407 Class Notes 5, 2010 [Compatibility Mode]

407 Class Notes 5, 2010 [Compatibility Mode] - Thermal...

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Thermal Properties . eat capacity A. Heat capacity Heat capacity, C, is a measure of a material’s ability to absorb heat from the external surroundings, or = Q T C = dQ/dT where dQ is the energy required to produce a dT temperature change ormally specified as heat capacity per mole (e g cal/mol ) Normally specified as heat capacity per mole (e.g. cal/mol-K) -- specific heat is also used; represents the heat capacity per unit mass (cal/g-K) -- measured either by maintaining a constant specific volume, C V , or by maintaining a constant external pressure, C P most solids, thermal energy absorption occurs rimarily y imparting In most solids, thermal energy absorption occurs primarily by imparting vibrational energy to the atoms -- due to atomic bonding, vibrations are coordinated in such a way that travelling lattice waves (sound waves) are generated, which have very high frequencies -- quantum mechanics allows certain energy values, called phonons.
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The heat capacity at constant volume, C V , is zero at 0º K but rises rapidly with temperature. At low temperatures = AT 3 C V AT where A is a temperature-independent constant bove the Debye temperature C ecomes relatively independent of Above the Debye temperature θ D , C V becomes relatively independent of temperature and attains a value of about 3R. In other words, the quantity of energy required to produce one-degree temperature change is constant. Fig. X - Temperature dependence of the heat capacity at constant volume; θ D is the Debye temperature.
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B. Thermal expansion Thermal expansion is a measure of the extent to which a material expands upon heating or contracts upon cooling, or Δ / 0 = α Δ T where α is called the linear coefficient of thermal expansion The expression for the volumetric expansion, α v , is similar -- in many materials, the value of α is anisotropic, i.e. it v depends on crystallographic direction
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407 Class Notes 5, 2010 [Compatibility Mode] - Thermal...

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