Found physics: frozen treat arcade crane

20101223796
http://www.flickr.com/photos/waiferx/5384723889/
Originally uploaded by Waifer X

20101223797
http://www.flickr.com/photos/waiferx/5384724079/
Originally uploaded by Waifer X

Frozen desert crane, Fun Factory arcade, Pearlridge Shopping Center, 'Aiea, HI. Photo by Cuesta College Physical Sciences Division instructor Dr. Patrick M. Len.

Physics clicker question: blackbody versus silverbody

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

Cf. Giambattista/Richardson/Richardson, Physics, 1/e, Conceptual Question 14.22

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

[0.6 participation points.] What should the outside of your jacket be like, in order to stay as warm as possible when hiking at night (in the dark)?
(A) Shiny, reflective.
(B) Black, matte.
(C) (It depends on how cold it is outside.)
(D) (It doesn't matter what type of jacket is worn outside in the dark.)
(E) (I'm lost, and don't know how to answer this.)

Sections 4987, 4988
(A) : 11 students
(B) : 5 students
(C) : 0 students
(D) : 15 students
(E) : 0 students

Correct answer: (A)

The rate of heat absorbed/emitted by an object depends on its emissivity: 1 for a perfect blackbody, and 0 for a perfect silverbody. While a blackbody will be an efficient absorber, it will also be a perfect emitter, which is desirable when cooling is an issue (e.g., car radiators, bottom tiles on the Space Shuttle), but at night, when heat loss by radiation needs to be minimized, a jacket approximating a silverbody would be better.

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

Physics clicker question: latent heat

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

Cf. Giambattista/Richardson/Richardson, Physics, 1/e, Multiple-Choice Question 14.4

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

[0.6 participation points.] The heat capacity of a material undergoing a phase change (such as melting/freezing, or boiling/condensing) is typically:
(A) 0.
(B) ∞.
(C) some finite number.
(D) (Not enough information is given.)
(E) (I'm lost, and don't know how to answer this.)

Sections 4987, 4988
(A) : 8 students
(B) : 7 students
(C) : 12 students
(D) : 1 student
(E) : 0 students

Correct answer: (B)

The heat capacity is given by:

C = Q/delta(T).

When heat is put into (or taken from) a system undergoing a phase change, the temperature remains constant, and thus the denominator is zero, making the heat capacity infinite. Thus a system undergoing a phase change cannot have a defined heat capacity, as the heat goes into breaking or making bonds, instead of increasing/decreasing the amount of thermal energy in the system. Instead, the latent heat is defined:

L = Q/m,

where m is the amount of material undergoing a phase change.

Vacuum in an air-filled room

Dinosaur Comics, by Ryan North
www.qwantz.com
April 16, 2008 (excerpt)

Physics 8A learning goal Q12.1

Ryan North comments on popular unlikely disasters, among them the statistical improbability of experiencing a vacuum in a corner of an air-filled room, and swallowing a black hole.

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