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## ENTC 370: Announcements

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**ENTC 370: Announcements**• Homework assignments No.4: • Assigned Problems: • 4.13, 4.23, 4.32, 4.37, 4.44, 4.46, 4.52, 4.81 4.85. • Due Thursday, October 9th before 10:50 am • For more information, go to: • http://etidweb.tamu.edu/classes/entc370**ENTC 370: Announcements**• Exam I: • Thursday, Oct 23rd • Chapters 1 – 5 • Homeworks1 – 5 • Closed book/closed notes • Students will be allowed to bring own equation sheet • Double-sided is ok • Size: 8½ x 11**Polytropic Process**For an ideal gas under going a polytropic process, the boundary work is PVn = C Isothermal case →**Example of Boundary Work**• A piston-cylinder device contains 0.4 m3 of air initially at 100 kPa and 80 °C. The air is now compressed to 0.1 m3 in a way that the temperature remains constant. Determine the work done during the process.**1st Law of Thermodynamics for Closed Systems**Textbook →**Energy Balance for Closed Systems**(kJ) In Rate Form: (kW) Per unit mass:**Closed System Undergoing Cycle**For closed system undergoing a cycle:**Example**Example: An unconstrained piston-cylinder device contains 25 g of saturated water vapor that is maintained at a constant pressure of 300 kPa. A resistance heater within the cylinder is turned on, and current passes (0.2 A) for 5 min from a 120-V source. Heat loss is 3.7 kJ. Show that (a) boundary work and internal energy can be combined into one term, DH. (b) Determine the final temperature of the Steam. (H = U + PV)**Example**• A rigid tank is divided into equal parts. One side of the tank contains 5 kg of water at 200 kPa and 25 °C. The other side is empty. The partition that divides the tank is removed and water expands into the entire two parts. Heat transfer is allowed so the entire tank can reach a temperature of 25 °C. (a) Determine the volume of the tank, (b) final pressure, and (c) amount of heat transfer. http://brod.sfsb.hr/test/testhome/vtAnimations/animations/chapter05/closed/processes/nonuniformmixing/index.html**Example**Observation: No work crosses the system boundary during the expansion process, only heat**Specific Heat**• Indicator of energy storage capabilities of various substances (physical property) • Defined as the energy required to raise the temperature of a unit mass of a substance by one degree • Two types: • Specific Heat at constant volume, CV • Specific Heat at constant pressure, CP**These are property relations or independent of the type of**process Valid for any substance Units: kJ/kg-K or kJ/kg-° C Subscripts v and P stand for constant specific volume and constant pressure, respectively. Formal Definitions of Specific Heat**Internal Energy (u), Enthalpy (h) and Specific Heats (cp &**cv) of Ideal Gases • For gases, u and h only depends on Temperature: u = f(T) In the case of air: Use Table A-17**Example**For 5 min • Determine the final temperature, T2 • Identify type of fluid, type of device, energy flows • What remains constant? • Write down 1st Law and solve for unknown (T2)