Determine the diffusion constant for an aroma in air.
This project may be done by teams of 2-3 individuals. The goal is to measure the diffusion constant for some sort of gaseous substance in air. Be sure to read Section 14.4 and question 14.47 (Fourth Edition) before you start.
First, you should follow the hints in Problem 14.47 to convert the diffusion equation into the simple form
L*L t = --- 2D where t = time for substance to diffuse across a room (s) L = distance from substance has to cross (m) D = diffusion constant (m*m/s)Write down each step of the conversion, starting with the ordinary diffusion equation (Fick's Law), and ending with the version above.
Now for the experiment. You should choose a sample of some substance which smells strong and may be detected across a room; you might try cigarette smoke, or strong perfume, or dead skunk -- whatever is convenient. Make sure that all members of your team can identify the smell unambiguously. Choose one person to be the timekeeper, and the other(s) to be the smeller(s).
A trial: Make sure that the timekeeper has washed his hands thoroughly, and is wearing clean clothes, so that no part of him smells like the substance. Find a room which has no open windows or doors, and no air circulation -- so no active heating vents. Have the smeller(s) enter the room first, and measure the distance from the door to the far corner. He should stand in the far corner, facing the door, with eyes shut. The timekeeper now enters the room, carrying a sample of the substance. He starts his watch, and (if necessary) sprays the substance into the air. As soon as a smeller notices the aroma, he should open his eyes (but remain silent). The timekeeper should mark down the time. If there is a second smeller, wait for him to notice the smell, too; but try not to give away the fact that the first smeller has already noticed it.
Make at least 3 trials. Move to a different room each time, and allow at least 20 minutes to pass from one trial to the next. Make sure that the timekeeper washes thoroughly and wears a new outer layer of clothing each time. The smeller(s) should clear his nose by walking around outside in between trials.
For each trial, calculate the diffusion constant for your substance in air. Are the values similar to each other? If there are differences, try to explain them. Watch out for air currents.
Submit a report which contains a description of your substance, the dimensions of each room, all the raw data, and the calculations of the diffusion constant. You may also include interesting items you discovered during the course of the procedure.
This page maintained by Michael Richmond. Last modified Jan 14, 1998.
Copyright © Michael Richmond. This work is licensed under a Creative Commons License.