1 / 11

Lecture 7 – The Second Law (Ch. 2) Wednesday January 23 rd

Lecture 7 – The Second Law (Ch. 2) Wednesday January 23 rd. Entropy – a function of state The Clausius inequality The combined 1st and 2nd laws Calculation of entropy for irreversible processes The TdS equations. Reading: All of chapter 2 (pages 25 - 48)

brand
Télécharger la présentation

Lecture 7 – The Second Law (Ch. 2) Wednesday January 23 rd

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Lecture 7 – The Second Law (Ch. 2) Wednesday January 23rd • Entropy – a function of state • The Clausius inequality • The combined 1st and 2nd laws • Calculation of entropy for irreversible processes • The TdS equations Reading: All of chapter 2 (pages 25 - 48) Homework 2 due on Friday (25th) Assigned problems, Ch. 2: 6, 8, 16, 18, 20 Homework assignments available on web page I will post the next assignment on Friday

  2. Entropy and the 2nd Law Heat reservoir at temperature T2 > T1 Efficiency (h): Q2 Heat Engine Q1 Heat reservoir at temperature T1 < T2

  3. The Clausius Inequality and the 2nd Law Divide any reversible cycle into a series of thin Carnot cycles, where the isotherms are infinitesimally short: P v • We have proven that the entropy, S, is a state variable, since the integral of the differential entropy (dS = dQ/T), around a closed loop is equal to zero, i.e. the integration of differential entropy is path independent!

  4. The Clausius Inequality and the 2nd Law Therefore: Leading to: The 2nd expression is the quite general Clausius inequality for which the equality applies only to completely reversible processes. For an irreversible process

  5. The Clausius Inequality and the 2nd Law Consider the following cyclic process: P 2 Irreversible Reversible 1 V Recalling the definition of entropy:

  6. The Clausius Inequality and the 2nd Law For an isolated system, đQ = 0, therefore This leads to the following statement: • The entropy of an isolated system increases in any irreversible process and is unaltered in any reversible process. This is the principle of increasing entropy. The Clausius equality leads to the following relation between entropy and heat: This mathematical statement holds true for any process. The equality applies only to reversible processes.

  7. The combined 1st and 2nd Laws The 2nd law need not be restricted to reversible processes: • đQ is identifiable with TdS, as is đW with PdV, but only for reversible processes. • However, the last equation is valid quite generally, even for irreversible processes, albeit that the correspondence between đQ & TdS, and đW & PdV, is lost in this case.

  8. The Tds equations

  9. Entropy associated with irreversible processes Universe at temperature T2 Q T1 T2 > T1

  10. The TdS equations These equations give: • heat transferred (đQ = TdS) in a reversible process; • the entropy by dividing by T and integrating; • heat flow in terms of measurable quantities; • differences in specific heat capacities, CP, CV, etc..; • relationships between coordinates for isentropic processes.

More Related