# Homework Help: What is the engine's thermal efficiency?

1. May 13, 2012

### Bigworldjust

1. The problem statement, all variables and given/known data

A heat engine using 110g of helium as the working substance follows the cycle shown in the figure.

http://session.masteringphysics.com/problemAsset/1074114/5/19.P59.jpg

What is the engine's thermal efficiency?

2. Relevant equations

n=W/Q_h

3. The attempt at a solution

Alright so up to this point I have figured out p1,p2,p3, T1,T2,T3, V1,V2, and V3.

I tried doing W = area of triangle = .5(5-1)atm*(5.0*10^-3-1.0*10^-3)m^3 and got .008J. I assumed Q_h was 2200K because that is what I got for the temperature between T2 and T3, and that is where the heat should be going in. So I divided and 36.36% but that is wrong. Any help here? Thanks!

2. May 13, 2012

### Bigworldjust

Bump any help?

3. May 13, 2012

### thepatient

Work is not a triangle in this case. You'll have to integrate dW = P*dV, where P = nRT/V.

Also, why is Q_h a temperature of 2200 K and not a heat transfering into the engine cycle?

Heat units = Joules

Temperature units = Kelvin

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