20080526

Physics clicker question: lava lamps

Physics 5A, Spring Semester 2008
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 1/e, Conceptual Question 14.8

Students were asked the following clicker question (Classroom Performance System, einstruction.com) near the start of their learning cycle:

[0.6 participation points.] Why do wax droplets rise in a lava lamp after being heated in the base?
(A) Heat rises.
(B) Weight decreases.
(C) Density decreases.
(D) (I'm lost, and don't know how to answer this.)

Sections 4987, 4988
(A) : 9 students
(B) : 0 students
(C) : 21 students
(D) : 1 student

Correct answer: (C)

A lava lamp is an excellent demonstration of the principles of thermal convection. The light bulb at the bottom of the lava lamp heats the wax, which causes it to expand, decreasing in density. The expanded wax then rises, as it is now less dense than the fluid it is suspended in. Later after cooling at the top of the fluid, the wax will decrease in volume, increasing in density, and later sink.


"How lava lamps work"
http://home.howstuffworks.com/lava-lamp.htm
http://video.google.com/videoplay?docid=1145452462666090515

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