Physics for Scientists and Engineers
6th Edition
ISBN: 9781429281843
Author: Tipler
Publisher: MAC HIGHER
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Chapter 20, Problem 38P
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To Show:The cost of air conditioning isproportional to
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A typical electric refrigerator has a power ratingof 400 W, which is the rate (in J/s) at whichelectrical energy is supplied to do the work neededto remove heat from the refrigerator. If therefrigerator releases heat to the room at a rate of900 W, at what rate (in watts) does it remove heatfrom the inside of the refrigerator?
A 50-m-long section of a steam pipe whose outer diameteris 10 cm passes through an open space at 15°C. The averagetemperature of the outer surface of the pipe is measured to be150°C. If the combined heat transfer coefficient on the outer surfaceof the pipe is 20 W/m2·K, determine (a) the rate of heat lossfrom the steam pipe; (b) the annual cost of this energy lost if steamis generated in a natural gas furnace that has an efficiency of 75percent and the price of natural gas is $0.52/therm (1 therm =105,500 kJ); and (c) the thickness of fiberglass insulation(k = 0.035 W/m·K) needed in order to save 90 percent of the heatlost. Assume the pipe temperature to remain constant at 150°C.
A residential heat pump has a coefficient of performance of 2.4. How much heating effect, in Btu/h, will result when 5 hp is supplied to this heat pump?
Chapter 20 Solutions
Physics for Scientists and Engineers
Ch. 20 - Prob. 1PCh. 20 - Prob. 2PCh. 20 - Prob. 3PCh. 20 - Prob. 4PCh. 20 - Prob. 5PCh. 20 - Prob. 6PCh. 20 - Prob. 7PCh. 20 - Prob. 8PCh. 20 - Prob. 9PCh. 20 - Prob. 10P
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- Check your Understanding Show that QhQh=QcQc for the hypothetical engine of Figure 4.10 The second property to be demonstrated is that all reversible engines operating between the same two reservoirs have the same efficiency. To this, stat with the two engines D and E of Figure 4.10 (a), which are operating between two common heat reservoirs at temperatures Th and Tc . First, assume that D is a reversible engine and that E is a hypothetical irreversible engine that has a higher efficiency than D. If both engines perform the same amount of work W per cycle, it follows from Equation 4.2 that QhQh . It then follows from the first law that QcQc . Figure 4.10 (a) Two uncoupled engines D and E working between the same reservoirs. (b) The engines, With D working reverse. Suppose the cycle of D is so that it operates as a refrigerator, and the two engines are coupled such that the work output of E is used to drive D, as shown in Figure 4.10(b). Since QhQh and QcQc , the net result of each cycle is equivalent to a spontaneous transfer of heat from the cold reservoir to the hot reservoir, a process second law does not allow. The original assumption must therefore be wrong, and it is impossible to construct an irreversible engine such that E is more efficient than the reversible engine D. Now it is quite easy to demonstrate that the efficiencies of all reversible engines operating between the same reservoirs are equal. Suppose that D and E are reversible engines. If they are as shown in Figure 4.10(b), the efficiency of E cannot be greater than the efficiency of D, or second law would violated. If both engines are then reversed, the same reasoning implies that the efficiency of D cannot be greater than the efficiency of E. Combining these results leads to the conclusion that all reversible engines working between same two reservoirs have the same efficiency.arrow_forwardThe energy output of a heat pump is greater than the energy used to operate the pump. Why doesn't this statement violate the first law of thermodynamics?arrow_forwardIn a quasi-static isobaric expansion, 500 J of work are done by the gas. The gas pressure is 0.80 atm and it was originally at 20.0 L. If the internal energy of the gas increased by 80 J in the expansion, how much heat does the gas absorb?arrow_forward
- Many decisions are made on the basis of the payback period: the time it will take through savings to equal the capital cost of an investment. Acceptable payback times depend upon the business or philosophy one has. For some industries, a payback period is as small as 2 yeas) Suppose you wish to install the extra insulation in the preceding problem. If energy cost $1.00 per million joules and the insulation was $4.00 per square meter, then calculate the simple payback time. Take the average T for the 120-day heating season to be 15.0 .arrow_forwardWhy don't we operate ocean liners by extracting heat from the ocean or operate airplanes by extracting heat from the atmosphere?arrow_forwardHow much thermal energy must be added to 3.00 moles of monatomic ideal gas tat 300 K kept at constant volume of 1.5 L in order to raise the temperature to 400 K ? How would the result change if the ideal gas in the example is diatomicarrow_forward
- A test is conducted to determine the overall heat transfer coefficient in anautomotive radiator that is a compact cross-flow water-to-air heat exchangerwith both fluids (air and water) unmixed. The radiator has 40 tubesof internal diameter 0.5 cm and length 65 cm in a closely spaced plate-finnedmatrix. Hot water enters the tubes at 90°C at a rate of 0.6 kg/s and leaves at65°C. Air flows across the radiator through the interfin spaces and is heatedfrom 20°C to 40°C. Determine the overall heat transfer coefficient Ui of thisradiator based on the inner surface area of the tubes.arrow_forwardCan improved engineering and materials be employed in heat engines to reduce heat transfer into the environment? Can they eliminate heat transfer into the environment entirely?arrow_forwardA thermodynamic system undergoes a process in which its internal energy decreses by 500 J at the same time ,220J of work is done on the system find the energy transferred by the heatarrow_forward
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