Just thought I would share one of my favorite new Physics demos with the blog. We're currently covering fluids and thermodynamics and I thought this was a really great and cheap way to demonstrate work, internal energy, and phase changes. All you need is one of those 12 oz disposable clear plastic water bottles that you have in your recycling bin (any kind).
Peel off the label so you can see through the bottle. Put 1/2 inch of water inside the bottle and put the lid on. Twist the bottle until it forms a little "bow tie" shape. It is important to do this in one fluid motion. This will make some alarming noises and feel a little scary - probably a good idea to point the lid away from people just in case.
Let go. The bottle should "bounce" back to its normal shape. Inside, you should see whitish fog form. If you take the lid off, you can make fog rings by squeezing the bottle. This will only work once per bottle.
What's happening? When you twist the bottle, you are compressing the gas inside it. This increases the internal energy of the gas, warming it up and also causing more water to enter the vapor phase. When you let go, the gas rapidly expands to fill the new larger volume of the water bottle. This quickly cools the gas and causes the water vapor to transform back into water, or fog.
Let me know if you give it a try!
5 comments:
Why does it only work once per bottle?
I suspect it is because the bottle is stretched out and deformed in the process, so you can't compress the gas as much the second time around. It will actually work a little bit the second time, but it's not great.
We'll try it when we have the necessary supplies. (I'm somewhat proud to say we don't have any plastic water bottles.)
Brings back memories of boiling water in an open screw-top metal container, then screwing on the top, submerging it in cold water, and watching the container crumple as the air inside cools and contracts.
-Parental unit in MA
Hey there, parental units. There's a little more to your "container crumpling" experiment than just air contracting, too. It is actually a measure of how strong the atmospheric pressure we're all experiencing all the time is - 14.7 lbs / square inch. That may not sound like a lot, but if you balance an additional 15 lbs on a single square inch of your skin, you'll see that it HURTS! That force is the driving force behind the crumpling container - if you did the same experience in space, it wouldn't crumple! Happy experimenting...
Physics?
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