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Thermodynamics: Entropy and specific heat

  1. Mar 14, 2006 #1
    Hi here!

    (before scriptum. Sorry for my lousy English and LaTex.)

    I am a bit confused about entropy change statements:

    In most textbooks is given:
    [tex] \Delta S = n (C_V ln \frac{ \ T_{2}}{T_1} + R ln \frac{ \ V_{2}}{V_1}) [/tex]

    where n - moles of gas. And dimension of entropy is [J/K]

    But now I have one book there entropy change is defined using specific heats:

    [tex]
    \Delta S = m (c_v ln \frac{ \ p_{2}}{p_{1}} + c_p ln \frac{ \ V_{2}}{V_{1}})
    [/tex]

    where m - mass of gass, and dimension is [J/(kmol*K)]

    I can't understand where this isobaric specific heat gets here!

    Ok, I understand that these entropies essentially are given for different measures of amount of gass. First is given for moles, but second for kilograms. (am I right??)

    But how to involve specific heats there is obscure for me.

    Thanks!
     
    Last edited: Mar 14, 2006
  2. jcsd
  3. Mar 14, 2006 #2
    Oh my!

    Am I blind or what?!

    Everything works out if take in mind that
    [tex]
    C_p - C_V = R
    [/tex]
    and

    [tex]
    c_v = \frac { \ C_{V}}{ \mu}
    [/tex]

    That's it! Sorry for buzzer!
     
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