Chapter 19: Problem 1
Could you cool the kitchen by leaving the refrigerator open? Explain.
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Chapter 19: Problem 1
Could you cool the kitchen by leaving the refrigerator open? Explain.
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Refrigerators remain among the greatest consumers of electrical energy in most homes, although mandated efficiency standards have decreased their energy consumption by some \(80 \%\) in the past four decades. In the course of a day, one kitchen refrigerator removes \(30 \mathrm{MJ}\) of energy from its contents, in the process consuming \(10 \mathrm{MJ}\) of electrical energy. The electricity comes from a \(40 \%\) efficient coal-fired power plant. The electrical energy a. is used to run the light bulb inside the refrigerator. b. wouldn't be necessary if the refrigerator had enough insulation. c. retains its high-quality status after the refrigerator has used it. d. ends up as waste heat rejected to the kitchen environment.
A Carnot engine extracts heat from a block of mass \(m\) and specific heat \(c\) initially at temperature \(T_{\mathrm{b} 0}\) but without a heat source to maintain that temperature. The engine rejects heat to a reservoir at constant temperature \(T_{c} .\) The engine is operated so its mechanical power output is proportional to the temperature difference \(T_{\mathrm{h}}-T_{\mathrm{c}}\) $$P=P_{0} \frac{T_{\mathrm{h}}-T_{\mathrm{c}}}{T_{\mathrm{h} 0}-T_{\mathrm{c}}}$$ where \(T_{\mathrm{h}}\) is the instantancous temperature of the hot block and \(P_{0}\) is the initial power. (a) Find an expression for \(T_{\mathrm{h}}\) as a function of time, and (b) determine how long it takes for the engine's power output to reach zero.
A cosmic heat engine might operate between the Sun's \(5800 \mathrm{K}\) surface and the \(2.7 \mathrm{K}\) temperature of intergalactic space. What would be its maximum efficiency?
In an alternative universe, you've got the impossible: an infinite heat reservoir, containing infinite energy at temperature \(T_{\mathrm{b}} .\) But you've only got a finite cool reservoir, with initial temperature \(T_{c 0}\) and heat capacity \(C .\) Find an expression for the maximum work you can extract if you operate an engine between these two reservoirs.
How much energy becomes unavailable for work in an isothermal process at \(440 \mathrm{K},\) if the entropy increase is \(25 \mathrm{J} / \mathrm{K} ?\)
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