1 / 15

Specific Heat

Specific Heat. Thermodynamics Professor Lee Carkner Lecture 8. PAL # 7 Work. Net work of 3 process cycle of 0.15 kg of air in a piston Isothermal expansion at 350 C from 2 MPa to 500 kPa: isothermal work = PVln(V 2 /V 1 ) Get V from PV = mRT

lali
Download Presentation

Specific Heat

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. Specific Heat Thermodynamics Professor Lee Carkner Lecture 8

  2. PAL # 7 Work • Net work of 3 process cycle of 0.15 kg of air in a piston • Isothermal expansion at 350 C from 2 MPa to 500 kPa: • isothermal work = PVln(V2/V1) • Get V from PV = mRT • V1 = (0.15)(0.287)(623) / (2000) = 0.01341 m3 • V2 = (0.15)(0.287)(623) / (500) = 0.05364 m3 • W = (2000)(0.01341)ln(0.95364/0.01341) =

  3. PAL # 7 Work • Polytropic compression with n =1.2 • Need the final volume • P2V2n = P3V3n • V3 = ((500)(0.05364)1.2 / 2000)(1/1.2) = • W = (P3V3-P2V2)/1-n = (2000)(0.01690)-(500)(0.05364) /(1-1.2) = • Isobaric compression: • W = PDV = (2000)(0.01341-0.01690) = • Net work = 37.18-34.86-6.97 =

  4. Internal Energy of Ideal Gases • We have defined the enthalpy as: • but Pv = RT, so: • So if u is just a function of T then h is too

  5. Temperature Dependence of cP

  6. Ideal Gas Specific Heats • We define the specific heat as: • So then we can solve for the change in internal energy du = cv dT • If the change in temperature is small: • Where cv is the average over the temperature range

  7. Linear Approximation of c

  8. Using Specific Heats • We can write a similar equation for h Dh = cpDT • Either specific heat: • Is tabulated • Are generally referenced to 0 at 0K

  9. cv is Universal

  10. Specific Heat Relations • We can relate cp and cv dh = du + RdT cp = cv + R • For molar specific heats • The specific heat ratio: k = cp/cv

  11. Solids and Liquids • Volume is constant • This means: • c still is temperature dependent

  12. Incompressible Solid

  13. Incompressible Enthalpy • We can write out the enthalpy change expression for constant v Dh = du + vdP + Pdv = du + vdP • For solids the pressure does not change much and so:

  14. Enthalpy of Liquids • Heaters (constant pressure) • DP = 0 • Pumps (constant temperature) • DT = 0

  15. Next Time • Test 1 • For Monday: • Read: 5.1-5.3 • Homework: Ch 5, P: 12, 15, 20

More Related