Todd: I do the cloud-in-a-bottle demo that you describe a lot, though the version without smoke initially is a nice variation that I haven't tried yet on students. I've often thought wistfully about how nice it would be to be able to measure the temperature and pressure inside the bottle during the experiment, to verify the claims I make about those quantities before and after releasing the stopper on the bottle. Do you know of (relatively cheap) ways to measure those things and make the measurements accessible to a classroom of students?
By the way, as a valuable adjunct to the standard cloud-in-a-bottle demo, I precede it with another demo. I commute to campus by bike, so I remove my front wheel, grab the hand pump attached to the bike, and bring them to class (along with a thermometer and the bottle and matches). You can guess what I'm going to do with the bike wheel, I'm sure, but here's the procedure that I use, roughly:
(1) Have several students feel the metal valve attached to the bike tube, and comment how warm or cold it feels.
(2) Give a student a thermometer and ask them to measure the room temperature. [Ideally I'd have a fan available. I'd turn it on and ask students to predict what will happen to the temperature measured by the thermometer if it's stuck in front of the fan. Although there's no significant change in temperature, the result tends to be counterintuitive for some students and can lead to a lengthy though interesting diversion about how thermometers work and what they actually measure--namely, their own temperature.]
(3) Ask students what they think the pressure is like inside the tire, compared to the pressure outside the tire. Tell students that I'm going to open the valve on the bike tube, whereupon the pressure difference between the inside of the tire and outside should push air out of the tire, and as air comes out of the tire the pressure on that air should decrease. I ask students to predict what, if anything, should happen to the volume (or density) of the emerging air as a result of the pressure decrease, and what might happen to the temperature of that air as a result. [They've had a reading assignment beforehand that should allow them to predict this.]
(4) Ask a student to hold the thermometer directly in front of the valve without touching it, and open the valve. Air streams out for a few seconds. Ask the student to read the thermometer. Ask the students who had felt the metal valve beforehand to feel it again and comment on the difference. Ask the thermometer-reader to comment on the observed temperature change, if any. [Can constrast this to results of putting thermometer in front of the fan earlier, if that was available.] [This might raise questions about why the valve cools off if it's air that is supposed to cool when the pressure decreases. With prior reading, some of them can identify the relevant process as conduction.]
(5) Summarize results of this experiment: when the pressure on air decreases, the air expands and cools.
(6) Commenting that repressurizing the tire is necessary so that I can get back home at the end of the day, I reverse part of the experiment by pumping air back into the tire, noting that pushing on the pump plunger puts pressure on the air inside the barrel of the pump, which compresses that air. Also, the pressure inside the pump becomes greater than the pressure inside the tire, and the pressure difference pushes air from the pump into the tire. I ask students what should be happening to the pressure inside the tire as a result of pumping more air into it. Pumping air into the tire warms me up quite a bit--those small hand pumps are a lot of work. It also warms up the barrel of the small pump, as students can verify by touching it. The same is true for the tire valve. Conclusion: Increasing the pressure of air compresses it and warms it up. [They might argue that friction of the plunger against the inside walls of the pump generates heat and is responsible for the warming of the pump and the pressurized air. I can't disprove that, so what I sometimes do is deflate the tire before class and do the repressurizing experiment first, let the air inside the tire and the valve cool to room temperature [this happens pretty fast], and then ask students to predict what will happen to the temperature of the air and valve when the valve is opened and air rushes out of the tire. Any predictions based on friction will fail, and the available alternative hypothesis (temperature changes result from changes in volume caused by changes in pressure) will prevail.
(7) During the cloud-in-bottle experiment, I ask several students to take turns pumping air into the bottle using the compressible bladder attached to the bottle stopper. [It gets progressively harder to compress.] I ask them to comment on what this might tell them about what is happening to the pressure inside the bottle. Then I ask them to predict what will happen to the pressure inside the bottle when we pop the stopper, and why, and predict what will happen to the temperature as well. [These are the changes I haven't figured out how to measure directly.]
Any suggestions about improving this are welcome!
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