Compression and Expansion Index

  • Thread starter EngNoob
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Main Question or Discussion Point

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
 

Answers and Replies

  • #2
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Compression & Expansion basically relate to a change in Volume i.e. [itex]\Delta V \ne 0[/itex]

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

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

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

[tex]
W = P\Delta V
[/tex]

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.

[tex]
\int^{W}_0 dW = \int^{V_f}_{V_i}PdV
[/tex]

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.
 

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