Obtain the form of thermodynamics at constant pressure.

Explanation

If a system consist of a gas enclosed in a cylinder with frictionless movable piston, then work is done on the gas for move up piston.

Suppose, a gas enclosed in a cylinder. By obtaining the values of volumes corresponds to different pressure by piston. The graph of P`to` V will be drawn. Now, consider an intermediate state in which pressure and volume of gas are P and V respectively. From this position suppose the piston moves slightly by `Deltax`, the pressure does not change appreciably.
Work done by the gas
`DeltaW=FDeltax`
`=PADeltax [ because F=PA]`
`=PDeltaV [ because ADeltax = DeltaV]`
From first law of thermodynamics ,
`DeltaQ=DeltaU+PDeltaW` be obtained .