Carnot Engine Problem: Answers to Heat, Entropy & Thermal Efficiency

[tresehulyo]

Homework Statement

A Carnot Engine operating between 900 Fahrenheit and 90 Fahrenheit produces 40,000 ft-lb of work. Determine (a) the heat supplied, (b) the change in entropy during heat rejection and (c) the thermal efficiency of engine. (answers should be in BTU)

Homework Equations

What formula for Change in S Entropy do i need to use in here
Am i leading to correct answers below?

The Attempt at a Solution

(a) 22.74 BTU
W=NQ_H
=.60(40000 ft-lb * (9.478* 10^-4 BTU / 1 ft-lb)
= 22.74 BTU

(c) 60%
N_th = 1 - (T_L/T_H)
= 1 - ((90+460)/(900+460))
=.59 = 60%

 

[gabbagabbahey]

What formula for Change in S Entropy do i need to use in here

Look up "Entropy in the Carnot Cycle".

(a) 22.74 BTU
W=NQ_H
=.60(40000 ft-lb * (9.478* 10^-4 BTU / 1 ft-lb)
= 22.74 BTU

You probably shouldn't use the rounded value of 0.6 for N, since this can lead to inaccuracy in your answer...Instead just use W=(1 - (T_L/T_H))Q, plug everything into your calculator, and only round your final answer.

(c) 60%
N_th = 1 - (T_L/T_H)
= 1 - ((90+460)/(900+460))
=.59 = 60%

Again, you can get a more accurate answer if you use $T_{\text{abs}}=T_{\text{Ferenheit}}+459.67$ instead.

Other than that, you answers to (a) and (c) look good to me!