Problem of the Day!!! 1/26 If you have a massive molecule and light molecule with regard to mass and they are at the same temperature in a closed container which one would create the most pressure?

Well actually they both would create the same pressure because the massive one would have a slower speed but would hit with more force and the lighter one would have a faster speed but hit with less force. The net result would be the massive one would hit less but with more force and the lighter one would hit more often but with less force so they both would contribute the same pressure per unit time.

What happened inside your can? A review of the Can Crusher Experiment

In your minds eye picture an empty aluminum can of soda that has been sitting on your kitchen table for a long time. Why doesn’t the can crush? Let’s not use the word pressure for now. Is the outside surface area of the can equal to the inside surface area of the can? Is the average temperature inside the same or different than outside the can? Is the average speed of molecules inside the can the same or different than outside the can? Does the inside of the can get hit more often by molecules than the outside of the can?

When you heat air does it become denser or less dense? When you heat air does the average speed of the molecules increase or decrease. Same question again: Why doesn’t the can crush? If you heated gas molecules in an empty soda can would their average speed increase or decrease? If you heated gas molecules in an empty soda can would they hit the insides of the can more or less frequently? If you heated gas molecules in an empty soda can would there be more molecules or less molecules in the can after heating?

If you heated gas molecules in an empty soda can would the number of hits on the inside of the can be equal to, greater than or less than the number of gas molecule hits on the outside? Same question again: Why doesn’t the can crush? Remember you can not use the word pressure. If you inverted a heated soda can in water could more gas molecules enter the can? If you inverted a heated can in water would the velocity of the molecules increase or decrease? Conclusion: Why does the can crush?

Can Crusher Explained Gas pressure is really a factor of number of hits and the force of those hits. There are 2 ways to increase the number hits, increase the number of molecules or increase the velocity of molecules or do both.

So okay let’s put it together to “see” why the heated empty can be crushed when inverted in the water. As the can was heated gas molecules began moving faster and faster in the can and a lot of them actually left the can through the opening. But the can did not crush because the few inside hitting the inside at tremendous speed equaled the many outside hitting at a much slower speed, so the pressure outside was equal to the pressure inside. Now at some point we remove the can from the heat and quickly invert it in water. This does two things, it causes the interior gas molecules to cool and slow down dramatically and it blocks the opening in the can so that gas molecules can not get back inside. Now there are a lot fewer gas molecules inside the can and they are going at relatively slow speeds. So now the number of hits on the inside is significantly less than the number of hits outside so much so that the can is crushed.

Thinking about gas pressure as number hits of molecules is a useful model. In addition it is useful to think of a gas as space that has mostly nothing in it. A “real empty space” that has a few things in it that are relatively far apart. This explains why it is so compressible. The space is maintained by the few particles in it hitting the walls enough times so that outer hits do not overwhelm them: if they are overwhelmed the space gets smaller until the “pressure” due to hits is equalized and the walls are kept away by the pressure of the gas molecules hitting them.

What if we pre-cooled the can before submerging it? If you were to pre-cooled the heated can before inverting it in the beaker of water. There was less of a dramatic difference in pressure inside and outside because the force and number of hits inside and outside were not that different.

Therefore the can slowly fills with water without it crushing. If you practice this one and get the timing right it will almost fill with water and slowly sink in the beaker. This is dramatic if you have pushed the unheated can down into the beaker of water before – because in that case the water is pushed out of the beaker with very little water entering the “empty” can.