P2207_fall10_lecture31 - Example: Refrigerator T H = 21C =...

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Today: Heat and work Heat engines Refrigerators, heat pumps The “drinking bird”: How does it work???
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What is the typical efficiency of a car petrol engine? Efficiency = (work done) / (total energy used) = ? A. ~15 % B. ~25 % C. ~35 % D. ~45 % E. >=55%
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Example: Auto Engine T H = 1350°C = 1623 K T C = 300°C = 573 K Carnot efficiency = 1 – 573/1623 = 0.65 (65 %) But real engine: ~ 25 % only ! Why? To get Carnot efficiency, you need: No friction Add and remove heat at constant gas temperature Must run cycle very slowly low power engine But: car need lots of power! efficiency will be low
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Unformatted text preview: Example: Refrigerator T H = 21C = 294 K T C = 4C = 277 K k max = (useful output) / (energy we pay for) = 277 / (294-277) = 16 >> 1 Example: Heat Pump k max = (useful output) / (energy we pay for) = |Q c | / |W| if operated as AC in summer k max = (useful output) / (energy we pay for) = |Q H | / |W| if operated as heater in winter A heat pump is used to heat a building during the winter . The outside temperature is 0C and the inside temperature 20C . What is the maximum possible coefficient of performance ? K max = ? A. 1/20 B. 1 C. 13.65 D. 14.65 E. 20...
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P2207_fall10_lecture31 - Example: Refrigerator T H = 21C =...

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