# Homework Help: PV diagram

1. Jul 26, 2008

### ally1h

1. The problem statement, all variables and given/known data
A heat engine follows the cycle shown in the figure. a) How much net work is done by the engine in one cycle? b) what is the heat flow into the engine per cycle?

Diagram:
http://farm4.static.flickr.com/3050/2703330973_26acb35c27.jpg?v=0

2. Relevant equations
W = -PΔV
ΔU = Q+W

3. The attempt at a solution
a)
W = -PΔV
W = (-3.00 atm)(0.600 m^3)
W = -1.8 J

b)
ΔU = Q+W
ΔU = 0, due to energy conservation
so...
0 = Q+W
Q = W
Q = 1.8

Everything look okay? I wasn't so sure about part b....

2. Jul 27, 2008

### Irid

Probably in part (b) the question asks for total heat added to the engine per one cycle, not the net heat gained (i.e. not [added]-[removed]). So, you should decide which part of the cycle corresponds to heating and then use

$$dQ = p\, dV + V\, dp$$

to calculate the heat.

3. Jul 27, 2008

### yellowgators

Your pressure needs to be converted into Pa to get the right units.