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U = q + w assumptions

Posted: Mon Jan 15, 2018 5:01 pm
by Lily Guo 1D
Does the equation (change in)U = q + w assume that there is constant pressure? Or does the pressure not have to be constant?

Re: U = q + w assumptions

Posted: Mon Jan 15, 2018 6:29 pm
by Nishma Chakraborty 1J
I don't think pressure is necessarily constant

Re: U = q + w assumptions

Posted: Mon Jan 15, 2018 7:43 pm
by 404995677
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.

Re: U = q + w assumptions

Posted: Mon Jan 15, 2018 7:45 pm
by Jeremiah Samaniego 2C
And if external pressure is constant, you would be able to substitute q with ∆H in the equation and w with P*∆V!

Re: U = q + w assumptions

Posted: Mon Jan 15, 2018 8:45 pm
by Jessica Jones 2B
Do we use different equations for constant pressure and constant volume?

Re: U = q + w assumptions

Posted: Tue Jan 16, 2018 5:16 pm
by Jessica Nunez 1I
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.