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# ch05_eu - ENGLISH UNIT PROBLEMS SOLUTION MANUAL CHAPTER 5...

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ENGLISH UNIT PROBLEMS SOLUTION MANUAL CHAPTER 5

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Borgnakke and Sonntag CHAPTER 5 SUBSECTION PROB NO. Concept-Study Guide Problems 173-177 Kinetic and Potential Energy 178-181 Properties from General Tables 182-184 Simple Processes 185-196 Multistep Processes 197-198 Solids and Liquids 199-203 Ideal Gas 204-211 Polytropic Processes 212-215 Energy equation in Rate Form 216-218 and 213 Review problems 219-223
Borgnakke and Sonntag Concept Problems

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Borgnakke and Sonntag 5.173 E What is 1 cal in english units, what is 1 Btu in ft-lbf? Look in Table A.1 for the conversion factors under energy 1 Btu = 778.1693 lbf-ft 1 cal = 4.1868 J = 4.1868 1055 Btu = 0.00397 Btu = 3.088 lbf-ft
Borgnakke and Sonntag 5.174 E Work as F x has units of lbf-ft, what is that in Btu? Look in Table A.1 for the conversion factors under energy 1 lbf-ft = 1.28507 × 10 -3 Btu

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Borgnakke and Sonntag 5.175 E Look at the R-410a value for u f at -60 F. Can the energy really be negative? Explain. The absolute value of u and h are arbitrary. A constant can be added to all u and h values and the table is still valid. It is customary to select the reference such that u for saturated liquid water at the triple point is zero. The standard for refrigerants like R-410a is that h is set to zero as saturated liquid at -40 F, other substances like cryogenic substances like nitrogen, methane etc. may have different states at which h is set to zero. The ideal gas tables use a zero point for h as 77 F or at absolute zero, 0 R.
Borgnakke and Sonntag 5.176 E An ideal gas in a piston-cylinder is heated with 2 Btu during an isothermal process. How much work is involved? Energy Eq.: u 2 u 1 = 1 q 2 1 w 2 = 0 since u 2 = u 1 (isothermal) Then 1 W 2 = 1 Q 2 = m 1 w 2 = 2 Btu

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Borgnakke and Sonntag 5.177 E You heat a gas 20 R at P = C. Which one in table F.4 requires most energy? Why? A constant pressure process in a control mass gives (recall Eq.5.29) 1 q 2 = u 2 u 1 + 1 w 2 = h 2 h 1 C p T The one with the highest specific heat is hydrogen, H 2 . The hydrogen has the smallest mass, but the same kinetic energy per mol as other molecules and thus the most energy per unit mass is needed to increase the temperature.
Borgnakke and Sonntag Kinetic and Potential Energy

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Borgnakke and Sonntag 5.178 E A piston motion moves a 50 lbm hammerhead vertically down 3 ft from rest to a velocity of 150 ft/s in a stamping machine. What is the change in total energy of the hammerhead? Solution: C.V. Hammerhead The hammerhead does not change internal energy i.e. same P,T E 2 – E 1 = m(u 2 – u 1 ) + m( 1 2 V 2 2 - 0) + mg (h 2 - 0) = 0 + [ 50 × (1/2) ×150 2 + 50 × 32.174 × (-3)] / 32.174 = [562500 – 4826]/32.174 = 17 333 lbf-ft = ( 17 333 778 ) Btu = 22.28 Btu