< Physics 201 MWF10:20 Fall 1997 (Ford)
Exam IV Chapts. 14--19 in SZY College Physics

Show all your work. Partial credit will be given if earned. Write your answers in the blanks provided.


(16 pts) 1. An aluminum pan has a circular bottom with area 0.030 m² and thickness 0.0020 m. The pan is placed on an electric stove and filled with 0.50 kg of water that has a temperature of 100 °C. The thermal conductivity of aluminum is 205 J/s·m·C°. If the surface of the pan in contact with the water is maintained at 100 °C and the bottom surface in contact with the stove is at a uniform 106 °C,

a) what is the heat current in the bottom of the pan?

Ans. ___1.84 × 10⁴ J/s___

b) how much time does it take for all the water to boil away?

Ans. ___61.1 s___


(14 pts) 2. A glass volumetric flask used in a chemistry laboratory is filled to a reference mark on the long stem of the flask with exactly 300 cm³ of benzene. Initially both the flask and the benzene are at a temperature of 20 °C. The cross-sectional area of the column of liquid in the stem is 0.080 cm². The coefficient of volume expansion for benzene is 1.2 × 10^-4 (C°)^-1 and for the glass it is 2.5 × 10^-5 (C°)^-1. If the flask and benzene are both heated to 35 °C,

a) what is the change in volume of the benzene?

Ans. ___0.54 cm³___

b) will the liquid rise in the stem or fall in the stem?

Ans. ___rises___

c) what distance will the liquid rise or fall?

Ans. ___5.35 cm___


(18 pts) 3. What mass m of ice at an initial temperature of -15 °C must be placed in 0.50 kg of water that is initially at 40 °C to make the final temperature of the system 10 °C? Assume that the water is in a container of negligible mass and that no heat is lost to the surroundings.

Ans. ___0.155 kg___


(12 pts) 4. Two moles of an ideal gas undergo a reversible isothermal expansion at a temperature of 300 K. During this expansion the gas does 2100 J of work. What is the change of entropy of the gas?

Ans. ___7 J/K___


(16 pts) 5. During each cycle a Carnot refrigerator removes 30 J of heat from a cold reservoir that is at a temperature of 240 K. During each cycle the magnitude of the mechanical work required to operate the refrigerator is 70 J.

a) In each cycle how much heat energy is discharged to the high temperature reservoir?

Ans. ___100 J___

b) What is the temperature of the high temperature reservoir?

Ans. ___800 K___


(24 pts) 6. A heat engine takes 0.30 mol of an ideal gas around the cycle shown in the sketch. For this gas C_p = 29.1 J/mol·K. Process 1---->2 is at constant volume, process 2---->3 is adiabatic, and process 3---->1 is at constant pressure. The temperature of the gas in states 1, 2, and 3 is T₁ = 300 K, T₂ = 600 K, and T₃ = 455 K.

a) Calculate the work done by the gas in each process.

Ans. 1---->2 ___0___

2---->3 ___+905 J___

3---->1 ___-387 J___

b) In one complete cycle, what is the net heat flow Q?

Ans. ___+518 J___

c) In one complete cycle does a net amount of heat flow into or out of the gas?

Ans. ___into___