answer key assignment, Assignments of Thermodynamics

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ANSWER KEY IN

THERMODYNAMICS

1 (ASSIGNMENT 1)

Prepared by: Adam C. Macapili

Problem 1-7. A 3-kg plastic tank that has a volume of is filled with liquid water. Assuming the density of water is , determine the weight of the combined system. Assumptions The density of water is constant throughout. Properties The density of water is given to be. Analysis The mass of the water in the tank and the total mass are

Problem 1-9. At 45° latitude, the gravitational acceleration as a function of elevation z above sea level is given by , where a and. Determine the height above sea level where the weight of an object will decrease by 1 percent. Analysis The weight of a body at the elevation z can be expressed as

Problem 1-10E. A 150-lbm astronaut took his bathroom scale (a spring scale) and a beam scale (compares masses) to the moon where the local gravity is. Determine how much he will weigh ( a ) on the spring scale and ( b ) on the beam scale. Analysis ( a ) A spring scale measures weight, which is the local gravitational force applied on a body: Analysis (b) A beam scale compares masses and thus is not affected by the variations in gravitational acceleration. The beam scale will read what it reads on earth,:

Problem 1-16C. A can of soft drink at room temperature is put into the refrigerator so that it will cool. Would you model the can of soft drink as a closed system or as an open system? Explain. Answer: A can of soft drink should be analyzed as a closed system since no mass is crossing the boundaries of the system.

Problem 1-17C. What is the difference

between intensive and extensive

properties?

Answer: Intensive properties do not depend on the size (extent) of the system but extensive properties do.

Problem 1-19C. What is a quasi-equilibrium process? What is its importance in engineering? Answer: A process during which a system remains almost in equilibrium at all times is called a quasi- equilibrium process. Many engineering processes can be approximated as being quasi-equilibrium. The work output of a device is maximum and the work input to a device is minimum when quasi- equilibrium processes are used instead of non quasi-

Problem 1-20C. Define the isothermal, isobaric, and isochoric processes. Answer: A process during which the temperature remains constant is called isothermal; a process during which the pressure remains constant is called isobaric; and a process during which the volume remains constant is called isochoric.

Problem 1-27C. What are the ordinary and absolute temperature scales in the SI and the English system? Answer: They are celsius(°C) and kelvin (K) in the SI, and fahrenheit (°F) and rankine (R) in the English system.

Problem 1-29. The deep body temperature of a healthy person is 37°C. What is it in kelvins? Analysis The Kelvin scale is related to Celsius scale by

Problem 1-32E. The temperature of a system drops by 45°F during a cooling process. Express this drop in temperature in K, R, and °C. Analysis This problem deals with temperature changes, which are identical in Rankine and Fahrenheit scales. Thus, The temperature changes in Celsius and Kelvin scales are also identical, and are related to the changes in Fahrenheit and Rankine scales by

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