20110906

Physics quiz question: sorptivity units

Physics 205A Quiz 1, fall semester 2011
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 1.39

The equation for the volume V of liquid absorbed over a time t by a dry porous medium is V = AS⋅sqrt(t). The cross-sectional area A of the medium in contact with the liquid is measured in units of m2. The units for the sorptivity[*] of the medium, S are:
(A) m ⋅ s-1/2.
(B) m-1 ⋅ s1/2.
(C) m-2 ⋅ s1/2.
(D) m2 ⋅ s1/2.

[*] wki.pe/Capillary_force.

Correct answer (highlight to unhide): (A)

Replacing the quantities in the equation with their m and s units, except for the sorptivity S:

V = AS⋅sqrt(t),

(m3) = (m2)⋅S⋅(s)1/2,

then solving for S in terms of these units:

m ⋅ s-1/2 = S.

Sections 70854, 70855
Exam code: quiz01unI7
(A) : 24 students
(B) : 7 students
(C) : 13 students
(D) : 13 students

Success level: 33%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.69

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