|MadSci Network: Physics|
Doug, here are a few examples that may work for you. 1) automobile engine. The volume of the cylinder is constant (piston at rest) when the gas/air mixture is initially ignited. The pressure rises in this region because several molecules of gaseous products are formed from a smaller number of molecules of air and gasoline. the force of the gas pressure pushed the piston out (volume increasing) until the end of the stroke (envision the crankshaft). Another set of cylinders fires at this point, driving the piston upwards decreasing the volume and compressing the gas in the cylinder. this "push" removes the combustion products. This example shows the balance between pressure and volume. 2) 2L soda bottle crush. Pass around an empty (and sealed) 2L soda bottle. Ask students to crush it...they'll have a problem doing it. Pour boiling water in, cap the bottle quickly, and immerse in a cold water bath. Try this out before demo-ing, but it should work. I've seen this done with paint thinner cans too. What happens is that the water vapor condenses, dropping the pressure inside the bottle. The volume of the bottle changes to accomodate the pressure difference. 3) potato chips on the airplane. Airplane cabins are pressurized at less than atmospheric pressure. At 35000 feet (fairly typical cruising altitude for a large airliner), atmospheric pressure is probably about 50% or less that at sea level. Food that is packaged at sea level and sealed will then expand against the lower pressure in the cabin of the plane. make sure to check out: UofWisconsin It is a great UofWisconsin site with descriptions of many useful demos. The expanding marshmallow is a favorite of mine. Shakashiri's book (referenced there in many places) is the classic book on chem demos. Hope this works for you. feel free to email me at email@example.com if you have further questions. Best Regards, Mike
Try the links in the MadSci Library for more information on Physics.