1.6 Mechanisms of heat transfer  (Page 11/27)

Summary

• Heat is transferred by three different methods: conduction, convection, and radiation.
• Heat conduction is the transfer of heat between two objects in direct contact with each other.
• The rate of heat transfer P (energy per unit time) is proportional to the temperature difference $T_{\text{h}}-T_{\text{c}}$ and the contact area A and inversely proportional to the distance d between the objects.
• Convection is heat transfer by the macroscopic movement of mass. Convection can be natural or forced, and generally transfers thermal energy faster than conduction. Convection that occurs along with a phase change can transfer energy from cold regions to warm ones.
• Radiation is heat transfer through the emission or absorption of electromagnetic waves.
• The rate of radiative heat transfer is proportional to the emissivity e . For a perfect blackbody, $e=1$ , whereas a perfectly white, clear, or reflective body has $e=0$ , with real objects having values of e between 1 and 0.
• The rate of heat transfer depends on the surface area and the fourth power of the absolute temperature:
  
  $P=\sigma eA{T}^4,$
  
  where $\sigma =5.67\times {10}^{-8}\text{J/s}·{\text{m}}^2·{\text{K}}^4$ is the Stefan-Boltzmann constant and e is the emissivity of the body. The net rate of heat transfer from an object by radiation is
  
  $\frac{Q_{\text{net}}}{t}=\sigma eA\left(T_2{}^4-T_1{}^4\right),$
  
  where $T_1$ is the temperature of the object surrounded by an environment with uniform temperature $T_2$ and e is the emissivity of the object.