## using the first law

$\Delta U=q+w$

Rita Chen 1B
Posts: 112
Joined: Sat Jul 20, 2019 12:15 am

### using the first law

When will I need to factor in both q and w? I’m a little confused.

haileyramsey-1c
Posts: 105
Joined: Thu Jul 25, 2019 12:18 am

### Re: using the first law

Typically the problem will indicate it. For example if the problem gives you delta H at constant pressure then delta H = q or if the problem says explicitly that 50J heat transferred. Or, the problem may have two steps and in one you find w and in the order you use w to find q.

Hailey Kim 4G
Posts: 110
Joined: Sat Jul 20, 2019 12:16 am

### Re: using the first law

When the volume is constant, w=0. If the problem gives you the amount of energy that increased/decreased (q) after doing a certain amount of work (w) is when you can incorporate both q and w for the change in internal energy.

Dina Marchenko 2J
Posts: 54
Joined: Thu Jul 25, 2019 12:16 am

### Re: using the first law

With constant pressure or volume, there is no expansion or compression work being done so w can be ignored. You don't really ignore q unless it's specifically stated that there is no heat transfer.