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Kelvin and Absolute Zero

Kelvin and Absolute Zero. Separated 1/15/2015 from ppt Init 12/17/2007 by Daniel R. Barnes.

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Kelvin and Absolute Zero

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  1. Kelvin and Absolute Zero Separated 1/15/2015 from ppt Init 12/17/2007 by Daniel R. Barnes WARNING: Some images appearing in this presentation may have been swiped without permission from the world wide web. Equally as heinous as this, I completely ignore significant figure conventions when I round numbers.

  2. Let’s say you’ve got a balloon, and it’s freezing cold out, but just barely freezing. The sun comes out and warms up the balloon to about room temperature. What should happen to it? 21oC 57oC 0oC Wow! Did you see how big it got? J/K. It only got about 2% wider, but it did expand a little. Okay, now let’s pretend it gets REALLY hot, as hot as the highest recorded temperature for Death Valley. Okay. This is a little bit underwhelming, but it has expanded to 6% bigger than it was in the beginning.

  3. The balloon expands as it gets hot, but not by much. Maybe we need to REALLY heat things up. Let’s say the sun heats the balloon up to the boiling point of water. Darn. It’s only expanded to 10% wider by the time it’s at the boiling point of water. This is a real let-down. 427oC 260oC 21oC 57oC 100oC 0oC Let’s heat it up to the hottest temperature setting of many ovens, 500oF. What is that in Celsius? Golly. The balloon is as hot as an oven on full power and it’s only epanded 25% beyond its original width. I’m not impressed. Let’s say we heat it up to the temperature on the sunny side of the planet Mercury.

  4. Looking at how little the balloon seems to expand, it’s hard to imagine that the fiercest storms are caused by air expanding and rising. It doesn’t really seem to expand all that much, but it does, and it does have dramatic consequences. 427oC 260oC 21oC 57oC 100oC 0oC WARNING: This thought experiment has some flaws in it, so don’t take it TOO seriously. For one thing, Charles’ Law is only valid in situations of constant pressure. In an expanding balloon, the rubber’s elastic force increases with stretching, adding to the pressure in the balloon. The pressure inside a limp garbage bag always equals that of the surrounding atmosphere, but the pressure inside of a balloon is always higher than the outside air pressure. (That’s why air always shoots out of a balloon when it’s allowed to escape.) This difference in pressure increases as the balloon inflates, since inflation requires the rubber to stretch more and more. I’m sure this makes the math more complicated somehow. I suppose I should have used a limp garbage bag in the example instead of a balloon, but garbage bags aren’t as pretty as balloons, so I used a balloon. So sue me.

  5. Gases do expand when you heat them. This principle is known as “Charles’ Law.” 2.5 If you heat up a body of gas and measure its volume as you heat it up, you notice that it does expand with heat. 2.0 Volume / L 1.5 Gases expand when you heat them because molecules speed up as they get hotter, causing them to bang around harder and push outward. 1.0 0.5 0.0 0 100 200 300 400 Temperature / oC

  6. Gases expand with heat, so if the volume of a gas is not zero, then it stands to reason that is does not have zero heat energy. 2.5 Zero Celsius is not the coldest possible temperature. 2.0 Volume / L 1.5 Even at 0oC, gas molecules are still zipping around pretty fast, banging into stuff pretty hard. 1.0 0.5 How cold would a gas have to get in order to shrink down to zero volume? 0.0 0 100 200 300 400 Temperature / oC

  7. To determine this, we need to . . . EXTRAPOLATE 2.5 2.0 Volume / L 1.5 1.0 0.5 0.0 0 100 200 300 400 Temperature / oC

  8. EXTRAPOLATE 2.5 If you draw a line through the experimental data points, it hits the x-axis at about -273oC. 2.0 Volume / L 1.5 1.0 0.5 0.0 -400 -300 -200 -100 0 100 200 300 400 Temperature / oC

  9. Celsius is a great temperature scale if your goal is to have an easy time memorizing the freezing and boiling points of water. However, it seems like it would be more appropriate if zero degrees really meant zero heat energy, which means zero molecular motion.

  10. To reflect this molecular motion view of temperature, a third temperature scale was invented. The scale is named after the scientist who first theorized the idea of a coldest possible temperature.

  11. In the Kelvin temperature scale, zero really means zero. The coldest possible temperature . . . Is known as . . .

  12. Please notice that a Kelvin is just as big as a degree Celsius. Because of this, the Kelvin temperature is always . . . 273 higher than the Celsius temperature.

  13. To convert Celsius to Kelvins . . . add 273.

  14. oC = K - 273 To convert Kelvins to Celsius . . . subtract 273.

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