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## Homework Statement

The figure shows the cycle for a heat engine that uses a gas having gamma =1.25. The initial temperature is T

_{1}=300K, and this engine operates at 20 cycles per second. What is the engine's thermal efficiency?

[URL]http://session.masteringphysics.com/problemAsset/1074111/3/knight_Figure_19_54.jpg[/URL]

## Homework Equations

[PLAIN]http://upload.wikimedia.org/math/7/a/1/7a19ba3fdf776ed7bf92a998cb72996c.png=W[SUB]out[/SUB]/W[SUB]in[/SUB] [Broken]

[PLAIN]http://upload.wikimedia.org/math/7/a/1/7a19ba3fdf776ed7bf92a998cb72996c.png=1-Q[SUB]c[/SUB]/Q[SUB]H[/SUB] [Broken]

## The Attempt at a Solution

The first question had asked what was the power output of the engine and I managed to get the correct answer for that, so I'm very sure that my values for Q12, Q23, and Q31 are correct. Since Q23 was a positive value, I figured it was the work put in. I then used the power output that I had calculated and divided it by Q23, multiply by a hundred, which gave me 18% which turned out to be wrong.

I then tried the second equation and using the same value (Q23) for Q

_{H}and adding the absolute values of Q12 and Q31 together to get Q

_{c}, divide Q

_{c}by Q

_{H}, and subtracted the result from 1. It still turned out to be 18%, which I already knew was wrong.

So I'm not sure what I'm doing wrong here. Any feedback would be greatly appreciated.

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