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Homework Help: Thermodynamics - entropy

  1. Dec 10, 2006 #1
    1. The problem statement, all variables and given/known data
    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)

    2. Relevant 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.

    3. 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?
  2. jcsd
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