**Boyle’s law**states that: The absolute pressure of a gas varies inversely as the volume, provided the temperature remains constant.

**p V = a constant**

where: p = pressure in bar, V = volume in m

^{3}.

**Charles’ law**states that the volume of a gas under constant pressure, or the pressure of a gas under constant volume, varies as the absolute temperature. Therefore V varies as T, and p varies as T where T is the absolute temperature.

If the two laws are combined, we get:

**p V / T = constant**

The constant is usually denoted by R and therefore:

**p V = R T**

It can be shown that the value of the constant R applicable to air is 287.0 J/(kg K).

The relation between the pressure and volume of air

*may be represented by:*

__during its expansion and compression__**p V**

^{n}= R Twhere ‘n’ has value which

*.*

__depends on the addition or subtraction of heat during the process__When the temperature remains constant during compression or expansion they is said to be

**isothermal**and the value of ‘n’ is one. In order to obtain pure isothermal compression it would be necessary to remove heat from the air at the same rate as heat is produced by the work done on the gas. When a gas expands and when no heat passes during expansion or contraction they is said to be

**adiabatic**.

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