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Chapter 15: 15-4Q (page 412)
Explain why the temperature of a gas increases when it is compressed adiabatically.
During compression, the work on the gas system and the change in internal energy are positive. As internal energy is a function of temperature, the temperature of the gas increases during adiabatic compression.
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Which of the following possibilities could increase the efficiency of a heat engine or an internal combustion engine?
(a)Increase the temperature of the hot part of the system and reduce the temperature of the exhaust.
(b) Increase the temperatures of both the hot part and the exhaust part of the system by the same amount.
(c) Decrease the temperatures of both the hot part and the exhaust part of the system by the same amount.
(d) Decrease the temperature of the hot part and increase the temperature of the exhaust part by the same amount.
(e) None of the above; only redesigning the engine or using better gas could improve the engine's efficiency.
(II) Sketch a PV diagram of the following process: 2.5 L of ideal gas at atmospheric pressure is cooled at constant pressure to a volume of 1.0 L, and then expanded isothermally back to 2.5 L, whereupon the pressure is increased at constant volume until the original pressure is reached.
Question: Entropy is often called the‘time’s arrow’ because it tells us the direction in which natural processes occur. If a movie is run backward, name some processes that you might see that would tell you that time is ‘running backward’.
A cooling unit for a new freezer has an inner surface area of\({\bf{8}}{\bf{.0}}\;{{\bf{m}}{\bf{2}}}\), and is bounded by walls 12 cm thick with a thermal conductivity of\({\bf{0}}{\bf{.050}}\;{\bf{W/m}} \cdot {\bf{K}}\). The inside must be kept at -15°C in a room that is at 22°C. The motor for the cooling unit must run no more than 15% of the time. What is the minimum power requirement of the cooling motor?
(II) A gas is enclosed in a cylinder fitted with a light frictionless piston and maintained at atmospheric pressure. When 254 kcal of heat is added to the gas, the volume is observed to increase slowly from to \({\bf{16}}{\bf{.2}}\;{{\bf{m}}^{\bf{3}}}\).Calculate (a) the work done by the gas and (b) the change in internal energy of the gas.
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