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Homework Help: Entropy calculation.

  1. Sep 30, 2011 #1
    1. The problem statement, all variables and given/known data

    A gold block initially at 273 K (state 1) is placed in a microwave oven where the electric and magnetic fields create eddy currents that dissipate energy into the gold block.The microwaves acting on the electrons in the gold is an example of a work transfer into the block.

    The microwaves are shut off and the isolated block is allowed to come to internal thermal equilibrium. The temperature of the block in this final state is 375 K (state 3). What is the change in the energy and entropy of the Au block in process 1->3?

    h) What is the entropy transferred from the magnetron to the gold by the microwaves?
    i) What is the entropy generated in the gold block in process 1->3?

    2. Relevant equations

    del S= mc ln(t2/t1)

    (c= specific heat capacity of the solid)

    3. The attempt at a solution

    Since the action of the eddy currents is considered to be work,and h=the entropy transfer by work on a system is zero, answer to part (h) is zero.

    The entropy is a state function and since its a solid, and the volume and pressure changes can be neglected, we could calculate the answer to part (i) from the equation del S= mc ln(t2/t1)...does this sound right?
  2. jcsd
  3. Sep 30, 2011 #2

    rude man

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    Homework Helper
    Gold Member

    Probably 100% good.

    What are your values for T1 and T2 in your last formula? I know you don't mean states 1 and 2. And I would add that c = cp as opposed to cv. Probably makes little if any difference, though.
  4. Oct 9, 2011 #3
    Thanks rude man, my T2= 375K and T1=273 K.... :-)
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