U = q + w assumptions


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Lily Guo 1D
Posts: 64
Joined: Fri Sep 29, 2017 7:03 am

U = q + w assumptions

Postby Lily Guo 1D » Mon Jan 15, 2018 5:01 pm

Does the equation (change in)U = q + w assume that there is constant pressure? Or does the pressure not have to be constant?

Nishma Chakraborty 1J
Posts: 51
Joined: Fri Sep 29, 2017 7:04 am

Re: U = q + w assumptions

Postby Nishma Chakraborty 1J » Mon Jan 15, 2018 6:29 pm

I don't think pressure is necessarily constant

404995677
Posts: 82
Joined: Fri Sep 29, 2017 7:07 am

Re: U = q + w assumptions

Postby 404995677 » Mon Jan 15, 2018 7:43 pm

On page 262 and 263, it says that work is calculated under the assumption that the external pressure is constant, so I think we assume constant pressure for (change)U= q+w.

Jeremiah Samaniego 2C
Posts: 33
Joined: Fri Sep 29, 2017 7:05 am

Re: U = q + w assumptions

Postby Jeremiah Samaniego 2C » Mon Jan 15, 2018 7:45 pm

And if external pressure is constant, you would be able to substitute q with ∆H in the equation and w with P*∆V!

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

Re: U = q + w assumptions

Postby Jessica Jones 2B » Mon Jan 15, 2018 8:45 pm

Do we use different equations for constant pressure and constant volume?

Jessica Nunez 1I
Posts: 58
Joined: Thu Jul 13, 2017 3:00 am

Re: U = q + w assumptions

Postby Jessica Nunez 1I » Tue Jan 16, 2018 5:16 pm

According to page 273, the first law of thermodynamics (delta)U = q + w mentions that the internal energy of an isolated system is constant; however, I am unsure if we can assume pressure is constant.


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