Thermodynamics

First law of thermodynamics: Heat energy given to a system is used in the following two ways:

(i)                  In increasing the temperature and hence internal energy of the system.

(ii)                In doing work by the system.

If    ΔQ = heat energy given to the system

ΔU = increase in the internal energy of the system.

ΔW – work done by the system

Then, ΔQ= ΔU + ΔW is the mathematical statement of first law of thermodynamics.

First law of thermodynamics is equivalent to principle of conservation of energy.

Isothermal Process: If the changes are taking place in a system in such a way that temperature of the system remains constant throughout the change, then the process is said to be an isothermal.

Adiabatic process: If the changes are taking place in a system in such a way that there is no exchange of heat energy between the system and the surrounding then the process is said to be adiabatic process.

If carbon dioxide is suddenly expanded, it is changed into dry ice. This is an example of adiabatic process.

Second Law of Thermodynamics: The first law of thermodynamics guarantees that in a thermodynamic 

process, energy will be conserved. But this law doe’s not tell whether a given process in which energy is conserved will take place or not. The second law of thermodynamics gives the answer.

Through this law can be stated in many forms, the following two forms are worth mentioning:

Kelvin’s statement: whole of the heat can never be converted into work.

Clausious statement: Heat by itself cannot flow from a colder body to a hotter body.

Heat Engine: Heat energy is a device which converts heat energy into mechanical work continuously through a cyclic process. Every heat engine basically consists of the three parts: (i) source (a hot body) (ii) sink (a cold body) and (iii) a working substance.

Heat engine may be devided into two types:

(i)                  Internal combustion engine: in this engine, heat is produced in the engine itself. Example: Otto engine or petrol engine (efficiency = 52%), Diesel engine (efficiency = 64%)

(ii)                External Combustion Engine: in this engine heat is produced outside the engine. Steam engine is an example of external combustion engine. (Efficiency = 20 %)

Refrigerator or Heat Pump: A refrigerator is an apparatus which transfers heat energy from cold to a hot body at the expense of energy supplied by an external agent. The working substance here is called refrigerant.

In actual refrigerator, vapours of Freon (CCI₂ F₂) acts as refrigerant.