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Thermodynamics Notes3

Thermodynamics Notes3 - ME 311 FALL 2007 CHAPTER 2...

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ME 311 THERMODYNAMICS S. Masutani FALL 2007 CHAPTER 2 Focus for time being on closed systems. 1 st Law: Conservation of Energy Energy : Can be stored (accumulated) Can be transformed Can be transferred across system boundaries For closed systems: The transfer of energy occurs via work and heat transfer Closed system (control mass; no mass exchange across system boundaries) Production = Outflows – Inflows + Storage E Q W \_____________/ Q + W Be careful in analysis clearly to define system (and environment). When motions are involved, the definition of system must include a reference frame in which these motions will be measured. Laws of mechanics hold in inertial frames (free particle moves at constant velocity) REVIEW: WHAT IS ENERGY? Property of matter or system Convenient to divide into microscopic and macroscopic representations: Microscopic (Internal energy, U ; the macroscopic manifestation of microscopic processes) 1. Matter consists of fundamental particles; motions described by laws of mechanics. 2. Energy, at the microscopic level, resides in a number of modes: Translational For polyatomics: rotational, vibrational At low temperature, the energy of gas molecules are associated with translational and rotational modes. 1
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ME 311 THERMODYNAMICS S. Masutani FALL 2007 Also: 3. Electron orbitals; electron spin; [molecular binding forces; atomic binding forces; nuclear binding forces] Æ potential energy 4. magnetic and electric dipole moments--interaction with B and E fields 5. rest mass energy (changes very small in non-nuclear reactions) We assign the label “internal energy” to represent all relevant contributions of microscopic energy modes. Macroscopic Energy Representations If we were to provide a description of the system consisting of values of all microscopic energy modes of all atoms or molecules comprising system Æ too much information; i.e., ,... , i rot i transl E E ; i = 1, N where N is the number of atoms and/or molecules in the system. Alternatively, we can provide values of the total translational, rotational, vibrational, nuclear, etc. energy Æ usually still more information than necessary for engineering analysis; i.e., N i i rot N i i transl E E ,.... , Assign total energy possessed by system considering all energy forms; i.e.,
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Thermodynamics Notes3 - ME 311 FALL 2007 CHAPTER 2...

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