Question

Second law of thermodynamics states that complete conversion of energy of one kind into another is not possible as some energy is always lost in the form of some other energy. For example, in a car, the heat energy produced by the combustion of a fuel is not completely converted into mechanical work as some energy may be used up in overcoming friction of wheels, etc. Besides other factors, the efficiency depends upon the nature of the working substance (fuel) used in the engine (called heat engines). These studies were made by Sadi Carnot in 1820. 

Now answer the following questions. 

(i) How does second law of thermodynamics explain the working of heat engine (steam, gasoline, diesel) used in all types of vehicles (motorcycles, cars, trucks, ships, aeroplanes, etc)? 

(ii) How does the second law of thermodynamics explain the working of refrigeration (refrigerators and air conditioners)? 

(iii) If we push a car even with a great force, it stops after some time. Do you think it moves the distance we expected? Why or why not? How does the second law of thermodynamics explain it?

Answer 1

(i) All standard heat engines (steam, gasoline, diesel) work by supplying heat to a gas. The gas then expands in a cylinder and pushes a piston to do its work. But the heat and/or the gas must somehow then be dumped out of the cylinder to get ready for the next cycle. As a result, some heat is always lost and is not completely converted into work. That is why efficiency is never 100%. 

(ii) In refrigeration, heat flows from higher temperature to lower temperature in a compressor with mechanical work which is done by the compressor on the system. The system consists of four main components, compressor, condenser, expansion valve and evaporator. The following cycle is repeated over and again :

(iii) No, it does not move the distance we except because the energy spent by us in pushing the car is not completed converted into mechanical work but a part of it is is used up in overcoming friction between the tyres and the pavements .