## Delta U at Constant V/P

Letty Liu 2C
Posts: 105
Joined: Fri Aug 09, 2019 12:16 am

### Delta U at Constant V/P

What is the internal energy equation at constant volume and pressure? Is it U = q for constant volume and U = q + w at constant pressure?

Sofia Ban
Posts: 80
Joined: Fri Sep 28, 2018 12:25 am

### Re: Delta U at Constant V/P

Yes, at constant volume, delta U=qv and at constant pressure, delta U=qp+w (using qp=delta H, we can rewrite it as delta U=delta H-P*deltaV.

Charisse Vu 1H
Posts: 101
Joined: Thu Jul 25, 2019 12:17 am

### Re: Delta U at Constant V/P

At constant volume, delta u = q because there is no work being done. Work requires that there is a change in volume, as the book defines work as "the process of achieving motion against an opposing force". At constant pressure, delta u = q - P*delta V because you can rearrange the equation delta h = delta u + P*delta V (change in enthalpy at constant pressure) to find delta u.

Altamash Mahsud 1I
Posts: 50
Joined: Fri Aug 09, 2019 12:16 am

### Re: Delta U at Constant V/P

You are correct. For constant volume, since there is no volume change, there is no work being done, so internal energy would just equal heat. (U=q) For constant pressure, there can be both heat transfer and work being done, so then U=q+w. Since in this case there is constant pressure, then heat is just equal to enthalpy (q= delta H). So then U=delta H + work, which is also U= (delta H) - P*(delta V)

Leila_4G
Posts: 114
Joined: Sat Sep 14, 2019 12:17 am

### Re: Delta U at Constant V/P

I'm kind of confused, what does -P delta V equal?
Thank you!