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Equations of state (状态方程)
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A equation to express the relationship between state variables and quantity of the substance. For example,  |
 is the volume expansion and k is the compressibility.
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For ideal gas (理想气体):  |
Gas constant:  . |
In the ideal gas model, we ignore the volume of molecules and neglect the attractive forces between molecules. When these can not be ignore, it is the van der Waals gas. |
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a depends on intermolecular attractions and b represents the volume of a molecule.
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Variation of pressure as elevation changes |
Example: Find the variation of atmospheric pressure with elevation in the earth’s atmosphere. Ignore the variation of g with elevation.
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Solution: Consider an imaginary cylinder with cross-section area A and height  .
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pV diagrams—isotherms (等温线) |
pV diagram: the graph of pressure as a function of volume.
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Isotherm: the curve describing the behavior of gas at a constant temperature.
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Kinetic molecular model of an ideal gas |
Number of collision with A during dt, |
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Collisions between molecules |
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Suppose only one molecule is moving,
Number of collision per unit time,
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If all the molecules are moving,  |
Average time between collision:  |
Mean free path:  |
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