1. In describing the diffusion of particles, often the net particle flux is said to be proportional to the particle density gradient, with the constant of proportionality being the diffusion constant (or "diffusivity") D:
Our text gives D as . From a physical point of view, explain/argue why you think this functional form for the diffusion constant D is plausible. That is, why should D be proportional to the mean free path l and to the mean speed?
Also, consider the expressions/definitions for the mean free path and the mean speed. Explain how the diffusion constant D depends on more fundamental parameters like the mass M of the particles, the temperature T of the gas, etc. and once again briefly justify why D should depend on these parameters as it does.
(Note: We'll be doing this same sort of analysis of thermal conductivity in class next time.)
Enter your answer here:
2. What's the approximate order of magnitude of D at room temperature and atmospheric pressure? (Be sure to include units for D!)
Not required, but interesting to contemplate: If you'd like to boggle your mind, now consider that the mean squared displacement along any particular direction can be expressed as (mean squared displacement) = 2 D t, where t is the time of travel. In the last preflight, you calculated that it would take an unhindered gas molecule at room temperature and pressure only a millisecond or so to traverse the length of a typical room. About how long does it really take a molecule to diffuse from one end of a room to the other?
3. Our text defines the thermal conductivity K as , where CV is the heat capacity per volume. The (MKS) units of K must be:
Check the correct answer: a) J/(m2 sec K)b) J/(m sec K)c) J/(sec K)d) J/(kg m2 sec K) Below is a space for your thoughts, including general comments about today's assignment (what seemed impossible, what reading didn't make sense, what we should spend class time on, what was "cool", etc.): You may change your mind as often as you wish. When you are satisfied with your responses click the SUBMIT button.
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