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Lecture04A - Lecture 4 1 Why do gases attract(16.6 16.7 For...

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Unformatted text preview: Lecture 4 1 Why do gases attract? (16.6, 16.7) For gas molecules, repulsion is (relatively) easy to explain: electron cloud (charge density) overlap (for closed shell systems) gives rise to a strong repulsive force (~1/r 12 or hard sphere). But what causes attraction? Assume we have neutral species (not a plasma, but a gas!) Simple answer: electrostatics. Consider 2 interacting dipoles: 1 μ 2 μ { 12 unit vector ˆ r r r = ⋅ r ( 29 ( 29 ( 29 1 2 12 1 12 2 int 2 3 ˆ ˆ 3 Interaction energy = 4 o r r u r μ μ μ μ πε ⋅- ⋅ ⋅ = r r r r But, our gas molecules have freedom of movement (translate, rotate), so on average: ( 29 ( 29 2 2 2 int 1 2 2 6 2 1 3 4 dd B o B u u r k T r k T μ μ πε = = - Average over all orientations Fig. 16.16 Lecture 4 2 Example: CO at 80K-31 For CO, is small (0.1D or 3.3 10 ) C m μ × ⋅ 23 1 21 B At T = 80K, k 1.38 10 80 1.10 10 Thermal Energy T JK K J--- = × ⋅ = × = ( 29 ( 29 ( 29 ( 29 4 31 4 4 2 2 6 1 2 2 2 10 4 2 2 21 3.3 10 2 2 3 3 4 1.11 10 1.10 10 dd o B C m r u k T C J m J μ μ πε----- ×- = = × × 6 82 6 5.8 105....
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Lecture04A - Lecture 4 1 Why do gases attract(16.6 16.7 For...

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