Which of the following is a TRUE statement about heat transfer:A) If an object is a good absorber of radiation, it is a poor emitter of radiation.B) Humans constantly radiate and absorb thermal radiation but on average, in winter, they radiate more heat than they absorb.C) Heat transfer by thermal radiation propagates slower than thermal conduction.D) Heat transfer by conduction propagates at speed of light.E) Convection takes place in gases and solids.F) Radiation needs medium to propagate.
Question
Which of the following is a TRUE statement about heat transfer:A) If an object is a good absorber of radiation, it is a poor emitter of radiation.B) Humans constantly radiate and absorb thermal radiation but on average, in winter, they radiate more heat than they absorb.C) Heat transfer by thermal radiation propagates slower than thermal conduction.D) Heat transfer by conduction propagates at speed of light.E) Convection takes place in gases and solids.F) Radiation needs medium to propagate.
Solution
The true statement about heat transfer is B) Humans constantly radiate and absorb thermal radiation but on average, in winter, they radiate more heat than they absorb.
Here's why:
A) This is false. An object that is a good absorber of radiation is also a good emitter of radiation. This is known as Kirchhoff's law of thermal radiation.
B) This is true. Humans, like all objects, constantly exchange heat with their environment by radiating and absorbing thermal radiation. In winter, because the environment is colder, humans tend to lose more heat than they absorb.
C) This is false. Heat transfer by thermal radiation can propagate at the speed of light, which is faster than thermal conduction.
D) This is false. Heat transfer by conduction does not propagate at the speed of light. It depends on the thermal conductivity of the material and the temperature difference.
E) This is false. Convection only takes place in fluids, which includes gases and liquids, but not solids.
F) This is false. Radiation does not need a medium to propagate. It can travel through a vacuum.
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