The first law of thermodynamics is the application of law of conservation of energy, adapted for thermodynamic systems. The law of conservation of energy states that the total energy of an isolated system is constant; energy can be transformed from one form to another, but cannot be created or destroyed.
The First Law says that the internal energy of a system has to be equal to the work that is being done on the system, plus or minus the heat that flows in or out of the system and any other work that is done on the system. For example
1- In an Automobile Engine chemical energy converted to kinetic energy
2- In an Heater or Furnace chemical energy converted to heat energy
3- In an Hydroelectric process gravitational energy converted to electrical energy
4- In an Battery chemical energy converted to electrical energy
5- In an Foods chemical energy converted to heat and kinetic energy
So, this is daily life example of according to first law of thermodynamics statement that energy can be converted from one form to another, but neither be created nor destroyed.
First law states that the internal energy of a closed system is equal to the amount of heat supplied to the system, minus the amount of work done by the system on its surroundings. Equivalently, perpetual motion machines of the first kind are impossible.
Thermodynamics system is an equilibrium state possess by state variable called the internal energy (E). Between two equilibrium states, the change in internal energy is equal to the difference of the heat transfer into the system and work is done by the system.