with so little friction as to not disturb it. I know... it's counter-intuitive, but it's true and can be easily demonstrated in a chemistry lab. -Mike Rodriguez <mikey@ma*.co*> -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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