Thermodynamics - entropy

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Homework Statement


A shipyard worker drops a hot steel rivet (mass 125g, temperature 350°C) into a river at temperature 5°C, a distance of 30m below. Stating any assumptions you make, calculate the entropy change of the universe as a result of this event. (Specific heat capacity of steel ~ 0.4 J/(gK)



Homework Equations


I'm not entirely sure. Think I need Q = mcΔT. I also reckon I need an entropy equation like dS=dQ/T, but I don't think thats the correct form.

I think PE = mgh could also be used as well.



The Attempt at a Solution


Assumptions: no air resistance

So far I've used Q = mcΔT to work out the change in heat when the rivet is dropped in the water (-17.25 J) and PE = mgh to work out the change in energy as the rivet is dropped. Would the best bet from there be to say that dS = dU/T where U = Q + W, with W being the PE?

But as I don't have a constant temperature I must not be able to use dS=dU/T can I?
 

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