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thermal-properties - CHAPTER 19 ISSUES TO ADDRESS How does...

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Chapter 19- ISSUES TO ADDRESS... How does a material respond to heat ? 1 How do we define and measure... --heat capacity --coefficient of thermal expansion --thermal conductivity --thermal shock resistance How do ceramics, metals, and polymers rank? CHAPTER 19:
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Chapter 19- 2 General: The ability of a material to absorb heat. Quantitative: The energy required to increase the temperature of the material.    C = dQ dT heat capacity (J/mol-K) energy input (J/mol) temperature change (K) Two ways to measure heat capacity: -- C p : Heat capacity at constant pressure. -- C v : Heat capacity at constant volume. HEAT CAPACITY
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Chapter 19- 3 Heat capacity... --increases with temperature --reaches a limiting value of 3R Atomic view: --Energy is stored as atomic vibrations. --As T goes up, so does the avg. energy of atomic vibr. Debye temperature (usually less than T room ) T (K) Heat capacity, C v 3R θ D C v = constant gas constant = 8.31 J/mol-K Adapted from Fig. 19.2, Callister 6e . HEAT CAPACITY VS T
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Chapter 19- 4 Why is c p significantly larger for polymers? Selected values from Table 19.1, Callister 6e . HEAT CAPACITY: COMPARISON Polymers Polypropylene Polyethylene Polystyrene Teflon c p (J/kg-K) at room T Ceramics Magnesia (MgO) Alumina (Al 2 O 3 ) Glass Metals Aluminum Steel Tungsten Gold 1925 1850 1170 1050 900 486 128 138 increasing c p c p : (J/kg-K) C p : (J/mol-K) material 940 775 840
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Chapter 19- 5 Materials change size when heating.
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