ENSC 388 (F09) - Entropy

Flashcards covering key vocabulary and concepts from the lecture notes on Entropy.

Entropy

A property defined by the second law of thermodynamics related to the direction of processes.

Clausius Inequality

A thermodynamic inequality stating that the cyclic integral of δQ / T is always less than or equal to zero (≤ 0).

Entropy (dS)

A thermodynamic property that measures the degree of molecular disorder or randomness in a system (kJ/K). dS = (δQ / T)int,rev

Increase of Entropy Principle

States that the entropy of an isolated system during a process always increases or, in the limiting case of a reversible process, remains constant.

Entropy Generation (Sgen)

The entropy created during an irreversible process; it is always a positive quantity or zero (for reversible processes).

Entropy Balance

Expresses the increase of entropy principle: Entropy change = Entropy transfer + Entropy generation; ΔSsystem = Stransfer + Sgen

Mechanisms of Entropy Transfer

Heat transfer (Q/T) and mass flow (ms), where s is the specific entropy.

Entropy Transfer with Heat

The ratio of heat transfer (Q) to absolute temperature (T) at a location: Sheat = Q/T

Isentropic Process

A process in which entropy remains constant (s2 = s1), which implies it is both reversible and adiabatic.

Gibbs Equations

Two equations that relate thermodynamic properties: Tds = du + Pdv and Tds = dh – vdP

Entropy Change of Solids and Liquids

Approximated by: Δs = cave * ln(T2 / T1), assuming incompressible substances.

Entropy Change of Ideal Gas

Can be calculated using: s2 – s1 = c v,ave ln(T2 / T1) + R ln(v2 / v1) or s2 – s1 = c p,ave ln(T2 / T1) - R ln(P2 / P1)

Reversible Steady-Flow Work

Expressed as: -wrev = ∫vdP + Δke + Δpe, where the integral is evaluated from state 1 to state 2.

Bernoulli Equation

For a steady-state flow of a liquid with no work interactions: v(P1 – P2) + (V1^2 – V2^2)/2 + g(z1 – z2) = 0