Winter 2013 Midterm: Question #4

[megan_thi_2j]

For part A of this question, it asks to calculate the work for each step:
A to B
B to C
C to A
Can someone clear up why for B to C, we use the reversible work formula, while for C to A, we use the irreversible work formula?
I can't seem to picture visually from the graph why one is reversible and one is not reversible

Thanks!

[Rika Jitosho 2E]

For B to C, by looking at the y-axis of the graph, the pressure decreases, so in other words, pressure is not constant. We know from class that under irreversible conditions, pressure is constant, but since that is not the case, we have to use the work equation for variable pressure (reversible work formula). By this logic, we can see from C to A that pressure remains the same, so in this case we would use the work equation for constant pressure (irreversible work formula).

[204751840 2E]

Just to clarify, if the pressure changes, but the volume does not, no work is done, right?

[Emily Wang 1H]

Yes, if there is no change in volume, then there is no work done by expansion because delta V will be 0.