2010-04-12 Chapter 19 and 20 Thermal _ Magnetic Properties

2010-04-12 Chapter 19 and 20 Thermal _ Magnetic Properties...

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Chapter 19 - 1 ISSUES TO ADDRESS. .. How does a material respond to heat ? 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: Thermal Properties
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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. 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 > C v • Specific heat has typical units of K kg J dT dQ C =
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Chapter 19 - 3 Heat capacity. .. -- increases with temperature -- reaches a limiting value of 3 R Atomic view: -- Energy is stored as atomic vibrations. -- As T goes up, so does the avg. energy of atomic vibr. Heat Capacity vs T Adapted from Fig. 19.2, Callister 7e . 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 19 - 4 Energy Storage How is the energy stored? Phonons – thermal waves - vibrational modes Adapted from Fig. 19.1, Callister 7e .
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Chapter 19 - 5 increasing c p Why is c p significantly larger for polymers? Selected values from Table 19.1, Callister 7e . 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 900 486 138 128 c p : (J/kg-K) C p : (J/mol-K) material 940 775 840 Heat Capacity: Comparison
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Chapter 19 - 6 Thermal Expansion Materials change size when heated. ) T T ( L L L initial final initial initial final - α = - coefficient of thermal expansion (1/K or 1/°C) T init T final L final L init Atomic view: Mean bond length increases with T . Adapted from Fig. 19.3(a), Callister 7e . (Fig. 19.3(a) adapted from R.M. Rose, L.A. Shepard, and J. Wulff, The Structure and Properties of Materials , Vol. 4, Electronic Properties , John Wiley and Sons, Inc., 1966.) Bond energy Bond length ( r ) increasing T T 1 r ( 5 ) 1 T 5 bond energy vs bond length curve is “asymmetric”
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Chapter 19 - 7 Thermal Expansion : Comparison Q: Why does α generally decrease with increasing bond energy? Polypropylene 145-180 Polyethylene 106-198 Polystyrene 90-150 Teflon 126-216 Polymers at room T Ceramics Magnesia (MgO) 13.5 Alumina (Al 2 O 3 ) 7.6 Soda-lime glass 9 Silica (cryst. SiO 2 ) 0.4 Metals Aluminum 23.6 Steel 12 Tungsten 4.5 Gold 14.2 Material Selected values from Table 19.1, Callister 7e . Polymers have smaller α because of weak secondary bonds
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Chapter 19 - 8 Thermal Expansion: Example Ex: A copper wire 15 m long is cooled from 40 to -9°C. How much change in length will it experience? 1
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This note was uploaded on 09/30/2011 for the course PHYSICS 1410 taught by Professor Falco during the Spring '08 term at Texas State.

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2010-04-12 Chapter 19 and 20 Thermal _ Magnetic Properties...

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