General Introduction to Heat Transfer
When two objects at different temperatures are brought in thermal contact (not necessarily physical contact) with each other, energy flows from one object to the other. This energy, which flows by virtue of temperature difference between the two objects or due to temperature gradient within the same object, is called “heat.” Thus, heat is a form of energy that comes into play only in the presence of temperature gradient. Heat has meaning only during the transfer process. Once it has flowed into a body, it manifests as internal energy of the body. Because heat is relevant only during the transfer process, it is often referred to as “energy in transit” (energy in the process of being transferred). In accordance with the second law of thermodynamics, heat flows in the direction of decreasing temperature (from high temperature to low temperature). A good understanding of heat transfer is important in a number of applications involving composite materials.