Thermodynamic Processes

Thermodynamic Processes

Thermodynamics deals with changes that occur in the properties of a system when the system goes from one equilibrium state to another. Such a change in the values of properties of a system is called a process.
Alternatively, when a thermodynamic system changes from one state to another, the operation is called a thermodynamic process.

Types of thermodynamic processes Thermodynamic processes are as follows.

1. Isothermal Process

A system is said to be isothermal if the temperature of the system remains constant during each stage of the process.

i.e., adiabatic processes are those process in which,

  • temperature remains constant, i.e., dT = 0
  • heat is exchanged with surrounding.
  • System is placed in a constant temperature bath, i.e., thermostate.
  • Change in internal energy is zero ( only for ideal gases) i.e., ΔU = 0
    [ ∵ As, ΔU = nCvΔT
    since, ΔT = 0  ⇒ΔU = 0 ]

Examples of isothermal processes : refrigerators, heat pump.

  • In mechanism of refrigerator, the heat energy is removed and transmitted to the surrounding environment. Thus, a set of changes takes place but the temperature inside the refrigerator remains constant.
  • By the use of heat pump, the house can be kept at the desired temperature. The heat pump can remove heat from the house and dump it out and it can also bring heat inside the house.
2. Isobaric Process
  • Isobaric processes are those process in which pressure remains constant during each stage of the process.
    i.e., for an isobaric process, dP = 0
  • Volume and temperature are variable.
  • System should be placed in a open vessel.
  • For exampleheating of water to its boiling point and its vapourisation takes place at the same atmospheric pressure.
3. Isochoric Process

Isochoric process are those process in which,

  • volume remains constant, i.e., dV = 0
  • Pressure and temperature are variable.
  • Work, W = – PdV = 0   (as dV=0 ∴ W=0)
    System should be placed in a closed vessel.
  • For exampleheating of a substance in a non-expanding chamber.
4. Adiabatic Process

A process is said to be adiabatic if no heat enters or leaves the system during any step of the process.

i.e., adiabatic processes are those process in which,

  • No exchange of heat takes place, i.e., q = 0
  • The temperature of the system varies. ( If the process is exothermic, the heat evolved will remain in the system and therefore, the temperature of the system rises. On the other hand, if the process is endothermic, heat absorbed is supplied by the system itself and hence the temperature of the system falls ) .
  • The system is thermally isolated (this can be done by keeping the system in an insulated container such as ‘thermos bottle’ )
    High vacuum and highly polished surfaces help to achieve thermal insulation.
  • Example of adiabatic processpumping air into a bicycle tire rapidly. When the tire fills up, the air inside the tire compresses rapidly and causes a rise in temperature. Note that this rise in temperature occurs without any heat being transferred into or out of the system.
5. Cyclic Process
  • When a system undergo a number of different processes and finally returns to its initial state, then the overall process is termed as cyclic process.
  • For a cyclic process, change in all state functions will be zero. i.e.,
    dE=0 , dH=0
6. Reversible Process
  1. Process in which all changes occuring at any part of time are exactly reversed when changes in variables are carried out in opposite direction.
  2. Process takes place in infinitesimal small steps and take infinite time to complete the process.
  3. It is an ideal process.
  4. It is a slow process.
  5. System is in virtual equilibrium at any state, i.e., each step is in equilibrium with previous step or equilibrium exists always.
  6. Driving force should be infinitesimally greater than opposing force.
7. Irreversible Process
  1. Process in which direction of change cannot be reversed by small changes in variables.
  2. It takes finite time and takes place in single step i.e., irreversible process takes place in short time.
  3. All natural processes are irreversible.
  4. It is a fast process.
  5. System is in equilibrium only at initial and final state not in the between.
  6. Driving force is greater than the opposing force.

 

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