09-12 - E45 Lectures Chapter 17 Chapter 17 Thermal...

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Chapter 17 - E45 Lectures 11/13/09, 11/18/09
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Chapter 17 - 2 ISSUES TO ADDRESS... How do materials respond to the application of heat ? How do we define and measure... -- heat capacity? -- thermal expansion? -- thermal conductivity? -- thermal shock resistance? How do the thermal properties of ceramics, metals, and polymers differ? Chapter 17: Thermal Properties
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Chapter 17 - 3 Quantitatively: The energy required to produce a unit rise in temperature for one mole of a material. heat capacity (J/mol-K) energy input (J/mol) temperature change (K) Heat Capacity Two ways to measure heat capacity: C p : Heat capacity at constant pressure. C v : Heat capacity at constant volume. C p usually > C v • Heat capacity has units of ° - F mol lb Btu K mol J dT dQ C = The ability of a material to absorb heat
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Chapter 17 - 4 Heat capacity... -- increases with temperature -- for solids it reaches a limiting value of 3 R From atomic perspective: -- Energy is stored as atomic vibrations. -- As temperature increases, the average energy of atomic vibrations increases. Dependence of Heat Capacity on Temperature Adapted from Fig. 17.2, Callister & Rethwisch 3e . R = gas constant 3 R = 8.31 J/mol-K C v = constant Debye temperature (usually less than T room ) T (K) θ D 0 0 C v
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Chapter 17 - Heating a body => excitation of motion of its molecules by collisions Dulong and Petit: C is the same for all solids: i.e., amt of heat required to raise T by 1°C/gm-at wt 6 cal/°C. gr at wt i.e., C is dependent on how many atoms and not their identity Also, C solid = 2C gas => C depends less on the kind of atoms than on their state of aggregation
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Chapter 17 - Ideal gas: atoms = independent mass points collisions allow atoms to exchange energy E total of gas = 3/2RT Since PE = 0, KE = 3/2RT =>C gas 3/2R
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Chapter 17 - In solid, mass points are not independent motion is confined to vibrations about fixed points Atom of solid = mass attached to a spring = harmonic oscil Now E total
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