# Homework Help: Carnot engine

1. Apr 6, 2014

### Maylis

Hello,

I have the problem statement attached in the thumbnail. I am confused about this question. I looked at the solution manual, and they are saying that the pressure at states 3 and 4 are the same, as well as at states 1 and 2. I don't see how this is. Looking at the carnot cycle PV diagram, clearly all 4 states are at different pressures.

Anyways, I did the calculation with the same pressures. Now I am working on part C, and I thought since step 1-->2 is an isotherm, I could use the equation Q=RT ln(P2/P1). However, if P2 and P1 are the same, then Q is zero. What is going on here?

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Last edited: Apr 6, 2014
2. Apr 6, 2014

### Simon Bridge

Do we not need figure 8.2 as well?
I can see how a carnot cycle can operate between three pressures but not just two.
Are you sure you are reading the correct key - what you describe sounds like a simple steam engine cycle.

3. Apr 6, 2014

### Maylis

My apologies but figure 8.2 is a TS diagram of the Carnot cycle which I drew in my attempt, I am certain I am reading the correct key

Last edited: Apr 6, 2014
4. Apr 6, 2014

### Maylis

Here is a photo of the part of the solution regarding the pressure. Table F.1 is a saturated steam table

5. Apr 6, 2014

### Simon Bridge

Oh the states include a liquid state and a bunch of saturated vapor states.
Looks like changes of phase are happening during the cycle.
You would be expected to account for this in your analysis.

Of course it may just be that the pressures are the same to 3dp (i.e. the isotherms are almost flat).
Or that the answer key is just wrong.