THE SECOND LAW OF THERMODYNAMICS

THE SECOND LAW OF THERMODYNAMICS

When a body at 100*c is kept in contact with a similar body at 0*c, heat flows from the hotter body to the colder body and both come to 50*c. Is the reverse process possible ? That is, if we put two similar bodies both at 50*c in contact, can heat flow from one body to the other so that one body reaches 0*c and the other 100*c ? A block moving at a speed v0 on a rough table eventually stops and the table and the block warm up. The kinetic energy of block appears as the internal energy of the table and the block. Can the reverse process be possible ? That is, we heat the block and the table and put the block on the table. Can the bodies cool down and the block start sliding with speed on the table converting the internal energy into kinetic energy ? Consider a container with rigid walls divided in two parts by a partition having a valve. A gas is put in one part and vacuum is created in the other part. The valve is now opened. The gas eventually occupies both the part of the container. Is the reverse process possible ? We put the gas distributed in both the parts with the valve open. Can the gas go into one part evacuating the other part all by itself ?

The answer to all these questions is NO. of course, the first law of thermodynamics is not violated in any of these proposed reverse processes. The energy is conserved in the direct process as well as in the reverse process. still the reverse process is not possible. There must be a law nature other than the first law which decides, whether a given process, allowed by the first law, will actually take place or not. This law is the second law of the thermodynamics. This law may be stated in various ways. We give here one statement in terms of working of heat engines. We know that a heat engine taken Q1 amount of heat energy from a hot body, converts a part of it into mechanical work and rejects the rest amount Q2 to a cold body. The efficiency of the engine is 1 - Q2 / Q1. The efficiency would be 1, that is, 100% if Q2 = 0./ Such an engine would not need any "low- temperature body" to which it needs to reject heat. Hence, it needs only one body at a single temperature, from which it will take heat and covert it completely into mechanical work. This temperature can even be temperature of the surrounding and hence we will not have to burn any fuel to prepare steam or gases at high temperature to run the engine. A scooter could be run by an engine taking heat from the body of the scooter without needing any petrol. A ship could be run by an engine taking heat from the ocean. However, all attempts to construct such a 100% efficient engine failed. In fact, it is not possible to have such an engine and this is one from of the second law of thermodynamics stated more precisely as follows:

It is not possible to design a heat engine which works in cyclic process and whose only result is to take heat from a body at a single temperature and convert it completely into mechanical work.

This statement of the second law is called the Kevin-planck statement.

One can convert mechanical work completely into heat but one cannot convert heat completely into mechanical work. In this respect, heat and work are not equivalent. We shall now study some other aspects of the second law of thermodynamics.