The First Law of Thermodynamics states that work and heat are mutually convertible. The present tendency is to include all forms of energy. The First Law can be stated in many ways:
1. Energy can neither be created nor destroyed; it is always conserved. However, it can change from one form to another.
2. All energy that goes into a system comes out in some form or the other. Energy does not vanish and has the ability to be converted into any other form of energy.
3. If the system is carried through a cycle, the summation of work delivered to the surroundings is equal to summation of heat taken from the surroundings.
4. No machine can produce energy without corresponding expenditure of energy.
5. Total energy of an isolated system in all its form, remain constant
The first law of thermodynamics cannot be proved mathematically. Its validity stems from the fact that neither it nor any of its corollaries have been violated.
