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DMS Graduation 1997:
Pressure Release

This is my talk for the Dunn Middle School graduation ceremony on 6 June 1997, delivered outdoors on the steps of the Middle School. I explain how to do it at the end.

It's my custom to do a science experiment for my graduation speech. It's always hard to come up with an experiment that will work outside for a large audience, and will generate a metaphor for the occasion. I asked several kids (and a parent) what I should do, and the universal answer was "Just blow something up!" This happened last year too. I'm becoming too predictable!

So I came prepared. {Hold up gas can.} Actually I'm not going to blow anything up. I'm going to blow it in. I'll cause an implosion rather than an explosion. If it works, then we can blow it up afterwards.

I'm going to put about one inch of water in the can, and put it on the stove to boil. Note that the cap is open, so when it boils, the steam will escape, just like making tea.

{Put an inch or so of water in gas can and place on stove. This will take awhile to boil. Keep talking.}
It's become a joke to say "Children, don't try this at home." Well, the whole point today is that you're not children any more, and of course you can try this at home. But don't use an empty gas can like I'm doing. We actually lit a coffee can full of gasoline in science class last fall, and it just burned and boiled. But an apparently empty can that contains gas vapors really can explode. This was an empty can that's been sitting in my garage for years, and I soaked it overnight. A better alternative is to use a soda can.
{Put a 1/4 inch or so of water in soda can and put on stove. It will boil in a few minutes, but keep talking.}
A watched pot never boils, so while it's heating, think about what's happening inside the can. When the water boils, it changes state from a liquid to a gas. Heat expands and rises. So the vapors will push the air out of the can, and we'll see a cloud of steam escape from the opening. Even though steam is escaping, the pressure inside the can is roughly the same as the pressure outside, because it's open, and the can is in a state of equilibrium. If the pressure in the can suddenly became much greatere, the can would bulge, maybe even explode. But what if the pressure suddenly became much less?

We don't usually think about the weight of air, because we don't usually feel it. But air does have weight. It's pushing on us all over with a force of almost 15 pounds per square inch. We don't feel it because we push back with the same force. This is an example of equilibrium, two opposing forces that are equal.

Sometimes we do notice the weight of the air. I live in the mountains about 1500 feet higher than school. So the weight of the atmosphere at home is a bit less than it is here since there's not as much air above me. We usually use our ears and sinuses to equalize pressure inside and out. But sometimes if I have a cold, my ears don't pop when I drive down to school, and I feel the extra pressure all day long. It's even more obvious in an airplane or under water.

{The water should be boiling now, so do the experiment. Cap the gas can, remove it from the heat, and pour cold water over it. It will crunch right up with a cool sound. Put it back on the stove (still capped) to reheat it and "blow it up" again. You can repeat this a few times unless a seam opens up.}

{Also do it with the soda can. The can will be hot, so use a cloth glove or towel to hold it or just move real fast. The trick is to pick up the can and quickly invert it into a shallow tray of water all in one motion. This is impressive!}

When the can is boiling, it's full of steam at normal pressure. The water vapor pushes the air that was inside the can, out of the can. Then I cap the can and cool it rapidly.

All of the vapor inside the can quickly turns into a few drops of water when it cools, which takes up much less space. The interior pressure drops and the outside atmospheric pressure pushes in and crunches it. Normal atmospheric pressure is nearly 15 pounds per square inch, and the can has lots of square inches! That's a lot of force, and we see the effect.

That's my experiment. So what's the point?

What's the real shape of a can? It depends! It depends on the lid and the fire, the strength of the can (especially the seams), and the pressure inside and out. So too I think who a person is depends on pressures, both inside and out.

Popeye says "I yam what I yam." And who we are certainly depends on our goals and ambitions, the choices we make, our own internal pressures.

But that's not the end of it. We also feel pressures from the outside, from friends and ideas, the expectations of other people and institutions.

We can equalize pressures inside and out, and maintain. We can expand and grow. We can also crunch or burst if the pressures become too unbalanced, inside or out.

You graduating seniors, we've had full years together here at Dunn Middle School, and I wish you full years ahead. We've kept the pressure on you up here at school. And you'ce met it in your own ways. Keep your pressures high! Like the sign under the oak tree says, Carpe Diem -- seize the day, take the opportunity, expect a lot. Regulate your pressure when you have to, so you don't get hurt. That's part of the process. And prosper.


How to do it:

What I used for the speech:

This is a classic "science trick" described in many books. The soda can works really well because the sides are so thin, but the gas can is more visible. Be sure the gas can has no gasoline vapors! Mine was old, and I soaked it in water overnight just to be sure. The gas can will implode on it's own when capped and taken off the heat. But pouring cold water on it (and into the tray) makes the effect more impressive.


last modified .

Marc Kummel / mkummel@rain.org