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What is Entropy? Tds Relations and Δ S of Liquids and Solids

What is Entropy? Tds Relations and Δ S of Liquids and Solids. What is entropy?. Cannot define or explain what it is physically to my satisfaction. Doesn’t detract from its usefulness. Can be viewed as a measure of molecular disorder or molecular randomness.

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What is Entropy? Tds Relations and Δ S of Liquids and Solids

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  1. What is Entropy?Tds Relations andΔS of Liquids and Solids

  2. What is entropy? • Cannot define or explain what it is physically to my satisfaction. • Doesn’t detract from its usefulness. • Can be viewed as a measure of molecular disorder or molecular randomness. • Lowest in solid phase and highest in gas phase • Boltzmann relation: S = k ln p. • p is called thermodynamic probability = total number of possible microscopic states of a system.

  3. Perfect order →its entropy is zero (Third law of thermo).

  4. Disorganized energy is not much use. Gas molecules have a lot of energy but it is not organized. Therefore, the gas cannot turn a paddle wheel and produce work.

  5. Raising a weight by a rotatiing shaft does not create any disorder (entropy) so energy is not degraded. (no friction) There is no entropy transfer associated with energy transfer as work. Therefore, this work is reversible.

  6. This work, done on a gas, increases the level of disorder (entropy) of the gas, and thus energy is degraded. so this process is not reversible

  7. Quality of energy transferred decreases because it has less value at lower temperature. The increase in entropy of the cold body more than offsets the decrease in entropy of the hot body.

  8. Entropy principles apply to our every day lives. Efficient people lead low- entropy (highly organized) lives. They have a place for everything (maximum uncertainty). A minimum of energy to find anything.

  9. The entropy of my office is large.

  10. From an energy balance with substitutions of δQ = TdS and δW = PdV, we get the “Tds” equations: Can solve for ds and integrate for ideal gases Both are good for both systems

  11. Entropy Change of Liquids and Solids • Can be approximated as incompressible substances. • gets rid of the Pdv part of the first Tds equation. • Leaves: ds = du/T = CdT/T • Which when integrated gives: s2 – s1 = Cavln(T2/T1) • For substances that expand a lot with Temp, must take Δv into account. • Also says that isentropic processes are also isothermal with incompressible substances. • See Table 6 – 1 page 322.

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