Heat engine is a device which converts heat into work continuously through a cyelic process.
The essential parts of a heat engine are
(1) Source: It is a reservoir of heat at high temperature and infinite thermal capacity. Any amountof heat can be extracted from it without changing its temperatre
(2) Working substance: Steam, petrol etc.
(3) Sink: It is a reservoir of heat at low temperature and infinite thermal capacity. Any amount of heat can be given to the sink.
The working substance absorbs heat Q1 from the source, does an amount of work W, returns the remaining amount of heat to the sink and comes back to its original state and there occurs no change is its internal energy.
By repeating the same cyele over and over again, work is continuously obtained.
The performance of heat engine is expressed by means of “efficiency” ŋ which is defined as the ratio of useful work obtained from the engine to the heat supplied to it.
Refrigeratior or Heat Pump
A refrigeratior or heat pump is basically a heat engine run in reverse direction.
It essentially consists of three parts
(1) Source: At higher temperature T1.
(2) Working substance: It is called refrigerant liquid ammonia and Freon works as a working substance.
(3) Sink: At lower temperature T2.
The performance of a refrigerator is expressed by means of “coefficient of performance” which is defined as the ratio of the heat extracted from the cold body to the work needed to transfer it to the hot body.
Second Law of Thermodynamics
First law of thermodynamics merely explains the equivalence of work and heat. It does not explain why heat flows from bodies at higher temperatures to those at lower temperatures. It cannot tell us why the converse is not possible. It cannot explain why the efficiency of a heat engine is always less than unity. It is also unable to explain why cool water on stirring gets hotter whereas there is no such effect on stirring warm water in a beaker. Second law of thermodynamics provides answers to therse questions. Statement of this law is as follows
(1) Clausius statement: It is impossible for a self acting machine to transfer heat from a colder body to a hotter one without the aid of an external agency.
(2) Kelvin’s statement: It is impossible for a body or system to perform continuous work by cooling it to a temperature lower than the temperature of the coldest one of its surroundings. A Carnot engine cannot work it the source and sink are at the same temperature because work done by the engine will result into cooling the source and heating the surroundings more and more.
(3) Kelvin-Planck’s statement : It is impossible to design an engine that extracts heat and fully utilize into work without producing any other effect.