First law of thermodynamics

It is considered as the general law & nature.  It states the conservation of energy.  That is Heat and Work are mutually convertible.

                              Q = ∆E + W

This equation shows that when we apply a certain amount of heat into a system or taken out a corresponding work is takes place and the rest of the heatis converted as the internal energy of the system.

Internal Energy

Internal Energy is of 2 types.  Macro and Micro.

Macro

The macro comprises of the kinetic energy and potential energy of all the atoms in a system.

                  E_Macro = E_KE+E_PE

Micro

It is composed of all the small forms of energy like rotational, translational, vibrational etc.

Consider an IC engine.  When combustion in IC engine takes place heat is forme and with the help of this heat expansion stroke takes place ie. the work takes place.  Here a lot of energy is lost that is we cannot convert the complete heat into work, because heat is a low grade energy and work is a high grade energy.

We cannot convert low grade energy to high grade energy completely.  But we can convert the high grade energy to low grade energy completely with a small amount of loss.