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Old Feb 17th 2018, 03:44 AM   #1
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Question on entropy, please help me. urgent, thanks!!!!

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Last edited by xdyx; Feb 18th 2018 at 07:49 AM.
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Old Feb 17th 2018, 04:34 AM   #2
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For question A & B, it's just S= Q/T right? thank you so much!!
If you correctly identify the system in each case and get your signs right.

I need some guidance on this question. Particularly im unsure on part C and D.
Why are the answers to A and B different?
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Old Feb 17th 2018, 04:47 AM   #3
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Originally Posted by studiot View Post
If you correctly identify the system in each case and get your signs right.



Why are the answers to A and B different?
Hi! Appreciate your reply. The answers will be different because they have different temperature? Is my line if thought correct?

For interior the sign will be negative
For exterior the sivn will be positive.

But as for the entropy generated in the wall, i really need your guidance on this! Please advise. Thanksss
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Old Feb 17th 2018, 10:40 AM   #4
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OK so here are some (hopefully helpful) notes.

1) The question does not mention time or wall area, perhaps there was more to it?

So 300kJ are conducted through the walls from an interior temp of 296K to and exterior temp of 273K.

We can assume there is a heat source within the house satisfying the first law and maintaining the steady state temperature.

The outside is assumed to be an infinitely large reservoir so any heat input will not affect its temperature.

So both the transfers of heat across the boundary are made at constant temperature (isothermal) and are therefore reversible.

The the entropy transferred will be 300000/296 and 300000/273 respectively.

The inside has no net change in entropy because it gains this back from its heat source. However it transfers -300000/296 entropy units to the wall.

This entropy passes through the wall as heat flux and the wall in turn transfers
-300000/273 (negative from the point of view of the wall since it is entropy evolved) entropy units to the outside From the poit of view of the outside this becomes a positive input gain in entropy.

As you point out the wall outputs more entropy than it inputs.

So there must be some entropy generation within the wall to make up this difference, since the wall has no heat source but is in a steady state so does not change (loose) any of its own entropy.

This is an example of irreversible heat transfer, which is where the entropy generation comes in to play.

Finally part (d) should only refer to the entire universe which will only increase in entropy by that generated within the wall. The question refers to "this process" which transfers entropy (as heat) down a thermal gradient, thereby generating additional entropy.
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