Thursday, June 16, 2011
Wednesday, June 15, 2011
http://www.av8n.com/physics/gas-laws.htm
Constant S and N
For our next scenario, we arrange to have “no friction” (or very little), and “no heat leaks” (or very little).
To say the same thing more formally, we want to keep the entropy constant. If you don’t know what entropy is, don’t worry about it too much right now, and follow the scenario as described in the previous paragraph.
If the tank is large, this is relatively easy to arrange; we just need to make sure the experiment is done neither too quickly nor too slowly.
If the volume is changed too quickly, there will be an irreversible increase in entropy due to internal friction in the gas as it sloshes around.
If the experiment is done too slowly, there will be a gain or loss of entropy due to heat flowing through the walls of the tank.
If the tank is not large and not thermally insulated, the window between “too quickly” and “too slowly” may be inconveniently small (or perhaps nonexistent). Conversely, if we make the tank large enough and provide reasonable thermal insulation, there will be plenty of time to change the volume with minimal dissipation, and plenty of time to make the desired measurements.
In this scenario, we observe that smaller volume is associated with higher pressure (and vice versa). Also, the gas cools as it expands (and heats up as it is compressed). The point describing the system moves from point [1] to point [2] along a contour of constant entropy, as shown in fig
For our next scenario, we arrange to have “no friction” (or very little), and “no heat leaks” (or very little).
To say the same thing more formally, we want to keep the entropy constant. If you don’t know what entropy is, don’t worry about it too much right now, and follow the scenario as described in the previous paragraph.
If the tank is large, this is relatively easy to arrange; we just need to make sure the experiment is done neither too quickly nor too slowly.
If the volume is changed too quickly, there will be an irreversible increase in entropy due to internal friction in the gas as it sloshes around.
If the experiment is done too slowly, there will be a gain or loss of entropy due to heat flowing through the walls of the tank.
If the tank is not large and not thermally insulated, the window between “too quickly” and “too slowly” may be inconveniently small (or perhaps nonexistent). Conversely, if we make the tank large enough and provide reasonable thermal insulation, there will be plenty of time to change the volume with minimal dissipation, and plenty of time to make the desired measurements.
In this scenario, we observe that smaller volume is associated with higher pressure (and vice versa). Also, the gas cools as it expands (and heats up as it is compressed). The point describing the system moves from point [1] to point [2] along a contour of constant entropy, as shown in fig
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