Thermodynamic equations

Rachel Yoo 1F
Posts: 65
Joined: Fri Sep 28, 2018 12:24 am

Thermodynamic equations

I'm struggling with trying to understand which equation to use when I'm reading the homework problems so does anyone have any tips or advice on how to memorize which work,q, or delta u equation to use for the different types of different processes?

Kathryn Wilhem 1I
Posts: 60
Joined: Fri Sep 28, 2018 12:27 am

Re: Thermodynamic equations

The heat capacity equation varies depending on whether there is constant pressure or constant volume. You use q=nCT, but for constant pressure, C=(5/2)R, and for constant volume, C=(3/2)R.

W = - P x ∆V is used to find work, which is then used in the equation ∆U = q + w.
For ∆U = q + w, if volume is constant, the equation becomes ∆U = q. If pressure is constant it is ∆U = q + w which can also be written as ∆U = ∆H - P∆V.

w= -nRTln(V2/V1) is used in isothermal reversible expansion.

PV=nRT can be used to find n, or P∆V.

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