Heating mechanisms
In the problems in this section, clearly identify the system and objects in the environment that interact with the system. Label the work as WEnvironment on System, heating as QEnvironment to System, and the change in internal energy as ΔUint.
Insulating a house You insulate your house using insulation rated as R-12, which will conduct 1/12 Btu/h of thermal energy through each square foot of surface if there is a 1 °F temperature difference across the material (R-12 insulation is said to have a thermal resistance R of 12 h . ft2 . °F/Btu). The conduction rate through a material of area A, across which there is a temperature difference T2 - T1, is
H Conduction = (1/R2) (T2 - T1)
While blowing across the bowl of soup in the previous problem, you wonder how efficiently the soup can cool by itself through evaporation. You notice that the bowl of hot soup loses 0.40 g of water by evaporation in 1 min. What is the average evaporative heating rate of the soup during that minute? Assume that soup is primarily water.
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