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I Relation between 'P' and 'V' for an Adiabatic process

  1. Jun 19, 2016 #1
    Hi All,

    I have a little query concerning the derivation of PV γ = constant. In my text book of Physics, first they give the equation for adiabatic process using the first law of Thermodynamics, as;

    dEint = W → (1)​

    ΔEint ⇒ change in internal energy and W ⇒ workdone
    Then, they used the relation:

    Cv = Q / ndt​
    Q ⇒ heat
    'n' ⇒ no. of moles
    Cv ⇒ Molar Heat capacity at constant volume
    dT ⇒ Change in temperature

    Now, for an isochoric process;
    Q = dEint
    dEint = nCvdT → (2)​

    Substituting dEint from eq.(2) into eq.(1)

    ⇒ W = nCvdT​

    As thermodynamic work is given as W = -PdV
    ∴ -PdV = nCvdT
    ⇒ PdV = - nCvdT → (3)​

    Now, writing equation of state of the gas in differential form as;
    d(PV) = d(nRT)​
    ⇒ PdV + VdP = nRdT​
    Using eq.(3)
    ⇒ - nCvdT + VdP = nRdT
    ⇒ VdP = nCvdT + nRdT​

    Using the relation Cp = Cv + R into the above equation

    ⇒ VdP = nCpdT → (4)​

    Now, dividing eq.(4) by eq.(3)
    ⇒ VdP / PdV = -Cp / Cv
    or VdP / PdV = -γ
    where, γ ⇒ ratio of molar heat capacities

    Rearranging the above equation:
    dP / P = -γ dV / V​
    Integrating both sides with the initial state 'i' and final state 'f' being the lower and upper limits respectively, gives;

    dP / P = -γ dV / V​

    ⇒ PiViγ = PfVfγ

    which can be written as: PV γ = constant

    Now, my query was that,
    1. why have they substituted the internal energy from an isochoric process into an adiabatic process (Substituting dEint from eq.(2) into eq.(1))? Considering this substitution, can we say that heat absorbed in the isochoric process is equivalent to the work done in an adiabatic process?

    2. What is the physical significance of γ on a curve of PV γ = constant ? (Please do not go into the details of poltropic processes)
  2. jcsd
  3. Jun 19, 2016 #2

    For an isochoric process the work done is always zero.
    A typical example of an isochoric process is addition or removal of heat from a closed system.
    The volume remains constant but temperature and pressure change according to the process.
    thus leading to change in internal energy.

    An adiabatic system is perfectly insulated from external environment and there is no heat transfer in or out of the system.
    Work done in an adiabatic process completely results in change in the internal energy of the system.
    so the two processes can be related.

    The heat capacity ratio(gamma) for an ideal gas can be related to the degrees of freedom of a molecule of gas

    gamma = 1 + 2/(degree of freedom )
    for a monoatomic gas, degree of freedom is 3 so its value is (1 +2/3) = 1.67
    and for diatomic gas it (gamma) goes to 1.4 as the gas has five degrees of freedom.
    This ratio becomes important when one uses the gas in a cyclic process for conversion of heat into work. and gas is taken as working substance like in a heat ( carnot engine.) engine.
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