# Homework Help: Help with Heat Engines

1. Sep 16, 2009

### Ravey

1. What is the thermal efficiency for the heat engine shown in the figure?
What is the heat extracted from the hot reservoir for the heat engine shown in the figure?t

Figure shown here:

2.Wout = 1/2 B*H Also, Qc=Qh-Wout, efficiency: Wout/Qh

3. Wout = .5 * 200KPA* (400x10^-6m^3). I have no idea how to obtain Qc (cold reservoir) or Qh (hot)

2. Sep 16, 2009

### Mapes

Hi Ravey, welcome to PF. What's the significance of the heat flows marked on the image?

3. Sep 16, 2009

### Ravey

Thanks!

I believe that is heat leaving the engine*, but what do I do? Add them? According to some note Qc is the sum of the heat leaving the engine. So I assume so. However, qh is not 280-40 = 240. What's going wrong?

Last edited: Sep 16, 2009
4. Sep 16, 2009

### Ravey

Stumped too I see ;)

5. Sep 16, 2009

### Mapes

Hint: No energy is stored in the engine from cycle to cycle.

6. Sep 16, 2009

### Ravey

Got it!

Qh=Q_c+W_out = 280J + 40J = 320J.

40/320 = efficiency.

7. Sep 17, 2009

### Mapes

Sounds good!

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