Science In Real Life

SiRL: Experiments You Can Do At Home 2 November 19, 2009

Welcome, welcome, one and all. Gather round, take a seat, preferably in your own living room. This is part two of *coughcoughmumble* of the Experiments You Can Do At Home series here on Science In Real Life. Last time the experiments focused on electromagnetism; it can be found here in case you missed it. Today we’re under pressure – air pressure. So grab your gear, guys and gals, and let’s get sciencing!

Put That In Your Pipe and Smoke It

Okay, for this experiment you will need just three things: a campfire burner (butane will work nicely), a PVC pipe about 5 to 10 centimeters in diameter (we use the metric system here at SiRL, none of that imperial nonsense for us) and at least a meter long, and your wits. Your mission, should you choose to accept it, is to find some way to make sound with these two items, other than banging them into each other like a confused, angry monkey. Hint: turn the burner on. Okay, lemme know when you’ve figured it out.

I’ll wait.

… no? Okay. Here’s what you do. With the burner a-burnin’, hold the pipe in a vertical fashion so that its lower end is hovering just above the flame. Fiddle with it until you get the position just right. (And the award for Best That’s What She Said Moment in a Science Essay goes to …) If you have it vertical, and just the right distance above the flame, the tube will begin to sound off with a deep, resonant tone. Words don’t do it justice, honestly, but having heard it myself I can tell you that it is a much louder sound than you would expect these two objects to be able to produce. If you only try one experiment this year, make it this one.

Alright, enough cheese. Let’s suss out why whatever is happening here is happening. Flames, such as the one coming out of your burner there, are hot. I trust this is news to no one. That heat radiates, conducts, and convects itself out into the surrounding air. The air heated by the flame then rises upwards, drawing cooler air in from below. Lather, rinse, repeat. When there’s no pipe present, that’s the end of the story. But now introduce the pipe. I cannot stress how important geometry is to physics. The key concept here is that the shape of the pipe constrains the waves of heat coming off the flame. With air at one end of the pipe going like gangbusters, and traveling in the form of heat pulses up the length of the tube, there is what we call a fundamental mode (not to be confused with the natural state of being of Fred Phelps). It is the lowest energy oscillation possible given the source and geometry.

That fundamental mode, in this case, happens to occur at an audible frequency suitable for the likes of Mufasa, Darth Vader, or even James Earl Jones. Speaking of cool cats …

Schrödinger’s Cat

So for this one you will need an ordinary domestic shorthair feline, a box large enough to hold same, a small amount of radioactive substance (Americium from a household smoke detector will do), a geiger counter, and a switch controlling the release of — what? Really? Oh, fine. Ladies and gentlemen, the ASPCA has just informed me that I am not allowed to actually do this experiment. Fortunately I have a backup plan.

Stop the Presses

Get yourself a copy of one of those old fashioned print newspapers (before they die out for good), a thin plastic ruler (of the 12 inches variety, not Prince Charles), and meet me at the kitchen table (aw, here it goes). Take just one sheet and fold it in quarters. Place the ruler with half its length protruding from the table’s edge and the folded newsprint on top of the remaining half. Stand to one side and make sure no one you terribly care about is standing on that side of the table, because now you’re going to whack the protruding ruler as hard as you can. Wow, look at it go! You must have gotten a good 5 meters on that. Judges? 9.9, 9.8, and … 8.5? Oh well, there’s always the bobsled competition.

Let’s try this again, only this time you unfold the paper sheet to its full area and use it to cover the table-bound half of the ruler. Flatten it out as much as possible. This time you can get the whole family to gather round, because when you whack the ruler it is going to move about as much as the Californian legislation on gay marriage. What happened? Air pressure happened! The mass of the newspaper is tiny, so the only real difference between exhibit A and exhibit B is the surface area of the paper. By conscripting more area, you also conscript a lot more air, pressing it into the service of holding down the paper, which in turn holds down the ruler, all in the house that Jack built. Not only that, but you have vastly increased the paper-table contact area, which has vastly increased the friction resisting the paper’s motion. That, unfortunately, is a topic for another essay.

Now go forth, my readers, and do science to things!