Physics clicker question: bag of chips, from sea level to the mountains

Physics 205A, Spring Semester 2009
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Conceptual Question 9.9

Gas Bag
Posted by witch532
http://www.youtube.com/watch?v=wRFZXy-999I

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

An unopened bag of potato chips is purchased at sea level, and then is brought up on a car trip into the mountains. As a result, the bag "inflates" and becomes rigid. This is because of a(n):
(A) increase in pressure inside the bag.
(B) decrease in pressure outside the bag.
(C) (Both of the above choices.)
(D) (I'm lost, and don't know how to answer this.)

Sections 30880, 30881
(A) : 5 students
(B) : 22 students
(C) : 5 students
(D) : 0 students

This question was asked again after displaying the tallied results with the lack of consensus, with the following results. No comments were made by the instructor, in order to see if students were going to be able to discuss and determine the correct answer among themselves.

Sections 30880, 30881
(A) : 0 students
(B) : 31 students
(C) : 3 students
(D) : 0 students

Correct answer: (B)

The principal cause of the bag "inflating" is the decrease in pressure in the air surrounding the bag as it is brought to a higher elevation. The pressure inside the bag then is greater than the ambient pressure outside. The absolute pressure inside the bag could increase due to an increase in temperature (using the ideal gas law), which is what some students discussed in choosing (C).

Pre- to post- peer-interaction gains:
pre-interaction correct = 69%
post-interaction correct = 91%
Hake, or normalized gain <g> = 72%

Physics final exam question: mass density of gases

Physics 205A Final Exam, Fall Semester 2008
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 1/e, Conceptual Question 13.10

[10 points.] Suppose there are two tanks of identical volume, one containing H2 molecules and the other He atoms. The two gases are at the same temperature and pressure. Which has the higher mass per volume density (or are they equal)? Explain your answer using the ideal gas law and properties of ideal gases.

Solution and grading rubric:

• p = 10/10:
  Correct. Identical pressures, volumes, and temperatures means that both tanks hold the same number of particles. Since an He atom has more mass than an H_2 molecule, then the He tank will have the greater mass per volume density.
  
• r = 8/10:
  As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. May only indirectly imply that the number of particles is the same.
  
• t = 6/10:
  Nearly correct, but argument has conceptual errors, or is incomplete. Does not discuss how it is that the tanks have the same number of particles, directly arguing from greater molar mass for He atoms; or says that mass per volume will be the same due to the same number of particles for either tank.
  
• v = 4/10:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Uses equipartition (mode-counting) and/or fact that He and H_2 are monatomic and diatomic, respectively.
  
• x = 2/10:
  Implementation/application of ideas, but credit given for effort rather than merit.
  
• y = 1/10:
  Irrelevant discussion/effectively blank.
  
• z = 0/10:
  Blank.
  

Grading distribution:
Sections 70854, 70855
p: 9 students
r: 4 students
t: 16 students
v: 7 students
x: 2 students
y: 0 students
z: 1 student

A sample of a "p" response (from student 0013):

A sample of a "t" response (from student 1863) confusing number density with mass density:

A sample of a "v" response (from student 1001) appealing to equipartition:

Physics quiz question: mass, moles and molar mass of He

Physics 205A Quiz 7, Fall Semester 2008
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 1/e, Problem 13.36

[3.0 points.] How many grams of He atoms are there in 5.0 moles?
(A) 0.80 g.
(B) 1.3 g.
(C) 6.8 g.
(D) 20 g.

Correct answer: (D)

The molar mass of He is 4.0 unified atomic mass units; that is, there are 4.0 g per mole of He atoms. Since there are 5.0 moles of He atoms, then there are 20 g of He atoms. Response (A) is 4.0/5.0; response (B) is 5.0/4.0; response (C) is 4.0*1.7 (in reference to another sample of 1.7 moles of Ne atoms discussed in a later, related question).

Student responses
Sections 70854, 70855
(A) : 6 students
(B) : 5 students
(C) : 2 students
(D) : 23 students

"Difficulty level": 63%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.49