The science of heat transfer has evolved from three fundamental concepts or “laws” of thermodynamics, the first and second law being the most important.
The Zeroth Law of Thermodynamics
If objects A and B are separately in thermal equilibrium with a third object, C, then A and B are in thermal equilibrium with each other.
The First Law of Thermodynamics
It is known that energy must be “conserved” or exactly accounted for. Heat that flows to or from an object must go somewhere. Energy cannot be created or destroyed, only changed from one form to another.
The Second Law of Thermodynamics
The entropy of an isolated system which is not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium.
What that means in relation to our considerations is that heat always flows from a point of higher temperature (higher molecular activity) to one of lower temperature (lower molecular activity) until thermal equilibrium is achieved.
While there are only two real modes of heat transfer, conduction and radiation, we will look at the principles of convection and change of state in our considerations as they also have a profound impact on Infrared Thermography and infrared inspections.
The Third Law of Thermodynamics
This law can be written in a number of forms. The simplest law states that if one could reach absolute zero, all bodies would have the same entropy. In other words, a body at absolute zero could exist in only one possible state, which would possess a definite energy, called the zero-point energy. This state is defined as having zero entropy.
Basic Infrared Thermography Principles by Robert Wayne Ruddock