| MadSci Network: Engineering |
Hello Lim, You have some interesting questions! First, I would like to give a little background on how a thermometer works: The principle behind a mercury (or alcohol) thermometer is that the liquid expands at a precise rate with increases in temperature(and shrinks with decreasing temperature). In theory, you could take a kitchen measuring cup, fill it to the one-cup level with mercury,and measure temperature changes by looking at how the liquid level changes. The problem is that the volume change with moderate temperature changes is very small- Mercury has a 'volumetric thermal expansion rate' of only .000018 per degree C. This means, that for a 10 degree C rise, the volume will increase by only 10 x .000018 = .00018 times the original volume. Our measuring cup volume will increase to only 1.00018 cups! This would be impossible to measure. (Also, mercury is very hazardous to handle). So how does a thermometer solve this problem? A thermometer is simply a device that allows us to accurately measure very small changes in volume. The 'bulb' of the thermometer contains a volume of mercury. It is connected to a VERY small diameter 'capillary tube'. Because of the connection, when the temperature changes and the mercury expands, virtually ALL of the additional volume is forced into the capillary tube. This has the effect of greatly magnifying the ability to see the volume change. With this information, we can address your questions: a)If you have 2 thermometers, identical except for bulb size, what will happen to each with an equal temperature rise?: The mercury in the small bulb will expand a small percent of its original volume. The added volume will flow up into the capillary tube, some distance. The mercury in the large bulb unit would expand the same 'percentage', but will have a proportionately larger volume that will flow into the tube. This volume will fill a greater length of the tube, and will hit the end stop at the top of the thermometer sooner than the small bulb unit. THEREFORE, the small bulb unit will have a larger temperature range of measurement. b) Because the larger unit would move more fluid into the tube, it will give a faster, more precise (magnified) reading of a temperature change compared to the small unit. c)Because alcohol has a smaller volume expansion rate than mercury (.000011 vs. .000018)**, both our small and large bulb thermometers would have a smaller movement of the fluid in the tube for a given temperature change. The ratios would still be the same though, and the small bulb unit would still allow for a larger temperature range of measurement. The references below will give you more information. I hope that this helps! Regards, Jay Shapiro http://www.answers.com/topic/coefficient-of-thermal- expansion http://www.answers.com/topic/mercury-in-glass-thermometer-1 ** http://www. cord.edu/dept/physics/p128/lecture98_36.html
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