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Lecture27

Lecture27 - PHYSICS 220 Lecture 27 Thermodynamics Textbook...

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Lecture 27 Purdue University, Physics 220 1 Lecture 27 Thermodynamics Textbook Chapter 15 PHYSICS 220

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Lecture 27 Purdue University, Physics 220 2 Overview Last Lecture – Conduction - contact I = Q/t = κ A Δ T/d – Convection - fluid motion I = Q/t = h A Δ T – Radiation - electromagnetic radiation • I emit = Q/t = eA σ T 4 • I absorb = Q/t = eA σ T 0 4 Wien’s Law: λ max T = 2.898 × 10 -3 mK • Today – Thermodynamics – Thermodynamic Processes – Entropy
Lecture 27 Purdue University, Physics 220 3 First Law of Thermodynamics Δ U = Q - W Heat flow into system Increase in internal energy of system Work done by system Equivalent ways of writing 1st Law: Q = Δ U + W The change in internal energy of a system ( Δ U) is equal to the heat flow into the system (Q) minus the work done by the system (W) Energy Conservation

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Lecture 27 Purdue University, Physics 220 4 Signs You are heating some soup in a pan on the stove. To keep it from burning, you also stir the soup. Apply the 1 st law of thermodynamics to the soup. What is the sign of A) Q B) W C) Δ U Positive, heat flows into soup Negative, stirring does work on soup Positive, soup gets warmer
Lecture 27 Purdue University, Physics 220 5 W = P Δ V W > 0 if Δ V > 0 expanding system does positive work W < 0 if Δ V < 0 contracting system does negative work W = 0 if Δ V = 0 system with constant volume does no work Work Done by a System M M Δ y W = F d cos θ = P A d = P A Δ y = P Δ V

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Lecture 27 Purdue University, Physics 220 6 P-V Diagram Ideal gas law: PV = nRT • For n fixed, P and V determine “state” of system – T = PV/nR – U = (3/2)nRT = (3/2)PV • Examples: – which point has highest T? B – which point has lowest U? C – to change the system from C to B, energy must be added to system V P A B C V 1 V 2 P 1 P 3
Lecture 27 Purdue University, Physics 220 7 Special PV Cases Constant Pressure – isobaric Constant Volume – isochoric Constant Temp Δ U = 0 – isothermic • Adiabatic Q=0 (no heat is transferred) V P W = P Δ V (>0) 1 2 3 4 Δ V > 0 V P W = P Δ V = 0 1 2 3 4 Δ V = 0

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Lecture 27 Purdue University, Physics 220 8 V P 1 2 V 1 V 2 P 2 moles of monatomic ideal gas is taken from state 1 to state 2 at constant pressure
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