Work out entropy change of the universe

AI Thread Summary
To calculate the entropy change of the universe when a block is dropped into a lake, consider the block's potential energy transforming into kinetic energy upon impact, which then converts to thermal energy. The block and lake start at the same temperature of 10°C, meaning the block's entropy change is zero since it does not experience a temperature change. The lake, being much larger, experiences a negligible temperature increase, allowing the entropy change to be calculated primarily from the potential energy lost by the block using the formula mgh/T. The discussion highlights the importance of understanding energy transfer processes and the distinction between reversible and irreversible heat transfer in entropy calculations. Overall, the key takeaway is that the block's temperature remains constant, leading to no entropy change for it, while the lake's change is minimal.
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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