Help with Calculating Work Done by a Model Stirling Engine

[roam]

Homework Statement

I need some help with this problem:

A model Stirling engine uses n = 7.44 × 10^–3 mol of gas (assumed to be ideal) as a working substance. It operates between a high temperature reservoir at T_H= 95.0°C and a low temperature reservoir at T_c = 24.0°C. The volume of its working substance doubles during each expansion stroke. It runs at a rate of 0.6 cycles per second. Assume the engine is ideal.

How much Work does the engine do per cycle (include the sign)?

Homework Equations

$$W=nRT ln \frac{V_i}{V_f}$$

The Attempt at a Solution

Using the formula above I get

(7.44 × 10^-3)(8.314) T ln (v_i/2v_i)

But I don't know how to continue since I don't know the value for the initial volume or temprature and I don't know how to find them... I mean for the temprature at least, I'm given two different tempratures T_H and T_c and I don't see which one to use!

 

[ideasrule]

The volume doubles, so Vi/Vf = 1/2 for the expansion processes and Vi/Vf=2 for the compression process.

Take a look at this graph: http://upload.wikimedia.org/wikipedia/commons/2/25/Stirling_Cycle.svg

You have to calculate the work done for each leg and add them together to calculate total work. The expansion/compression legs are isothermal, so there's only one T.

 

[roam]

The volume doubles, so Vi/Vf = 1/2 for the expansion processes and Vi/Vf=2 for the compression process.

Take a look at this graph: http://upload.wikimedia.org/wikipedia/commons/2/25/Stirling_Cycle.svg

You have to calculate the work done for each leg and add them together to calculate total work. The expansion/compression legs are isothermal, so there's only one T.

THANK YOU! It makes perfect sense now. But I have another question; what if they ask 'what is the Power of the engine?'

Should we just divide the total work done (that we just calculated) by the number of cycles per second? I tried that but it didn't give me the correct answer...

 

[nonequilibrium]

You should multiply the work with the number of cycles per second, cause P = Q/t and Q = W(one cycle) * (cycles in a second) if t = 1 sec.