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Thermodynamics



7. Thermodynamic Processes



a) Quasi static Processes

:

  • In Quasi static process deviation of system from it's thermodynamic equilibrium is infinitesimally small.
  • All the states through which system passed during a quasi static process may be regarded as equilibrium states.
  • Consider a system in which gas is contained in a cylinder fitted with a movable piston then if the piston is pushed in a infinitely slow rate, the system will be in quiscent all the time and the process can be considered as quasi-static process.
  • Vanishingly slowness of the process is an essential feature of quasi-static process.
  • During quasi-static process system at every moment is infinitesimally near the state of thermodynamic equilibrium.
  • Quasi static process is an idealized concept and its conditions can never be rigoursly satisfied in practice.

(b) Isothermal Process

:

  • In isothermal process temperature of the system remains constant throughout the process.
  • For an iso-thermal process equation connecting P, V and T gives.
         PV = constant
    i.e., pressure of given mass of gas varies inversly with its volume this is nothing but the Boyle's law.
  • In iso thermal process there is no change in temperature, since internal energy for an ideal gas depends only on temperature hence in iso thermal process there is no change in internal energy.
    Thus,
         ΔU=0
    therefore,     ΔQ =ΔW
  • Thus during iso thermal process
         Heat added (or substacted) from the system = wok done by (or on) the system

(c) Adiabatic Process

:

  • Process in which no heat enters or leaves a system is called an adiabatic process
  • For every adiabatic process Q=0
  • Prevention of heat flow can be acomplished by surrounding system with a thick layer of heat insulating material like cork, asbestos etc.
  • Flow of heat requires finite time so if a process is perfomed very quickly then process will be pratically adiabatic.
  • On applying first law to adiabatic process we get
         ΔU=U2 - U1= - ΔW               (adiabatic process)
  • In adiabatic process change in internal energy of a system is equal in magnitade to the work by the system.
  • If work is done on the system contracts i.e. ΔW is nagative then.
         ΔU = ΔW
    and internal energy of system increases by an amount equal to the work done on it and temperature of system increases.
  • If work is done by the system i.e., ΔW is negative
         ΔU = -Δ W
    here internal energy of systems decreases resulting a drop in temperature.

(d) Isochoric process v:


  • In an isochoric process volume of the system remain uncharged throughout i.e. ΔV = O.
  • When volume does not change no work is done ; ΔW = 0 and therefore from first law
         U2 - U1 = ΔU =ΔQ
  • All the heat given to the system has been used to increase the intenal energy of the system.

(e) Isobaric Process

:

  • A process taking place at constant pressure is called isobaric process.
  • From equation (3) we see that work done in isobaric process is
         W = P(V2 - V1) nR (T2-T1)
    where pressure is kept constant.
  • Here in this process the amount of heat given to the system is partly used in increasing temperature and partly used in doing work.



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