What are the laws of thermodynamics?Explain in brief.

Thermodynamics: Chapter of the physics where phenomena which are ranging in temperature, have the primary role of the study. These phenomena involve the thermal motion of matter and energy transformation from one form to another.

Thermodynamics is based on experiences from where came out laws or principles of law, known as zero law, the first (first principle), the second law (second principle) and the third  law(third principle).

Zero law of thermodynamics
  Two thermodynamic systems put in contact and isolated from environment, reach themselves (eventually) the state of thermal equilibrium.
  Zero law is the basis for methods of measurement of body temperature.
First principle of thermodynamics
This principle is an extension of the law of energy conservation in the processes occurring thermal motion of matter.
The existence of heat as a form of energy transfer has been a particular problem in the history of thermodynamics. Only Joule's experiments  proved and confirmed the similar nature of heat and mechanical work. Subsequent experiments showed that mechanical work and heat are the only forms of energy transfer between a thermodynamic system and environment.
The first principle of thermodynamics introduces a new size of a state called internal energy, which is equal to the sum of all the kinetic energy of molecules of a body.
Thus, the first principle (Mayer formulation ) says that a system adiabatically isolated from the environment can change his internal energy  due to external mechanical work  performed (ΔL = ΔQ = U2-U1). If the system is adiabatically isolated and there is heat exchange between system and environment,then  ΔU = ΔQ-ΔL.

To conclude, the first law of thermodynamics is the law of conservation of matter and existing energy.

The first law of thermodynamics has a great importance in the study of thermal machines.  Thermal machine is called that device which takes heat from the outside making it into mechanical work.
 

The second principle of thermodynamics

Conservation laws are expressed mathematically in terms of the equalities. So, the first principle allows the transformation of energy from one form to another, as long as the total energy are preserved, therefore there is no restriction on converting heat into mechanical energy and vice versa.
From mathematical point of view, principle II is represented by an inequality and is not a conservation law.

Formulation of the second principle, that is most important in our study, is the formulation that includes a new size of state characterizing the organization of a system, called entropy.

Increasing entropy principle says:
In any adiabatic process, the entropy of any system  increases or remain constant, ΔS ≥ 0, where "=" is for reversible processes  and ">" for irreversible processes.

This principle is important precisely because it says that in nature there is a law of change whose directions are downward and not upward as evolution implies.
Classical thermodynamic says entropy increases for any physical process and the energy tends to lower levels of use.
Statistical thermodynamics says that every amount of energy has an associated size of the state called entropy which measures the degree of disorder of the system.