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Chapter2 Lecture - CHAPTER 2 Learning Objectives 1 Discuss...

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CHAPTER 2 Learning Objectives: 1. Discuss the different forms of energy 2. Energy transfer by heat 3. Energy transfer by work 4. Introduce the first law of thermodynamics
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E = E macroscopic + E microscopic Extrinsic: due to external effects imposed on the system Gravitational field Magnetic/electric field Motion of system relative to surroundings Intrinsic: due to ―internal‖ molecular effects Energy
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E macroscopic : Consider 2 effects Macroscopic kinetic energy Macroscopic potential energy 2 2 1 mV KE mgz PE These energies have arbitrary references, and hence are only relevant if they undergo change. ) ( 2 1 2 1 2 2 V V m KE Change in kinetic energy a body of mass m would experience if the velocity is changed from V 1 to V 2 ) ( 1 2 z z mg PE Change in potential energy a body of mass m would experience if the elevation is changed from z 1 to z 2 1 2 KE KE KE 1 2 PE PE PE Note that KE and PE can be positive or negative and that KE and PE can be specified without knowledge of how the mass reached a given velocity or position z (i.e. path independence). Hence KE and PE are (extensive) properties of the system. Energy
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E microscopic : due to ―internal‖ molecular effects E microscopic = U which we call internal energy U Sensible energy Latent energy Chemical energy Nuclear energy Translational Vibrational Rotational Molecular kinetic energies Energy stored in molecular potential energies intermolecular forces influenced by spacing between molecules. e.g. gas vs. liquid vs. solid. Interatomic bond energy chemical reaction breaks bonds and forms new ones. O H CO O CH 2 2 2 4 2 2 Heat released Internuclear bond energy Energy
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E = E macroscopic + E microscopic E = U + KE + PE The change in energy between 1 and 2 is: E= U + KE + PE ) ( ) ( 2 1 ) ( 1 2 2 1 2 2 1 2 1 2 z z mg V V m U U E E Extensive If we divide the expression by mass m we get: ) ( ) ( 2 1 ) ( 1 2 2 1 2 2 1 2 1 2 z z g V V m U m U m E m E ) ( ) ( 2 1 ) ( 1 2 2 1 2 2 1 2 1 2 z z g V V u u e e Intensive e = u + ke + pe Energy
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Energy Exchange for Systems Closed systems: Energy can be added or removed from a closed system through heat transfer or work. Open systems: Energy can be added or removed from an open system through heat transfer, work, or mass transfer. Mass entering Heat Work dt dm u dt du m dt d(mu) dt dU mu U energy U internal of change of rate he Consider t Energy change through mass transfer Energy transfer occurs at the boundary!!
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Heat Transfer Energy transfer between a system and the environment can occur through the transfer of heat driven by a temperature difference.
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