## U = q + w assumptions

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

### U = q + w assumptions

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

I don't think pressure is necessarily constant

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

### Re: U = q + w assumptions

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

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

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

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.