Physics quiz question: manometer

Physics 205A Quiz 5, Fall Semester 2008
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 1/e, Problem 9.24

A water manometer is attached to a flask containing a gas, where the other end is open to the atmosphere.

[3.0 points.] How does the pressure of the gas inside the flask compare to atmospheric pressure?
(A) P_flask = P_atm.
(B) P_flask > P_atm.
(C) P_flask < P_atm.
(D) (Not enough information is given to determine this.)

Correct answer: (C)

The manometer water level will be lower on the side that is at lower pressure, thus P_flask < P_atm. If the water levels were the same in either side of the U-tube, then P_flask = P_atm. If the water level was higher on the side of the U-tube open to the atmosphere, then P_flask > P_atm.

Student responses
Sections 70854, 70855
(A) : 2 students
(B) : 7 students
(C) : 31 student
(D) : 1 student

"Difficulty level": 75%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.15

[3.0 points.] How does the pressure at point [1] compare to the pressure at point [2]?
(A) P_1 = P_2.
(B) P_1 > P_2.
(C) P_1 < P_2.
(D) (Not enough information is given to determine this.)

Correct answer: (A)

According to Pascal's principle, "same fluid, same level, same pressure." In fact, P_1 = P_2 = P_atm.

Student responses
Sections 70854, 70855
(A) : 30 students
(B) : 6 students
(C) : 5 students
(D) : 0 students

"Difficulty level": 73%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.20