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)
Igloo thermal energy conduction A typical snow igloo (thermal conductivity about 1/10 of ice) is shaped like a hemisphere of radius 1.5 m with 0.36-m thick walls. What is the conductive heating rate through the walls if the inside temperature is 10 °C and the outside temperature is -10 °C? [Hint: Use the conductive heating equation from the previous problem, but replace 1/R by the thermal conductivity K.]
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