## different ways to calculate w

$w=-P\Delta V$
and
$w=-\int_{V_{1}}^{V_{2}}PdV=-nRTln\frac{V_{2}}{V_{1}}$

Chew 2H
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### different ways to calculate w

when do we use -PdeltaV and when do we use -nRTln(V2/V1) in calculating w? What's the difference between the two?

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### Re: different ways to calculate w

Generally you would use w=-nrtln(v2/v1) when it is an isothermal, reversible reaction.

Abigail Yap 2K
Posts: 51
Joined: Fri Sep 29, 2017 7:07 am

### Re: different ways to calculate w

Hi,

I believe the first equation is used when the system is at equilibrium (constant pressure) and the second is used when the pressure is changing.

Luis De La Cruz 1H
Posts: 37
Joined: Fri Sep 25, 2015 3:00 am

### Re: different ways to calculate w

Indeed, you would use the first equation for instances in which pressure is constant, since the equation itself seems to tell you there is no change in pressure, only a change in volume, hence the missing delta before pressure. While the second equation is a derived equation (which you don't need to know the derivation of) that is used for instances in which pressure is not maintained at a constant.

AtreyiMitra2L
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### Re: different ways to calculate w

We use the former with constant pressure and the latter when its not constant pressure. The first is irreversible expansion and the latter is irreversible, isothermal expansion.

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