161ASpring2010-Lecture8

161ASpring2010-Lecture8 - 1 State Variables The Physical...

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1 State Variables State Variables The Physical State of a system is completely described by a small number of Macroscopic Variables (N,V,T,U, concentrations… we will add more later on) These variables are called state variables and include pressure, volume, temperature, amount of substance, Internal Energy In a chemical reaction it is the amounts (concentrations) of the products and reactants, their phases (gas, liquid, solid), Temperature and Pressure.
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2 Internal energy of an ideal gas Partition is broken and gas is Allowed to expand in an adiabatic (no heat transfer container) Controlled expansion (Isothermal Reversible) the same final state No heat exchanged Heat exchanged Different paths the same initial state No work done Work done Is U is the same for both processes? Free Expansion
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3 a b c i f State Functions for a Mountain Trekker h a =∆ h b =∆ h c s a ≠∆ s b ≠∆ s c
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4 Is work a state function? P 1 P 2 V 1 V 2 i f Ideal Gas Is the amount of work done on each path the same? Imagine two different transformations from the same initial state (i) to the same final state f. One tr
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5 Is work a state function? P 1 P 2 V 1 V 2 Work on red path is w red = -p 2 (V 2 -V 1 ) . i f Ideal Gas Work is not a state function •Work on blue path is w blue = -p 1 (V 2 -V 1 ) Thus w red < w blue . Work done on system differs for different paths from i to f .
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6 Thermodynamics is concerned with equilbrium states. An equilibrium state is one in which macroscopic properties (E,V,P,T,n 1 ,n 2 ,...) do not change with time. Properties are Either extensive (E,V,n) or intensive (T,P) Equilibrium States i f State space
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161ASpring2010-Lecture8 - 1 State Variables The Physical...

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