# Homework Help: Heat Q flows spontaneously from a reservoir at 383 K into a reservoir

1. Sep 19, 2010

### copitlory8

Heat Q flows spontaneously from a reservoir at 383 K into a reservoir that has a lower temperature T. Because of the spontaneous flow, thirty percent of Q is rendered unavailable for work when a Carnot engine operates between the reservoir at temperature T and a reservoir at 234 K. Find the temperature T.

I have absolutely no idea. Please help me. I am getting desperate. I have to pull up my failing physics grade or else i will pretty much be disowned by my family.

2. Sep 19, 2010

### rock.freak667

Given this, what would the efficiency of the Carnot Engine?

When you get that, what is the expression for efficiency of a Carnot Engine?

3. Sep 19, 2010

### copitlory8

the efficiency i guess is 2/3.
e=1-(Tc/Th)

4. Sep 19, 2010

### copitlory8

so what next?

5. Sep 19, 2010

### rock.freak667

How did you get 2/3 ?

So when you get 'e' Th=T, and you were given Tc, can you get T?

6. Sep 19, 2010

### copitlory8

i got 2/3 because it says that 1/3 is unavailable therefore 2/3 is available

7. Sep 19, 2010

### rock.freak667

That thinking is correct but thirty percent is not the same as 1/3.

8. Sep 19, 2010

### copitlory8

oh no! im lost again what is the th?

9. Sep 19, 2010

### copitlory8

so the efficiency is actually .7

10. Sep 19, 2010

### rock.freak667

Right so, what is T?

11. Sep 19, 2010

### copitlory8

there are two Ts in the problem. do i use the higher one for Th?

12. Sep 19, 2010

### rock.freak667

You are only considering the two temperatures associated with the Carnot engine, the high temperature T and the lower temperature 234 K.

13. Sep 19, 2010

### copitlory8

so i use 234K?

14. Sep 19, 2010

### rock.freak667

Yes.

15. Sep 19, 2010

### copitlory8

so let me get this straight .7(234)=T where T is the answer im looking for? this seems too simple

16. Sep 19, 2010

### rock.freak667

e=1- (Tc/T), so you '0.7' would be '0.3'.

Nothing is being done to the heat when it is moving from 384K to T, so I don't think you'd need to worry about that for now.

17. Sep 19, 2010

### copitlory8

so .3=1-(234/t)

18. Sep 19, 2010

### rock.freak667

No, 0.7 = 1 -(Tc/T)

when you rearrange you'd get the '0.3'.

19. Sep 19, 2010

### copitlory8

alright i think i almost got this. so the T i am trying to solve for is equal to Tc/.3

20. Sep 19, 2010

### rock.freak667

Yes and you know th vaue of Tc.