Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Thermodynamics Question - Entropy etc

  1. Jan 13, 2007 #1
    1. The problem statement, all variables and given/known data
    I've attached a J-peg with the question in, laid out much more clearly than I could expect to type it here. I hope the attachment is ok.


    2. Relevant equations



    3. The attempt at a solution

    So far I have:

    (1) dS = dQ / T
    Entropy is a measure of a system's disorder.

    (2) I've drawn a flow diagram but can't get a picture of that on here. Suffice to say I have tried to do this part quite hard but can't get it to work.

    (3)

    (a) dS = dQ /T ... dQ = 3/2 N K dT ... so dS = 3/2 N K dT/T. Then I can work out the integral and do this first part for temperature going from 293 to 273? Does mass not matter here?

    (b) dS = dQ/T ... We know Q from the question as 333kJ/kg so S = Q/T goes to S = 333 * 1 kilogram of water / 273 Kelvin. Is this right?

    (c) I have no idea how to do this.

    The entropy change of the universe will increase during this process, as it will do for all processes above 0 Kelvin.

    (4) Coefficient of Performance = Qh / Work = Qh / Qh - Qc ... this can then be written as Th / Th - Tc = 323K / 323K - 273K = 6.46.

    To improve the coefficient of performance of the ice-maker you could decrease the temperature of the hot reservoir???? This doesn't seem to make sense...

    If anyone has the time to look at this and help me I would very much appreciate it as I have lost all my second year notes on thermodynamics and can't see how to do this question properly.

    DoubleB
     

    Attached Files:

  2. jcsd
  3. Jan 13, 2007 #2
    Can anyone help me?
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook