Help with Heat Engines


by Ravey
Tags: engines, heat
Ravey
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#1
Sep16-09, 05:42 PM
P: 5
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)
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Mapes
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#2
Sep16-09, 06:09 PM
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Hi Ravey, welcome to PF. What's the significance of the heat flows marked on the image?
Ravey
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#3
Sep16-09, 06:12 PM
P: 5
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?

Ravey
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#4
Sep16-09, 07:17 PM
P: 5

Help with Heat Engines


Stumped too I see ;)
Mapes
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#5
Sep16-09, 07:41 PM
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Hint: No energy is stored in the engine from cycle to cycle.
Ravey
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#6
Sep16-09, 09:52 PM
P: 5
Got it!


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

40/320 = efficiency.
Mapes
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#7
Sep17-09, 06:28 AM
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Sounds good!


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