Laws of thermodynamics
The laws of thermodynamics form a basis for the study of thermodynamics. The first and second laws summarize our experience with processes that occur in the world. The zeroth and third law provide definitions.
The first law of thermodynamics states that the energy of a closed system is constant. Considering the universe to be a large closed system leads to another version of the first law, that energy can neither be created nor destroyed.
The second law of thermodynamics states that it is impossible for a system to undergo a cyclic process whose sole effects are the flow of heat into the system from a heat reservoir and the performance of an equivalent amount of work by the system on the surroundings. An example of such an impossible system would be an engine which took in air, extracted some of the thermal energy from the air and used it to propel a vehicle, and returned cooler air to the atmosphere. Using the concept of entropy the second law can be stated as the entropy of the universe always increases.
The third law of thermodynamics states that the entropy of a perfect crystal at zero Kelvin (absolute zero) is zero.
A fourth law is often included to define the concept of teperature. It is called the zeroth law and states that two systems each in thermal equilibrium with a third system are in thermal equilibrium with one another. In essence this law justifies the use of a thermometer to measure the temperature of a system.
Humorous statements of the three laws of thermodynamics abound: 1. You can never win. You can only break even. 2. You can only break even at absolute zero. 3. You can never reach absolute zero.