APS News

Springtime Demos Illustrate Basic Physics Concepts

A study of a different kind of isolated object, perhaps to be used in future turbulence generating grids. A utility knife tip punctures the film. The sharp edges produce small vortices in a shear layer roll-up. This fine structure is convected into a much larger von Karman vortex wake.
On Sunday afternoon, physicists from Ohio State University entertained attendees of the APS/AAPT Spring Meeting with demonstrations illustrating physics principles in everyday objects. Sponsored by the Physics Instructional Resource Association (PIRA) and the AAPT Apparatus Committee, the session was very well attended. PIRA is an organization intended to serve the needs of physics educators around the country by providing means for sharing ideas about demonstrations, laboratory activities, and instructional resources in general to advance the quality of physics education at all levels.

Maarten Rutgers displayed his talents for building some of the world's largest soap films, which can approach heights of 20 feet. He has constructed an apparatus to study two-dimensional fluid flows, which are typical in planetary atmospheres, for example. The device consists of two vertical nylon wires hanging down from a single point, held taut by a weight to form the sides of the flow channel. A simple soap solution - typically one or two parts of Dawn dish soap detergent to 100 parts tap water - drips onto the wires at the top. As it dribbles down, the wires adhere together with the soap solution. They are then pulled apart, forming a soap film in between them. Gravity pulls the film down at a rate of approximately 10 miles per hour, and a bucket beneath the weight collects the solution.

Rutgers' laboratory films are typically about 8 to 10 feet tall and 2-6 inches wide, but "nothing stops you from making these films as large as you like," he said. He has constructed soap films 4 stories high and 14 feet wide in the atrium of the Carnegie Science Center in Pittsburgh, PA. A giant soap film reaching 7 stories (80 feet) and 4 feet wide was produced at the University of Chicago's James Franck Institute. Such tall films, says Rutgers, are 10,000,000 times taller than they are thick. The Xperiment Museum in Stockholm, Switzerland now has a permanent installation with a two-story flowing film.

Roderick Grant (a.k.a. "Chef Boy R.G."), a professor emeritus at Denison University and amateur chef, used simple cooking examples to demonstrate basic physics concepts. For example, he added a tablespoon of hot cocoa to a cup, filled it with hot water, and then tapped the cup's bottom with a spoon until the pitch stabilized. He then poured water into one tall glass, and club soda into a second, tapping the bottoms of each to compare the pitch. The pitch of the class of soda water changed after it was stirred gently. "Dissolved air released from mixing cocoa, or from the release of CO2 in soda, modifies the velocity of sound and consequently the perceived pitch," Grant explained. He also demonstrated conduction and convection cooling, using containers of water and soup, respectively, and supplied attendees with a reference list for a wide variety of articles discussing the culinary aspects of physics and chemistry.

Leonard Jossem and Richard Noll displayed unexpected electrical behavior in ordinary light bulbs, showing how to confine an electron in a Penning trap, as well as how to trap fluorescent particles. Edward Adelson demonstrated a spectrum of pastel colors obtained without the usually required prism or slit. Alas, a scheduled appearance by unicyclist Harris Kagan, intended to demonstrate that a ball thrown straight up in the air while on a moving unicycle will always land right in front of him, was canceled due to injury during practice.

Additional information and photos of giant soap films, as well as directions on how to make your own, links to other Web sites, and references to books and articles on the subject, can be found at http://www.physics.ohio-state.edu/~maarten and http://www.exploratorium.edu/ronh/bubbles/bubbles.html. More information about unicycling may be found at http://www.unicycling.org.

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Editor: Barrett H. Ripin