# Work out entropy change of the universe

## 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.

## 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 dont know how to begin..

entropy delta S = integral dQ/T... what do i do :S ?

Mapes
Homework Helper
Gold Member
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..

Mapes
Homework Helper
Gold Member
What if you just calculate the entropy change when the kinetic energy is added to the lake?

then i get the right answer..

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?

Mapes
Homework Helper
Gold Member
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?

Mapes