During the expansion of a gas in piston-cylinder the relation between the pressure and the volume is given as

The relationship between the pressure and the volume is given as P=K/V or PV= K. (Pressure increases as Volume decreases).

Explanation:

When gas is trapped in an enclosed medium for example a cylinder by a piston. When the piston is pushed in, the gas particles will have less room to move as the VOLUME the gas occupies has been DECREASED. This will automatically lead to frequent collision of the has particles with the wall of the container. The collision exerts forces which leads to INCREASED PRESSURE.

This obeys the Boyle's law which states that the volume occupied by a fixed mass of gas is inversely proportional to the pressure, provided temperature is kept constant. This can be mathematically expressed below:

PV=K, where K is constant

P1 V1=P2 V2

Where,

P1= initial pressure

V1= initial Volume

P2= final pressure

V2= final Volume.

Therefore, increasing pressure from initial pressure to final pressure means that initial volume will change to final volume, providing the temperature remains constant.