Lecture_05_02_08_2012

Lecture_05_02_08_2012 - The First Law of Thermodynamics y...

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The First Law of Thermodynamics on Reifenberger Ron Reifenberger Birck Nanotechnology Center Purdue University February 8, 2012 Lecture 5 1
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First Law of Thermodynamics Overlooks the fine microscopic details (which in many cases are irrelevant). Describes the conversion of one form of energy (heat) into other forms Important Consequences: Conservation of energy Allows estimates of maximum efficiency of energy conversion Completely General! (not just ideal gas) 1 st Law 2
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he Effect of Heat The Effect of Heat “ istributed” Distributed Energy http://mw2.concord.org/public/part2/heat/page8.cml 1 click 3
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Statement of 1 st Law (1842) E int = Q + W E int E int represents the distributed energy in a closed system. It can contain contributions from a variety of sources like thermal, chemical, etc. Sign conventionl 4
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he “system” Universal Appeal! The system The “system” can be a collection of matter in any form, separated from its surroundings by real or imaginary boundaries. The system may be a biological cell, the contents of a test tube, the gas in a piston, a thin film, or a cup of coffee. How you define a “system” depends on the problem you want to solve. 5
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ign Conventions When using the 1 st Law, it’s important determine the proper signs Work (W) has a sign associated with it: f work is done ON a system it is assigned a positive sign Sign Conventions to determine the proper signs If work is done ON a system, it is assigned a positive sign If work is done BY a system (external work or work on the environment), it is assigned a negative sign Note: This sign convention is consistent with definition in mechanics in which work done on a system (eg lifting a box) increases the mechanical (potential) energy of the system. By analogy, positive work on a system eat (Q) also has a sign associated with it:
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Lecture_05_02_08_2012 - The First Law of Thermodynamics y...

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