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)
After a vigorous workout, you stand in shorts in a 20 °C room in front of a fan that blows air past you. What are the signs (+ or -) of the convective and radiative heating rates? Will you perspire to keep cool? Explain your answer.
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