## Isothermal expansion against vacuum (winter 2010 midterm Q3)

$\Delta U=q+w$

Vinh Nguyen 3O
Posts: 3
Joined: Thu Dec 11, 2014 3:00 am

### Isothermal expansion against vacuum (winter 2010 midterm Q3)

On the Winter 2010 Midterm, Question 3, Part B, it asks to calculate q, w, $\Delta U$, and $\Delta S$ for an isothermal expansion against a vacuum. The explanation says," $\Delta S$ and $\Delta U$ depend on the initial and final state of the system, therefore the deltaS and deltaU of process a and b are the same." Can someone explain this in the context of the information given in the problem? What is the initial state and what is the final state?

Niharika Reddy 1D
Posts: 127
Joined: Fri Sep 26, 2014 2:02 pm

### Re: Isothermal expansion against vacuum (winter 2010 midterm

The initial and final pressures are the initial and final states. Since the gas is expanding under 2 different conditions but from the same initial pressure (1000. kPa) to the same final pressure (100. kPa), we can use the fact that entropy and internal energy are state functions to support the statement that the change in entropy and change in internal energy are the same for both processes since their initial and final states are the same.