Physics quiz question: mass diffusivity

Physics 205A Quiz 1, fall semester 2019
Cuesta College, San Luis Obispo, CA

The Sherwood number S_N is a dimensionless number that measures mass transfer and diffusion[*]:

S_N = h/(D·L).

The convective coefficient h is measured in units of m·s^–1, and the characteristic length L is measured in m. The units for mass diffusivity D are:
(A) s^–1.
(B) s.
(C) m^–2·s.
(D) m²·s^–1.

[*] wki.pe/Sherwood_number.

Correct answer (highlight to unhide): (D)

Starting with the expression for the Sherwood number S_N:

S_N = h/(D/L),

the dynamic pressure D can be expressed as:

D = h·L/S_N.

Then replacing the quantities in the equation with their m and s units, the units of D are:

D = m·s^–1·m/(1)),

D = m²·s^–1·m.

Sections 70854, 70855
Exam code: quiz01B34t
(A) : 13 students
(B) : 4 students
(C) : 2 students
(D) : 36 students

Success level: 65%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.44