Homework 5

Never enter units in your answers. Be sure to use the units indicated on the HW submission page. Enter numbers without commas. 23,546 should be 23546 Always enter a * for multiplication. g(H+h) should be g*(H+h).



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Submitted Answers will be graded W 10/15, Th 10/16, F 10/17, and M 10/20 at 12:00 noon.




Problem 5.1
	What you should learn: Reversible processes can happen in reverse and make sense physically.
	Problem: Which of the following processes is reversible or almost so? (Enter the letter of the best answer.) A. You light a match. B. You fold a paper. C. You set a book on a table. D. You clap your hands.
	Hints: What is a good test of reversibility?
	Results of previous submissions: Submission Score Your Answer Correct Answer Comments Database Results Wizard Error The operation failed. If this continues, please contact your server administrator.
	Answer Range: Submit HW answers.






Problem 5.2
	What you should learn: The efficiency of a heat engine is e = |W| / |Qin|. For a Carnot engine (but not in general), efficiency is a function only of the temperatures between which the engine operates. e = 1 – Tc / Th.
	Problem: A Carnot engine operating between temperatures Tc and Th has heat Q flowing into it in every cycle. How much work W does the engine produce per cycle?
	Constants and fixed variables:   Th = 480 °C   Q = 30.0 J
	Variables that must be changed with each submission: Database Results Wizard Error The operation failed. If this continues, please contact your server administrator.
	Hints: My answer isn't in range, but the problem seems easy.
	Results of previous submissions: Submission Score Your Answer Correct Answer Comments Database Results Wizard Error The operation failed. If this continues, please contact your server administrator.
	Answer Range: 16.0 - 20.0 J Submit HW answers.






Problem 5.3
	What you should learn: When run in cooling mode, the coefficient of performance COP of a refrigerator is |Qc| / |W|. (You don't need to memorize this relationship.)
	Problem: A freezer keeps its contents at Tc while the outside temperature is T0. It has a COP given below. How much work (W) must be put into the refrigerator to cause 1000 J of energy to be added to the room?
	Constants and fixed variables:   Tc = –20 °C   T0 = +20 °C
	Variables that must be changed with each submission: COP Database Results Wizard Error The operation failed. If this continues, please contact your server administrator.
	Hints: Be careful about identifying the correct Q. What does energy conservation tell us about refrigerators?
	Results of previous submissions: Submission Score Your Answer Correct Answer Comments Database Results Wizard Error The operation failed. If this continues, please contact your server administrator.
	Answer Range: 120 - 200 J Submit HW answers.






Problem 5.4
	What you should learn: Apply the basic laws of thermodynamics to a reversible heat engine.
	Problem: A heat engine has uses a monatomic ideal gas that goes through two constant pressure legs and two constant volume legs as shown in the figure. What is total work W for one cycle? (Consider W to be the absolute value of the work.)
	Constants and fixed variables:   Volume at A: VA = 1.24 x 10–3 m3   Volume at B: VB = 1.89 x 10–3 m3   Pressure at A: PA = 1.35 x 105 Pa
	Variables that must be changed with each submission: Pressure at C, PC Database Results Wizard Error The operation failed. If this continues, please contact your server administrator.
	Hints: A list of basic rules. Don't I need to know n or T?
	Results of previous submissions: Submission Score Your Answer Correct Answer Comments Database Results Wizard Error The operation failed. If this continues, please contact your server administrator.
	Answer Range: 35.0 - 50.0 J Submit HW answers.






Problem 5.5
	What you should learn: Apply the basic laws of thermodynamics to a reversible heat engine.
	Problem: Consider the cycle of Problem 5.4. What is QAB the heat flow from A to B? Be careful of the sign.
	Constants and fixed variables:   Volume at A: VA = 1.24 x 10–3 m3   Volume at B: VB = 1.89 x 10–3 m3   Pressure at A: PA = 1.35 x 105 Pa
	Variables that must be changed with each submission: Pressure at C, PC Database Results Wizard Error The operation failed. If this continues, please contact your server administrator.
	Hints: A list of basic rules.
	Results of previous submissions: Submission Score Your Answer Correct Answer Comments Database Results Wizard Error The operation failed. If this continues, please contact your server administrator.
	Answer Range: |QAB| is between 200 and 250 J Submit HW answers.






Problem 5.6
	What you should learn: Apply the basic laws of thermodynamics to a reversible heat engine.
	Problem: Consider the cycle of Problem 5.4. What is QBC the heat flow from B to C? Be careful of the sign.
	Constants and fixed variables:   Volume at A: VA = 1.24 x 10–3 m3   Volume at B: VB = 1.89 x 10–3 m3   Pressure at A: PA = 1.35 x 105 Pa
	Variables that must be changed with each submission: Pressure at C, PC Database Results Wizard Error The operation failed. If this continues, please contact your server administrator.
	Hints: A list of basic rules.
	Results of previous submissions: Submission Score Your Answer Correct Answer Comments Database Results Wizard Error The operation failed. If this continues, please contact your server administrator.
	Answer Range:  |QBC| is between 150 and 225 J Submit HW answers.






Problem 5.7
	What you should learn: Apply the basic laws of thermodynamics to a reversible heat engine.
	Problem: Consider the cycle of Problem 5.4. What is QCD the heat flow from C to D? Be careful of the sign.
	Constants and fixed variables:   Volume at A: VA = 1.24 x 10–3 m3   Volume at B: VB = 1.89 x 10–3 m3   Pressure at A: PA = 1.35 x 105 Pa
	Variables that must be changed with each submission: Pressure at C, PC Database Results Wizard Error The operation failed. If this continues, please contact your server administrator.
	Hints: A list of basic rules.
	Results of previous submissions: Submission Score Your Answer Correct Answer Comments Database Results Wizard Error The operation failed. If this continues, please contact your server administrator.
	Answer Range:  |QCD| is between 80.0 and 150 J Submit HW answers.






Problem 5.8
	What you should learn: Apply the basic laws of thermodynamics to a reversible heat engine.
	Problem: Consider the cycle of Problem 5.4. What is QDA the heat flow from D to A? Be careful of the sign.
	Constants and fixed variables:   Volume at A: VA = 1.24 x 10–3 m3   Volume at B: VB = 1.89 x 10–3 m3   Pressure at A: PA = 1.35 x 105 Pa
	Variables that must be changed with each submission: Pressure at C, PC Database Results Wizard Error The operation failed. If this continues, please contact your server administrator.
	Hints: A list of basic rules.
	Results of previous submissions: Submission Score Your Answer Correct Answer Comments Database Results Wizard Error The operation failed. If this continues, please contact your server administrator.
	Answer Range:  |QDA| is between 100 and 140 J Submit HW answers.






Problem 5.9
	What you should learn: Apply the basic laws of thermodynamics to a reversible heat engine.
	Problem: Consider the cycle of Problem 5.4. What is e, the efficiency of the engine.
	Constants and fixed variables:   Volume at A: VA = 1.24 x 10–3 m3   Volume at B: VB = 1.89 x 10–3 m3   Pressure at A: PA = 1.35 x 105 Pa
	Variables that must be changed with each submission: Pressure at C, PC Database Results Wizard Error The operation failed. If this continues, please contact your server administrator.
	Hints: What is Qh in the efficiency formula?
	Results of previous submissions: Submission Score Your Answer Correct Answer Comments Database Results Wizard Error The operation failed. If this continues, please contact your server administrator.
	Answer Range: 0.100 - 0.150 Submit HW answers.

 




Problem 5.10
	What you should learn: If you wish to calculate entropy changes for solids and liquids, use dQ = mcdT and dS = dQ / T.
	Problem: A piece of hot copper is placed into water 500 ml of water in a calorimeter. The initial temperatures Tc and Tw along with the final temperature Tf are given. Find the change in entropy (DeltaS) of the copper-water system in this process.
	Constants and fixed variables:   Mass of the water: mw = 0.500 kg   Initial water temperature: Tw = 12.0 °C   Initial copper temperature: Tc = 130.0 °C   Specific heat of water: cw = 4186 J/(kg °C)   Specific heat of copper: cc = 387 J/(kg °C)
	Variables that must be changed with each submission: Database Results Wizard Error The operation failed. If this continues, please contact your server administrator.
	Hints: It seems like there are too many unknowns. Can't I just use ΔS = mcΔT / T ?
	Results of previous submissions: Submission Score Your Answer Correct Answer Comments Database Results Wizard Error The operation failed. If this continues, please contact your server administrator.
	Answer Range: 9.50 - 21.0 J/K Submit HW answers.





Problem 5.11
	What you should learn: In a constant volume or constant pressure process, the change in entropy is calculated in much the same way as for solids and liquids. You need to use dQ = nCvdT or nCpdT along with dS = dQ / T. Then integrate to find ΔS.
	Problem: n  moles of ideal monatomic gas is heated isobarically from Ti to Tf. What is the change in entropy (DeltaS) of the gas?
	Constants and fixed variables:   Number of moles: n = 1.24   Initial temperature: Ti = 20.0 °C   Ideal gas constant: R = 8.314 J/(mol K)
	Variables that must be changed with each submission:  Database Results Wizard Error The operation failed. If this continues, please contact your server administrator.
	Hints: Be sure to put the temperature in kelvins.
	Results of previous submissions: Submission Score Your Answer Correct Answer Comments Database Results Wizard Error The operation failed. If this continues, please contact your server administrator.
	Answer Range: 15 - 20 J/K Submit HW answers.




Problem 5.12
	What you should learn: In a constant temperature process, the change in entropy is just Q / T.
	Problem: n  moles of ideal monatomic gas are compressed at constant temperature from Pi to Pf. What is the change in entropy (DeltaS) of the gas?
	Constants and fixed variables:   Number of moles: n = 1.24   Initial pressure: PI = 125000 Pa   Temperature: T0 = 210.0 °C    Ideal gas constant: R = 8.314 J/(mol K)
	Variables that must be changed with each submission:  Database Results Wizard Error The operation failed. If this continues, please contact your server administrator.
	Hints: What is Q for an isothermal process?
	Results of previous submissions: Submission Score Your Answer Correct Answer Comments Database Results Wizard Error The operation failed. If this continues, please contact your server administrator.
	Answer Range: –13.0 to –9.00 J/K Submit HW answers.





Problem 5.13
	What you should learn: Entropy is a state variable.
	Problem: What is the change in entropy around one complete cycle of the heat engine described in Problem 5.4 above. Use the same numbers as in Problem 5.4 to evaluate your answer.
	Results of previous submissions: Submission Score Your Answer Correct Answer Comments Database Results Wizard Error The operation failed. If this continues, please contact your server administrator.
	Answer Range: –10 to +10 J/K Submit HW answers.



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