The first law of thermodynamics states that energy is conserved. We may express it as follows: Increase in internal energy of a system is equal to heat put into the system with the deduction of work done by the system on its surroundings, or ΔU = ΔQ - ΔW, where U – internal energy Q – heat W - work A system can be anything. It is most convenient if it has well defined boundaries. DQ is positive if it is put into the system, and negative if it is taken out of the system. DW is positive if the system does work on its surroundings and is negative if work is done on the system. The internal energy is the sum of the kinetic and potential energies of atoms and molecules that make up the system. 2. The second law of thermodynamics Sadi Carnot (1796 -1832), Rudolf Clausius (1822-1888), Wiliam Thomson (Lord Kelvin 1824-1907) established the second law of thermodynamics. The second law is a statement that all processes go only in one direction to a state of higher and higher entropy (in other words, in the direction of greater and greater degradation of energy). An isolated system always goes from a less probable to a more probable configuration. We hence have the following statement for the second law. The first statement of the 2nd law: In any physical process, the entropy S for an isolated system never decreases; that is, we always have ΔS ≥ 0 Unlikely as it may sound, the second law is one of the few fundamental laws of physics that historically arose from very practical questions, in particular the need to understand the theory of heat engines. Carnot analyzed how much mechanical work could be extracted from heat, and what, in principle, is the most efficient heat engine that one could construct. His analysis was the beginning of the concept of entropy. It was only much later, in the work of Boltzmann, that there emerged a microscopic and more fundamental understanding of the principle of entropy. Based on the considerations of heat and work, we have a few other formulations of the second law. The second statement of the 2nd law: No mechanical work can be extracted from an isolated system at a single temperature. The third statement of the 2nd law: Heat cannot spontaneously flow from a cold body to a hot body. Although these formulations may seem to be a far cry from Statement I of the second law, it will be shown to be identical to it. Since only changes in entropy are defined, it was thought that there was an additive constant which would always be arbitrary. However, it was realized that this was not so. 3. The Third Law of Thermodynamics. The Third Law of Thermodynamics states that as temperature tends to absolute zero, so does entropy. In other words S(T) → 0 as T → 0