1) During the winter, when the outside temperature is -12.5 o C, inside of a roo
ID: 1515715 • Letter: 1
Question
1) During the winter, when the outside temperature is -12.5oC, inside of a room is maintained at 21.0oC. What is the rate of heat loss, in W, through a glass window of dimensions 60.0cm x 45.0cm and how much energy, in kWh, is lost in 30.0min? The thermal conductivity of glass is 0.80W/mK and the thinkness of the glass window is 5.00mm.
2)A Carnot heat engine absorbs QH amount of heat from a hot reservoir at 277oC and rejects heat in the cold reservoir that is maintained at 27.0oC.
i) What is the thermal efficiency of the engine?
ii) What is the energy QH absorbed from the hot reservoir?
iii) What is the work done, in J?
Explanation / Answer
1) heat transfer rate, Q/t =k A dT/dx
k = 0.80 W/mK for glass
A = 0.60 x 0.45 = 0.27 m^2
dx = 0.005 m
dT = 21 +12.5 = 33.5 deg C
t = 30 / 60 = 0.5 hr
Q / (0.5 h ) = 0.80 x 0.27 x 33.5 / 0.005 W
Q = 723.6 Wh = 0.72 KWh
2) efficiency, n = 1 - (Tc / Th)
n = 1 - ( (273 + 27) / (273 + 277)) =0.46
ii) no energy given (some data missing )
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