## Quiz 1 prep #1 Winter 2014 Question #11

$\Delta U=q+w$

Shannon Han 2B
Posts: 48
Joined: Fri Sep 26, 2014 2:02 pm

### Quiz 1 prep #1 Winter 2014 Question #11

For #11 on the winter 2014 quiz #1 preparation, the question asks to find the final volume for a system that has a two-step process. I get step 1, but on step 2 I'm confused why ∆U goes from -158 J from step 1 to 158 J on step 2. Is it because it says it returns to the original internal energy?

Neil DSilva 1L
Posts: 70
Joined: Fri Sep 26, 2014 2:02 pm

### Re: Quiz 1 prep #1 Winter 2014 Question #11

In the first step, the system is expanding, so energy is lost to the surroundings in the form of work (the energy of the system is decreasing).
In the second step, we can assume the system is being compressed, so energy is being forced into the system (the energy of the system is increasing).

Miriam Ifegwu 1D
Posts: 17
Joined: Fri Sep 20, 2013 3:00 am

### Re: Quiz 1 prep #1 Winter 2014 Question #11

Hi!!
$\Delta U = +$ because the question says " as it returns to the original internal energy". Its internal energy is increasing at it is returning to its original internal energy calculated in part 1. The key word in the question is returns which insinuates that some form of energy, is causing the internal energy to increase (+) or return. Another way to look at it is that returning implies the opposite of expansion (in Part 1): Compression. In compression, heat is typically added in the form of work done ON the system which is causing the overall internal energy to increase (be positive).
Hoped this helped :)