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Chapter 4: Engine and Refrigerators

Expert-verified
An Introduction to Thermal Physics
Pages: 122 - 148
An Introduction to Thermal Physics

An Introduction to Thermal Physics

Book edition 1st
Author(s) Daniel V. Schroeder
Pages 356 pages
ISBN 9780201380279

44 Questions for Chapter 4: Engine and Refrigerators

  1. Suppose that heat leaks into your kitchen refrigerator at an average rate of 300 watts. Assuming ideal operation, how much power must it draw from the wall?

    Found on Page 129
  2. What is the maximum possible COP for a cyclic refrigerator operating between a high-temperature reservoir at 1K and a low-temperature reservoir at 0.01 K ?

    Found on Page 129
  3. Explain why a rectangular P V cycle, as considered in Problems 1.34 and 4.1, cannot be used (in reverse) for refrigeration.

    Found on Page 129
  4. A heat pump is an electrical device that heats a building by pumping heat in from the cold outside. In other words, it's the same as a refrigerator, but its purpose is to warm the hot reservoir rather than to cool the cold reservoir (even though it does both). Let us define the following standard symbols, all taken to be positive by convention:(a) Explain why the "coefficient of performance" (COP) for a heat pump should be defined as Qh / W.(b) What relation among Qh , Qc, and W is implied by energy conservation alone? Will energy conservation permit the COP to be greater than 1 ?(c) Use the second law of thermodynamics to derive an upper limit on the COP, in terms of the temperatures Th and Tc alone.(d) Explain why a heat pump is better than an electric furnace, which simply converts electrical work directly into heat. (Include some numer

    Found on Page 130
  5. In an absorption refrigerator, the energy driving the process is supplied not as work, but as heat from a gas flame. (Such refrigerators commonly use propane as fuel, and are used in locations where electricity is unavailable.* ) Let us define the following symbols, all taken to be positive by definition:Qf= heat input from flameQc= heat extracted from inside refrigeratorQr= waste heat expelled to roomTf= temperature of flameTc= temperature inside refrigeratorTr= room temperature(a) Explain why the "coefficient of performance" (COP) for an absorption refrigerator should be defined as Qc / Qf.(b) What relation among Qf, Qc, and Qr is implied by energy conservation alone? Will energy conservation permit the COP to be greater than 1 ?(c) Use the second law of thermodynamics to derive an upper limit on the COP, in terms of the temperatures Tf

    Found on Page 130
  6. At a power plant that produces 1 GW109 watts) of electricity, the steam turbines take in steam at a temperature of 500o, and the waste heat is expelled into the environment at 20o(a) What is the maximum possible efficiency of this plant?(b) Suppose you develop a new material for making pipes and turbines, which allows the maximum steam temperature to be raised to 600o. Roughly how much money can you make in a year by installing your improved hardware, if you sell the additional electricity for 5 cents per kilowatt-hour? (Assume that the amount of fuel consumed at the plant is unchanged.

    Found on Page 134
  7. Derive a formula for the efficiency of the Diesel cycle, in terms of the compression ratio V1/ V2and the cutoff ratio V3/ V2. Show that for a given compression ratio, the Diesel cycle is less efficient than the Otto cycle. Evaluate the theoretical efficiency of a Diesel engine with a compression ratio of 18 and a cutoff ratio of 2.

    Found on Page 133
  8. The ingenious Stirling engine is a true heat engine that absorbs heat from an external source. The working substance can be air or any other gas. The engine consists of two cylinders with pistons, one in thermal contact with each reservoir (see Figure 4.7). The pistons are connected to a crankshaft in a complicated way that we'll ignore and let the engineers worry about. Between the two cylinders is a passageway where the gas flows past a regenerator: a temporary heat reservoir, typically made of wire mesh, whose temperatur

    Found on Page 133
  9. Calculate the efficiency of a Rankine cycle that is modified from the parameters used in the text in each of the following three ways (one at a time), and comment briefly on the results:

    Found on Page 137
  10. Imagine that your dog has eaten the portion of Table 4.1 that gives entropy data; only the enthalpy data remains. Explain how you could reconstruct the missing portion of the table. Use your method to explicitly check a few of the entries for consistency. How much of Table 4.2 could you reconstruct if it were missing? Explain.

    Found on Page 137

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