What is the Change in Internal Energy for a Cyclic Process on a PV Diagram?

[uchicago2012]

Homework Statement

For point A in the figure (See Figure 1), the pressure is 3 atm, the volume is .08 m3 and the internal energy is 750 J. At point C the values are 1.2 atm, .03 m3, and 420 Joules.
(a) For the cyclic process abcda, how much heat is exchanged with the environment?
(b) For the process abc, what is the change in internal energy?

Homework Equations

delta U = Q - W
where U = internal energy, Q = heat, and W = work

The Attempt at a Solution

for (a), I found Q = 9119.25 J because it's a cyclic process, so delta U = 0 and W_net = the area of the square. But for (b), it's not exactly clear how I'm supposed to find the change in internal energy. I can find W by finding the area under the curve, which equals 15198.75 J, but I don't know U now and I don't know Q either. All I can get is Q_da > 0, which isn't particularly helpful.

 

[p21bass]

Do you see what type of process A-B is? And B-C? Realizing what processes those are should help.

 

[uchicago2012]

I know A-B is constant pressure and B-C is constant volume. Thus, B-C does no work. But I know what the net work is. I need to know the net heat so I can figure out the change in internal energy and I don't know of any pertinent handy facts about constant volume or constant pressure processes. The only useful things about constant pressure processes in my book are for ideal gases, and this isn't an ideal gas. So I don't see what you're saying.