The branch of physics that deals with the relationships between heat and other forms of energy. Four basic laws have been established. &diamf3; The first law states that the amount of energy added to a system is equal to the sum of its increase in heat energy and the work done on the system. The first law is an example of the principle of conservation of energy. &diamf3; The second law states that heat energy cannot be transferred from a body at a lower temperature to a body with a higher one without the addition of energy. Thus, warm air outside can transfer its energy to a cold room, but transferring energy out of a cold room to the air outside requires extra energy (as with an air conditioner). &diamf3; The third law states that the entropy of a pure crystal at absolute zero is zero. Since there can be no physical system with lower entropy, all entropy is thus defined to have a positive value. &diamf3; The zeroth law states that if two bodies are in thermal equilibrium with some third body, then they are also in equilibrium with each other. This law has its name because it was implicitly assumed in the development of the other laws, and is in fact more fundamental than the others, but was only later established as a law itself.