When we want to study behaviour of gases and vapours under variations of temperature and pressure. Thermodynamics helps us to study heat and pressure energy.

Themodynamics is explained with two important Law's

1) First law of thermodynamics
2) Second law of thermodynamics

First law of thermodynamics

The law states that heat energy can be converted into mechanical energy and vice- versa. The rate of exchange of energy is constant and is measurable. This can be illustratued with example given below.

Heat energy to Mechanical energy

When fuel is burned in the piston, large amount of heat is generated. This heat energy is converted into mechanical energy by the help of piston and cranckshaft. This change of energy is measurable.

Mechanical energy to Heat energy

In the absence of lubrication, when two moving parts come in contact, large amount of heat is generated. This heat energy can be measured easily with temperature gauge.

Second law of thermodynamics

The law states that transfer of heat cannot take place from low temperature region to high temperature region by itself. It is only possible when an external energy source is applied. This can be explained with example given below.

Heat can easily flow from a radiator to open atmosphere as radiator is at higher temperaure than atmosphere. But if we want to lower the temperature of a region below the surrounding atmosphere we will have to add an external device called refrigerator. The refrigerator will keep the space cooler than surrounding atmosphere.

The Gas Laws

1) Boyle's Law

Under constant temperature condition, the volume of gas is inversely proportional to pressure.

                               P X V = K
P is absolute pressure
V is the volume occupied at pressure P.

2) Charles's Law

It is also known as Gay-Lussac's Law. The law states that under constant pressure, absolute temperature is directly proportional to the volume.

                                   V/T = K

The combined gas law expalines the relationship between volume, pressure and temperature. 

                          P1V1           P2V2
                          --------  =    --------
                            T1              T2

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