Lecture7 - Physics 7A Lecture 7 Winter 2010...

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Physics 7A – Lecture 7 Winter 2010 Prof. Robin D. Erbacher 343 Phy/Geo Bldg [email protected]
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No Quiz today, Quiz 5 is next week. Join this Class Session with your PRS clicker! Check Physics 7 website frequently for updates. Turn off cell phones during lecture.
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Review Multiple- Atom Systems: Particle Model of E bond , Particle Model of E thermal
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Example: H 2 O What is E bond in terms of KE and PE of individual atom (atom pair)? What is E thermal in terms of KE and PE of individual atom (atom pair)?
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Magnitude of E for a substance is the amount of energy required to break apart “all” the bonds
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Separation (10 -10 m) E bond ≈ -40 x 10 -21 Joules Energy required to break a single pair of atoms apart: +8x10 -21 J =5 bonds.
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A B KE A pair of atoms are interacting via a  atom-atom potential. Only these two  atoms are around. In two different  situations, the pair have a different  amount of  total energy .   In which  situation is E thermal  greater? E b)  Situation B has a greater  E thermal KE
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Initial Final Okay.  Now let us look at the  same problem from the  perspective of the  KE and PE of  individual atoms . Which situation is correct in  going from initial to final states?
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Initial Final Okay.  Now let us look at the  same problem from the  perspective of the  KE and PE of  individual atoms . Which situation is correct in  going from initial to final states?
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The energy associated with the random motion of particles is split between PE and KE .
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PE avg = KE avg = E tot /2 Energ y position E tot PE KE
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thermal not E .
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Equipartition of Energy
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If the atoms in the molecule do not move too far, the forces between them can be modeled as if there were springs between the atoms. Each particle in a solid or liquid oscillates in 3 dimensions about its equilibrium positions as determined by its single-particle potential.
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Another way of stating: Each particle has six “ways” to store the energy associated with its random thermal motion. We call this “way” for a system to have thermal energy a “ mode ”.
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Gas Liquids and Solids
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