Thermodynamic Processes

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 quiescent 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 rigorously satisfied in practice.
  • In practice, processes that are sufficiently slow and do not involve accelerated motion of the piston, large temperature gradient, etc. are reasonably approximation to an ideal quasi-static process.
  • The processes define below quasi-static processes only, except when stated otherwise.

(b) Isothermal Process

  • In isothermal process temperature of the system remains constant throughout the process.
  • For an isothermal process equation connecting P, V and T gives.
    $PV = constant$
    i.e., pressure of given mass of gas varies inversely with its volume this is nothing but the Boyle's law.
  • In isothermal process there is no change in temperature, since internal energy for an ideal gas depends only on temperature hence in isothermal process there is no change in internal energy.
    $\Delta U=0$
    therefore From First law of Thermodynamics
    $\Delta Q =\Delta W$
  • Thus during isothermal process
    Heat added (or subtracted) from the system = work done by (or on) the system
  • PV diagram for the Isothermal process is shown below
    P-V diagram for isothermal process
  • TV diagram for the Isothermal process is shown below
    T-P diagram for isothermal process

(c) Adiabatic Process

  • Process in which no heat enters or leaves a system is called an adiabatic process
  • For every adiabatic process $\Delta Q=0$
  • Prevention of heat flow can be accomplished 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 practically adiabatic.
  • On applying first law to adiabatic process we get
    $\Delta U=U_2 - U_1= - \Delta W$
    This is for adiabatic process
  • In adiabatic process change in internal energy of a system is equal in magnitude to the work by the system.
  • If work is done on the system contracts i.e. $\Delta W$ is negative then.
    $\Delta U = \Delta W$
    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., $\Delta W$ is negative
    $\Delta U = -\Delta W$
    here internal energy of systems decreases resulting a drop in temperature.
  • For adiabatic process of ideal Gas, the relation between Pressure and Volume is given by
    $PV^{\gamma}= Constant$
    Where $\gamma = \frac {C_p}{C_v}$
  • if an ideal gas undergoes a change in its state adiabatically from $(P_1, V_1)$ to $(P_2,V_2)$, we have
    $P_1V_1^{\gamma}= P_2V_2^{\gamma}$
  • PV diagram for the adiabatic Process is shown as below
    P-V diagram for adiabatic process
  • We can see the PV diagram of Isothermal Process and Adiabatic Process is similar. The graph of Isothermal is more tilted.
    Here is the P-V graph of an ideal gas for two adiabatic processes connected two isotherms
    P-V diagram for adiabatic  and Isothermal process combined

(d) Isochoric process

  • In an Isochoric process volume of the system remain uncharged throughout i.e. $\Delta V = O$.
  • When volume does not change no work is done , $\Delta W = 0$ and therefore from first law of Thermodynamics
    $U_2 - U_2 = \Delta U =\Delta Q$
  • All the heat given to the system has been used to increase the internal energy of the system.
  • For an Isochoric process equation connecting P, V and T gives.
    $\frac {P}{T}= constant$
    i.e Pressure increases as the temperature increased
  • PV diagram for the Isochoric Process is shown as below
    P-V diagram for Isochoric process
  • T-P diagram for the Isochoric Process is shown as below
    T-P diagram for Isochoric process
    It is straight line having some slope

(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
    $W = P(V_2 - V_1) =nR (T_2-T_1)$
    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.
    i,e $\Delta Q = \Delta U + \Delta W$
  • For an isobaric process equation connecting P, V and T gives.
    $\frac {V}{T}= constant$
    i.e Volume increases as the temperature increases
  • P-V diagram for the Isobaric Process is shown as
    P-V diagram for Isobaric process
  • V-T diagram for the Isobaric Process is shown as
    V-T diagram for Isobaric process

(e) Cyclic Process

  • In a cyclic process, the system returns to its initial state
  • Now since the system returns to it initial state,change in internal energy is zero
    $\Delta U=0$
  • Here in this process total heat absorbed is equal total work done by the system.
    i,e $\Delta Q = \Delta W$
  • P-V diagram for the Cyclic Process is shown as
    P-V diagram for Cyclic process

In Summary , we can represent Thermodynamics Process as Thermodynamics Processes Summary

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