Heat transfer processes

 
  Fig. 1-1 We can feel the heat of the sun even the Earth is separated from it by a great distance in vacuum.

We have learned in the last chapter that energy is transferred from a hot object to a cold object when they are in physical contact. We refer to the energy transfer due to temperature difference as heat. But do you notice that sometimes heat transfer can take place even when the hot and cold objects are not in contact? For example, we can feel the heat of the sun even the Earth is separated from it by a great distance in vacuum. The energy from the sun is obviously transferred to us by a different way.

In fact, there are mainly three ways of heat transfer, namely, conduction, convection and radiation. When a hot object and a cold object are in contact, heat is transferred by conduction. On the other hand, the energy of the sun reaches us by means of radiation. We will study these ways of heat transfer one by one in the following sections.

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