48. A
purpose is to extract heat from a cold reservoir. The same engine
running backward is called a heat pump if its purpose is to ex-
haust warm air into the hot reservoir. Heat pumps are widely used
for home heating. You can think of a heat pump as a refrigera-
tor that is cooling the already cold outdoors and, with its exhaust
heat QH, warming the indoors. Perhaps this seems a little silly, but
consider the following. Electricity can be directly used to heat a
home by passing an
is a direct, 100% conversion of work to heat. That is, 15 kW of
electric power (generated by doing work at the rate of 15 kJ/s at
the power plant) produces heat energy inside the home at a rate of
15 kJ/s. Suppose that the neighbor's home has a heat pump with
a coefficient of performance of 5.0, a realistic value. Note that
"what you get" with a heat pump is heat delivered, QH, so a heat
pump's coefficient of performance is defined as K = QH/ Win.
a. How much electric power (in kW) does the heat pump use to
deliver 15 kJ/s of heat energy to the house?
b. An average price for electricity is about 40 MJ per dollar. A
furnace or heat pump will run typically 250 hours per month
during the winter. What does one month's heating cost in the
home with a 15 kW electric heater and in the home of the
neighbor who uses a heat pump?
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- If the refrigerator door is left what happens to the temperature of the kitchen?arrow_forwardGive an example of a spontaneous process in which a system becomes less ordered and energy becomes less available to do work. What happens to the system's entropy in this process?arrow_forward(a) A woman climbing the Washington Monument metabolizes 6.00102kJ of food energy. If her efficiency is 18.0%, how much heat transfer occurs to the environment to keep her temperature constant? (b) Discuss the amount of heat transfer found in (a). Is it consistent with the fact that you quickly warm up when exercising?arrow_forward
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