## Calculating work and internal energy

$\Delta U=q+w$

Ashley Davis 1I
Posts: 57
Joined: Fri Sep 29, 2017 7:04 am

### Calculating work and internal energy

I'm having trouble determining when to use which equation. Could someone summarize when to use which equation to calculate work and internal energy?

I know that for work it can be calculated with w = -PΔV or w = $-nRT(V_2/V_1)$ so in what instances would we use these, i.e. how would we know using clues in the question when to use which? I thought w = -PΔV was for irreversible expansion only, and that the other equation is for reversible expansion?

In what instances would we use $U = 3/2 nRT$?
Last edited by Ashley Davis 1I on Mon Feb 12, 2018 6:27 pm, edited 1 time in total.

Jessica Jones 2B
Posts: 80
Joined: Fri Sep 29, 2017 7:04 am

### Re: Calculating work and internal energy

Yes w=-pdeltaV is for irreversible work and w=-nRTln(v2/v1) is used for isothermal, reversible expansion.
You would most likely use deltaU= w + q to calculate internal energy and would therefore need to solve for w and for q.

Ashley Davis 1I
Posts: 57
Joined: Fri Sep 29, 2017 7:04 am

### Re: Calculating work and internal energy

Thank you. Would we have to deduce on our own what kind of work is occurring or would it most likely say in the question what kind of expansion it is?

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