This simple but effective solar cooker uses the greenhouse effect and reflective material to trap and retain solar energy. Made of inexpensive, durable materials, it saves money and labor, and is of particular economic value in energy-poor developing countries. (credit: E.B. Kauai)
Conversely, dark space is very cold, about
, so that the Earth radiates energy into the dark sky. Owing to the fact that clouds have lower emissivity than either oceans or land masses, they reflect some of the radiation back to the surface, greatly reducing heat transfer into dark space, just as they greatly reduce heat transfer into the atmosphere during the day. The rate of heat transfer from soil and grasses can be so rapid that frost may occur on clear summer evenings, even in warm latitudes.
What is the change in the rate of the radiated heat by a body at the temperature
compared to when the body is at the temperature
?
The radiated heat is proportional to the fourth power of the
absolute temperature . Because
and
, the rate of heat transfer increases by about 30 percent of the original rate.
Career connection: energy conservation consultation
The cost of energy is generally believed to remain very high for the foreseeable future. Thus, passive control of heat loss in both commercial and domestic housing will become increasingly important. Energy consultants measure and analyze the flow of energy into and out of houses and ensure that a healthy exchange of air is maintained inside the house. The job prospects for an energy consultant are strong.
Problem-solving strategies for the methods of heat transfer
Examine the situation to determine what type of heat transfer is involved.
Identify the type(s) of heat transfer—conduction, convection, or radiation.
Identify exactly what needs to be determined in the problem (identify the unknowns). A written list is very useful.
Make a list of what is given or can be inferred from the problem as stated (identify the knowns).
Solve the appropriate equation for the quantity to be determined (the unknown).
For conduction, equation
is appropriate.
[link] lists thermal conductivities. For convection, determine the amount of matter moved and use equation
, to calculate the heat transfer involved in the temperature change of the fluid. If a phase change accompanies convection, equation
or
is appropriate to find the heat transfer involved in the phase change.
[link] lists information relevant to phase change. For radiation, equation
gives the net heat transfer rate.
Insert the knowns along with their units into the appropriate equation and obtain numerical solutions complete with units.
Check the answer to see if it is reasonable. Does it make sense?
Test prep for ap courses
Two ideal, black-body radiators have temperatures of 275 K and 1100 K. The rate of heat transfer from the latter radiator is how many times greater than the rate of the former radiator?
On a warm, sunny day, a car is parked along a street where there is no shade. The car’s windows are closed. The air inside the car becomes noticeably warmer than the air outside. What factors contribute to the higher temperature?
