Monday, November 21, 2011

Laws Of Thermodynamics

Thermodynamics is the branch of science which deals with the study of the flow of heat or any other forms of energy into or out of a system as it undergoes a physical or chemical change. The study of thermodynamics is based on three important laws or generalisations which is confirmed by well established experimental results. These laws are known as the Zeorth, First, Second and Third Laws of Thermodynamics.

Zeroth Law of Thermodynamics states that :  ''If two systems are each in thermal equilibrium with a third, they are also in thermal equilibrium with each other.''    

A system is said to be in thermal equilibrium when it experiences no net change in thermal energy. If A, B, and C are distinct thermodynamic systems, the zeroth law of thermodynamics can be expressed as:
  • "If A and C are each in thermal equilibrium with B, A is also in equilibrium with C."
The zeroth law was not initially recognized as a law, as its basis in thermodynamical equilibrium was implied in the other laws.  Once the importance of the zeroth law for the definition of temperature was realized, it was impracticable to renumber the other laws, hence it was numbered the zeroth law
First law of thermodynamics states that : A change in the internal energy of a closed thermodynamic system is equal to the difference between the heat supplied to the system and the amount of work done by the system on its surroundings.  E = q - W

The first law is in fact, an application of the principle known as the Law of Conservation of Energy to thermodynamic systems. Other statements of the first law are:

1. The total energy of an isolated system remains constant though it may change from one form to another.

2. Energy can neither be created nor destroyed, although it can be changed from one form to another.

3. Total energy of a system and surroundings remains constant.

4. It is not possible to construct a perpetual motion machine, i.e., a machine which can produce work without expenditure of of energy.

Second law of thermodynamics states that : ''Heat cannot spontaneously flow from a colder location to a hotter location.''

The need for the second law arise from the fact that for a particular process or change, the first law helps us to balance the internal energy, heat released and work done on the system or by the system. But, it does not sy anything about the thermodynamic possibility of the process to occur.

The second law explains that ''whenever a spontaneous or irreversible process takes place, it is accompanied by an increase in the total entropy of the universe.'' All spontaneous processes take place in the direction of increasing entropy. Entropy is a state quantity that is a measure of the randomness or disorder of the molecules of the system.

Third law of thermodynamics states that : As a system approaches absolute zero, all processes cease and the entropy of the system approaches a minimum value.

The third law does not give any any new concept. It only places a limitation to the value of entropy of a crystalline solid. The entropy of a substance varies directly with temperature. If we increase the temperature of a system, for example water, the molecules attain kinetic energy and starts moving restlessly resulting in an increasing entropy of the system. But, if we cool the system, the vibration of molecules slow down limiting the freedom of movement thereby decreasing the entropy. Finally, at absolute zero all molecular vibrations ceases resulting in nil disorder and the entropy of the system will be zero. i.e., S=0 at T=0K.

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1 comment:

joseph kj said...

good article, Thankyou.