Physics quiz question: lava lamp convection

Physics 205A Quiz 7, fall semester 2011
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Conceptual Question 14.8

Wax droplets in a lava lamp will rise after being heated in the base because their __________ increases, making their __________ decrease.
(A) density; mass.
(B) density; volume.
(C) mass; volume.
(D) mass; density.
(E) volume; mass.
(F) volume; density.

Correct answer: (F)

The light bulb at the base of the lava lamp heats wax suspended in fluid, which causes the wax to expand and increase in volume, 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 lamp, the wax will contract, decrease in volume, increasing in density, and subsequently sink.

Sections 70854, 70855
Exam code: quiz07h3A7
(A) : 1 student
(B) : 1 student
(C) : 0 students
(D) : 0 students
(E) : 0 students
(F) : 50 students

Success level: 96%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.12