How Are Compression and Expansion Used in Thermodynamics?

[EngNoob]

Could anyone tell me what the compression and expansion is used for in a Thermodynamics situation?

What is the notation for Compression and Expansion in a formula?

I am trying to calculate work done by a heat engine.

Thanks

 

[rohanprabhu]

Compression & Expansion basically relate to a change in Volume i.e. \Delta V \ne 0

Expansion is negative of Compression. Depending on your sign convention, either of them will be positive. Generally, \Delta V is taken as \Delta V = V_f - V_i and hence compression corresponds to a negative \Delta V i.e. -\Delta V_c > 0

Also, if a conservative force is applied for either of the process, the work done for the same |\Delta V| will have the same magnitude.

Under constant pressure, the work done in doing either of the process is given as:

<br /> W = P\Delta V<br />

and for non-constant pressure, we express it in a differential equation. Note that here, Pressure will be some function of Volume and may also depend on Temperature which in turn maybe another function of Volume.

<br /> \int^{W}_0 dW = \int^{V_f}_{V_i}PdV<br />

This holds only when there is no energy exchange from the system to the surrounding or vice versa in any form other than the force applied on the piston.