Work out entropy change of the universe

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Homework Help Overview

The problem involves calculating the entropy change of the universe when a block of mass 400g and thermal capacity 150J/K is dropped from a height of 100m into a lake, both initially at 10 degrees C. The discussion centers around the concepts of energy transfer, specifically potential and kinetic energy, and their effects on entropy.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants explore the relationship between potential energy loss and temperature changes, questioning how to calculate the entropy changes for both the block and the lake. There are discussions about whether the processes are reversible and the implications of energy transfer on entropy.

Discussion Status

Participants are actively engaging with the problem, raising questions about the assumptions made regarding temperature changes and the nature of energy transfer. Some guidance has been offered regarding the calculation of entropy changes, but there is still confusion about specific details and interpretations.

Contextual Notes

There is uncertainty regarding the treatment of the block's temperature change and the assumption of the lake's temperature remaining constant. The distinction between reversible and irreversible processes in the context of entropy calculations is also under discussion.

bon
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Homework Statement



A block mass 400g thermal capacity 150J/K at 10 degrees C is dropped from a height of 100m into a lake at 10 degrees C. Work out entropy change of the universe.


Homework Equations





The Attempt at a Solution



Not sure how to do this one..I guess we work out entropy change of block and lake separately, but i don't know how to begin..

entropy delta S = integral dQ/T... what do i do :S ?
 
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Rather than going straight for the equations, consider what happens in the problem. Does heating or cooling occur? Are there other types of energy transfer? Are the processes reversible? Can you make any simplifying assumptions?
 
Well...isn't it the case that as the block falls it loses PE therefore at point of contact with the water, we can model its temp as being 2.616K higher..

So is the entropy change of the block 1/283 C (285.616 - 283) ?

It's just that i keep getting the wrong answer..
 
What if you just calculate the entropy change when the kinetic energy is added to the lake?
 
then i get the right answer..

Please explain. I am really confused.

my attempt: as it falls, block loses PE, gains KE. As it strikes water, block loses KE gains thermal energy. Not block is at temp higher than 10 degrees C. Therefore block cools and lake heats up a tiny bit.. Therefore we need to calculate entropy change for both block and lake and add them...

what's the correct explanation?
 
The block hits the lake, and both are at 10°C. The kinetic energy is transformed entirely and irreversibly into thermal energy. But the lake is relatively large, so the temperature change is negligible. Thus, the entropy change is just mgh/T.
 
why is the entropy change of the block = 0? Doesn't it lose thermal energy?
 
bon said:
why is the entropy change of the block = 0? Doesn't it lose thermal energy?

The block starts and ends at 10°C.
 
ok thanks i see now
 
  • #10
PS if you are hot on your thermal phys please look here also: https://www.physicsforums.com/showthread.php?t=447675
 
  • #11
Also - what difference does it make in calculations of entropy if heat is transferred reversibly or irreversibly?
 

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