Explorers Club

At the end of April 2009, the Explorers investigated the properties of density, using what was surely one of the strangest contraptions to ever appear in (and above) their schoolyard.

As everyone knows, a hot air balloon rises because of…well, hot air. No helium or other “trick” is used, just the properties of air itself. How does this work? It all has to do with density.

Everything made of matter—science club students, teachers, and yes, even air—consists of ridiculously large numbers of ridiculously small clumps of matter known as molecules. Each different kind of material is made of different kinds of molecules. For example, the air we breathe consists mostly of molecules made of nitrogen and oxygen, with a touch of carbon dioxide, argon, and other building blocks thrown in.

Certain properties govern the behavior of all materials, and one of these is density. Density is a measurement of how closely packed the molecules making up a substance are: the more tightly packed the molecules the higher the density, and the more loosely packed the molecules the lower the density. As an example, consider a brick and an equal-sized chunk of styrofoam. While both have the same physical size, the brick has much more mass (and weight) than the styrofoam because the molecules in the brick are packed much more tightly together—in other words, brick is much more dense than styrofoam.

Air itself has density, but an additional interesting property is that when a substance such as air is heated, it expands. To expand a given amount of material grows in physical size, and to do that without adding any more material, the molecules have to move farther apart, becoming less dense. So, warm air has lower density—and looser-packed molecules—than cold air has.

A simple test can determine the relative densities of two materials. If one material has a higher density, it will sink through a material with a lower density, and vice versa—the material with the lower density will float atop one with a higher density. So, warm air will float—or rise—through colder air. Heat up the air contained within a balloon with a burner, and voila: a rising balloon.

Taking a hot air balloon ride would have made for an interesting field trip, but unfortunately that was a bit outside the scope of our club. So, we made do with the next best thing: the solar bag.

As described to the rather skeptical club members, the solar bag is essentially the world’s largest trash bag. Made out of very thin material, it is about 3 feet wide and stretches for a very bizarre-looking 50 feet. Once it was unrolled on the back playground area, the solar bag was filled with air by having pairs of students take turns running with it across the yard, then tying off the end so that it became more-or-less airtight.

What we hoped to have happen next was simple in theory. The bag is dark-colored, so that it absorbs the sun’s light and therefore rapidly heats up the air trapped inside. Evidence of this was seen as any loose and slack spots on the bag’s surface quickly disappeared as the air inside expanded and tightened the bag. Next came the big question: would it actually lift off the ground and float?

Well, yes and no. A prior test had confirmed that it could, but a certain set of conditions have to be met: strong sunlight and little or no wind. Unfortunately, by late afternoon on this day, some high clouds produced hazy skies, and worse were some fairly strong breezes. We tethered the bag with a string, and it repeatedly tried to rise, but gusts of wind frequently blew it sideways against the pull of the string, limiting its ability to move vertically.

So, this experiment fell into the “semi-successful” category. The bag never truly soared, but it clearly showed that it was trying to do so, and the expansion of the air inside was easy to see. Additionally, when the students touched the bag the heat of the air inside was fairly dramatic—not enough to burn anyone, but definitely hot. And in the end, it seemed to matter little to the students that the bag didn’t rise very high—they all still had a grand time chasing it around the schoolyard, bouncing it off their heads and doing the limbo underneath it.

To see photos from our afternoon with the solar bag, click on the word “Gallery” below. And no, some of those yellowish faces are NOT photo mistakes—some of the girls discovered the joys of smearing dandelions all over their faces while we were out in the yard. Mr. Ramsey just rolled his eyes and looked the other way—not being a 10 or 11 year old girl, such things were clearly beyond his understanding.

Gallery

NOTE: The solar bag is available online from several sources. The link below is where we got ours, but if you type “solar bag” into a Google search you’ll find some others.

Solar Bag supplier