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Homework Help: Basic Entropy Help

  1. Apr 20, 2009 #1
    1. The problem statement, all variables and given/known data

    Object A and object B are two identical microscopic objects. The table below shows the number of ways to arrange energy in one of these objects, as a function of the amount of energy in the object.

    E (joules) 4e-21 6e-21 8e-21 1e-20 1.2e-20 1.4e-20 1.6e-20
    # ways 6 20 37 60 90 122 148

    When there are 6e-21 joules of energy in object A, what is the entropy of this object?
    SA = ? J/K

    When there are 1e-20 joules of energy in object B, what is the entropy of this object?
    SB = ? J/K

    Now the two objects are placed in contact with each other. At this moment, before there is time for any energy flow between the objects, what is the entropy of the combined system of objects A and B?
    SAB = ? J/K

    2. Relevant equations

    omega = (q+N-1)!/q!(N-1)!
    S = kln(omega)

    3. The attempt at a solution

    I don't know how to get the quanta from the energy given...if that's how you do it? When i know the quanta i can find omega and and plug it into the entropy equation.
  2. jcsd
  3. Apr 20, 2009 #2
    E: 4e-21, 6e-21, 8e-21, 1e-20, 1.2e-20, 1.4e-20, 1.6e-20
    # ways: 6, 20, 37, 60, 90, 122, 148

    This might be easier to read
  4. Apr 20, 2009 #3


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    There is a more basic definition for S=k·ln(#), where # is the number of states it is possible for the system to be in.

    The formula using q and N seems to a way to calculate the number of states for a particular scenario ... however in this problem it is much easier to get #, just using the number of ways to arrange the given energy.

  5. Nov 28, 2009 #4
    Hey I have the same problem and I dont know how to do it as well
    Can someone please help me?
    What do you need to do after finding all of the S?
  6. Dec 1, 2009 #5
    S = k*ln(omega)

    k= boltzman's constant
    Boltzmann constant = 1.3806503 × 10-23 m2 kg s-2 K-1

    omega= (q+n-1)!/q!(n-1)! = Number of ways

    basically look at the energy asked and used the number of ways under for omega.

    SAB = SA + SB

    It worked for me hope i Helped
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