c (kg/m
3
)
x (cm)
0
5
t = 5 minutes
4
2
0
c (kg/m
3
)
x (cm)
0
5
t = 5 minutes
4
2
0
c (kg/m
3
)
x (cm)
0
5
t = 0 minutes
Column A
Column B
7.2 Diffusion Calculations
Objectives
•
Perform calculations for diffusion.
Figure 7.2.1 shows an example of diffusion that can occur when two metals are placed into
contact with each other. Normally we would graph the concentration versus distance as
shown in Figure 7.2.1a, and this graph represents what is called the
concentration gradient
,
which is the change in concentration as a function of position. We can calculate this
concentration gradient, how long it will take diffusion to occur, and what the concentration
will be for a particular diffusion situation. First, however, we have to understand the
possible diffusion situations that can occur. We will begin by considering two different
scenarios, shown in Figure 7.2.2.
Figure 7.2.1: Example of diffusion resulting when gold and copper are placed into contact.
The top row shows the change in concentration in the materials, while the bottom row
shows the resulting concentration gradient. Note that the materials would have to be
heated to very high temperatures for this to occur in a reasonable amount of time.
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Figure 7.2.2: Two different scenarios for how concentration can change over time due to
diffusion.
GI 7.2.1 In column A, what is the value for the concentration at x=5 cm and time=0?
GI 7.2.2 In column A, what is the value for the concentration at x=5 cm and time=5
minutes?
GI 7.2.3 In column B, what is the value for the concentration at x=5 cm and time=0?
GI 7.2.4 In column B, what is the value for the concentration at x=5 cm and time=5
minutes?
GI 7.2.5 In which column is the concentration independent of time? In which is it dependent
on time?
GI 7.2.6 Describe the difference between column A and column B in at least two different
ways.
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 Spring '08
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 Molecular diffusion, Fick

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