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Chemistry 51 Unit 2: Thermodynamics
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WORK DONE IN STRETCHING DNA
Piezo translator v Force Optical fiber bends when pulled x lo = contour length = 15mm r = x/lo = 1 i.e contour length (actual end-to-end separation) Force (pN) 40 80 120 160 0.6 0.8 1.0 1.2 1.4 1.6 r Application of F extends to contour length Structural transition Zero F Coiled state r < lo Irreversible damage Cluzel et al. Science 271:
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THE CONCEPT OF REVERSABILTY
P = 1 atm Pext = 1 atm pins P = 2 atm Pext = 1 atm (1) REMOVE PINS Irreversible Expansion Infinite number of steps Reversible Expansion P = 1 atm Pext = 1 atm P = atm Pext = atm Step 2 P = atm Pext = atm Step 1 (2) P = 2 atm Pext = 2 atm
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IRREVERSIBLE EXPANSION
Pext V1 V2 P V V1 V2 Pext w = -Pext(V2 – V1) P, V1 P, V2
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How does the pressure of an ideal gas vary with volume?
REVERSIBLE EXPANSION Pext = Pressure of gas. If the gas is ideal, then Pext = nRT/V How does the pressure of an ideal gas vary with volume? This is the reversible path. The pressure at each point along curve is equal to the external pressure. P V
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IRREVERSIBLE EXPANSION
Vi Vf Pi Pf A B The reversible path The shaded area is IRREVERSIBLE EXPANSION P V Vi Vf Pi Pf A B The shaded area is Pext = Pf Reversible expansion gives the maximum work
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REVERSIBLE COMPRESSION
Vf Vi Pf Pi A B The reversible path The shaded area is IRREVERSIBLE COMPRESSION V Vf Vi Pf Pi A B The shaded area is Pext = Pf Reversible compression gives the minimum work
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