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Lecture6

# Lecture6 - Physics 7A Lecture 6 Winter 2010...

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Physics 7A – Lecture 6 Winter 2010 Prof. Robin D. Erbacher 343 Phy/Geo Bldg [email protected]

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Quiz 4 is today, focused on material from DLMs 9-12. Join this Class Session with your PRS clicker! Quiz 3 Rubric has been posted. Check Physics 7 website frequently for updates. Turn off cell phones during lecture.
Particle Model of Matter

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Understand the particulate nature of matter
We are modeling real atoms of liquids and solids as oscillating masses and springs. r Goal : To understand macroscopic phenomena (e.g. melting, vaporizing) and macroscopic properties of matter such as phases, temperature, heat capacities, in terms of microscopic constituents and its behavior.

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Atom 1 (anchored) Atom 2 (bonded) ~ two atomic size particles interacting via “pair- wise potential” a.k.a. Lennard-Jones Potential
separation Flattening atoms have negligible  forces at large  separation. r Distance between the atoms, r Repulsive Atoms push apart as they  get too close. Equilibrium separation (bond length) r o P o t e n i a l E r g y

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ε Energy r (atomic diameters) r σ σ is the atomic diameter ε is the well depth (bond strength) σ σ pair-wise σ ~ 10 -10 m = 1Å Potential Energy between two atoms Potential Energy between two atoms “pair-wise potential” a.k.a. Lennard-Jones “pair-wise potential” a.k.a. Lennard-Jones Potential Potential
PE KE E Separation (10 -10 m) Energy (10 -21 J)

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The bond is an abstraction: Atoms that don’t have enough energy cannot escape the potential (force), so we treat them as bound until we add enough energy to free them.
If a bond is “broken” in an atom-atom potential, which of the following must be true: A. E tot < 0 B. E tot 0 C. PE < 0 D. PE 0 E. KE < 0 ε σ

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If a bond is considered stronger, it means that: A. A greater well depth ε and a greater σ B. A smaller well depth ε and a greater σ A. A smaller well depth ε and no constraint on σ D. A smaller well depth ε and a smaller σ E. A greater well depth ε and no constraint on σ ε σ
Multiple- Atom Systems: Particle Model of E bond , Particle Model of E thermal

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If the atoms in the molecule do not move too far, the forces between them
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Lecture6 - Physics 7A Lecture 6 Winter 2010...

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