Entropy and such

  • Thread starter Jayhawk1
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  • #1
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Here is the question:

8) [1.0/3.0] 1.6 kg of water at 34oC is mixed with 2.0 kg of water at 58o. in a well insulated container. a) What is the final temperature of the water (in degrees Celsius)? b) What is the approximate change of entropy? (Use the average temperature for the entropy calculation.)

Part a was easy enough... I got 47.3 oC

I've tried calculating part b in multiple ways, and I am getting a very small number- which is what my answer is actually suppose to be- BUT - it is not right. Any help?? Thanks.
 

Answers and Replies

  • #2
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hmm...well you know that deltaH = qt and qt/t = delta S...give that a try?
 
  • #3
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Nothing I've trired has worked... I keep coming up with a negative number... I need some definitive direction.
 
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  • #4
OlderDan
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Jayhawk1 said:
Nothing I've trired has worked... I keep coming up with a negative number... I need some definitive direction.
[tex] \Delta S = \frac{\Delta Q}{T}[/tex]

S is entropy, Q is heat, T is temperature. You are asked for an approximate value for the change in entropy using an average termperature. I interpret that to mean you should use the average temperature of the warm water to calculate its entropy loss and the average temperature of the cool water to calculate its entropy gain. There will be a net gain.
 
  • #5
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Still not working... what else should I try??
 
  • #6
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Can someone be a little more specific.... I am getting really frustrated. This is the last question on the last homework assignment for the year and I keep getting it wrong. Not to mention I don't have that many tries on it left. Please help me!!!!!!!!!
 
  • #7
OlderDan
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Jayhawk1 said:
Can someone be a little more specific.... I am getting really frustrated. This is the last question on the last homework assignment for the year and I keep getting it wrong. Not to mention I don't have that many tries on it left. Please help me!!!!!!!!!
How about showing us exactly what you did. I thought the last thing I told you was quite specific. What did you get for the amount of heat transferred from the warm water to the cold water, the entropy lost by the warm water, and the entropy gained by the cold water. If you are still getting negative answers for the net entropy change, you are doing something wrong in your calculations.
 
  • #8
ZapperZ
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Jayhawk1 said:
Can someone be a little more specific.... I am getting really frustrated. This is the last question on the last homework assignment for the year and I keep getting it wrong. Not to mention I don't have that many tries on it left. Please help me!!!!!!!!!
You cannot continue complaining that you are not getting the right answer. The way you have gone about with this appears as if you want someone to do the work for you.

Show EXACTLY what you did to get the wrong answer. Only then can someone point out what you did wrong. You learn MORE from mistakes like this than from the ones you did correctly.

Zz.
 
  • #9
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Same problem for me. I've tried getting Q by computing specific heat * g * delta T, but no luck with that.
 
  • #10
OlderDan
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squib said:
Same problem for me. I've tried getting Q by computing specific heat * g * delta T, but no luck with that.
What answer are you getting, how are you getting it, and what (if you know) are you supposed to get?
 
  • #11
Andrew Mason
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Jayhawk1 said:
Here is the question:

8) [1.0/3.0] 1.6 kg of water at 34oC is mixed with 2.0 kg of water at 58o. in a well insulated container. a) What is the final temperature of the water (in degrees Celsius)? b) What is the approximate change of entropy? (Use the average temperature for the entropy calculation.)

Part a was easy enough... I got 47.3 oC

I've tried calculating part b in multiple ways, and I am getting a very small number- which is what my answer is actually suppose to be- BUT - it is not right. Any help?? Thanks.
Since the temperature changes as the heat is absorbed/lost, you have to integrate.

see: https://www.physicsforums.com/showthread.php?t=77857

AM
 
  • #12
OlderDan
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Andrew Mason said:
Since the temperature changes as the heat is absorbed/lost, you have to integrate.

see: https://www.physicsforums.com/showthread.php?t=77857

AM
Except that the problem says
(Use the average temperature for the entropy calculation.)
These vague statements like "I've tried everything and can't get the answer" are pretty useless for figuring out where the difficulty lies.
 
  • #13
Andrew Mason
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OlderDan said:
Except that the problem says "average temperature"


These vague statements like "I've tried everything and can't get the answer" are pretty useless for figuring out where the difficulty lies.
Ok. Then the problem Jayhawk must be having is in the determination of average temperatures for the cool and warm water. Jayhawk, what are you using for average temperatures?

AM
 

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