Note on Isochoric, Isobaric, Reversible and Irreversible Process

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Isochoric Process

If a substance undergoes a process in which the volume remains constant, the process is called isochoric process.

Work done for Isochoric Process

Ideal gas equation for n-mole

$$\frac {P}{T} = \frac {n R}{V}$$

In this process volume of system is constant

$$\text {So,} \frac {P}{T} = K [\because \frac {nR}{T} is constant ]$$

work done

$$W = \int _{v_1}^{v_2}P dV$$

$$= 0[\because \text {no change in volume}]$$

Variation of pressure and volume in isothermal, adiabatic, isobaric and isochoric processes
Variation of pressure and volume in isothermal, adiabatic, isobaric and isochoric processes

Isobaric Process

If the working substance is taken in an expanding chamber in which the pressure is kept constant, the process is called isobaric process.

Work done for Isobaric Process

Ideal gas equation for n-mole

$$PV = n RT$$

$$\frac {V}{T} = \frac {n R}{P}$$

$$\frac {V}{T} = \text {constant} $$

work done

$$W = \int _{v_1}^{v_2}P dV$$

$$ =P \int _{v_1}^{v_2} dV$$

$$=P(V_2 - V_1) $$

Variation of pressure and volume during reversible process.
Variation of pressure and volume during reversible process.

Reversible process

A reversible process is one which can be retraced in the reverse order, so that it passes exactly through the same state as in the direct process.

P-V diagram for a reversible process is shown in the figure.

Examples of reverse process

  1. All mechanical processes taking place under the action of conservative force.
  2. An infinitesimally slow expansion and compression of an ideal gas at a constant pressure.
  3. All thermal processes taking place at an infinitesimally slow rate.

Conditions for Reversible Process

  1. All the process taking place in the cycle of operation must be infinitely slow.
  2. There should not be any loss of energy due to conduction or radiation during the cycle of operation.

P-V diagram of the irreversible process.
P-V diagram of the irreversible process.

Irreversible process

Those processes which can not be retracted in the opposite order by reversing the controlling factors are known as shown in the figure.

Examples of irreversible process

  1. Rusting of iron
  2. Dissolving of soap in water
  3. Decay of matter
  4. Flow of current through the conductor.

Limitations of the First Law of Thermodynamics

The first law of thermodynamics has the following limitations:

  1. First law of thermodynamics does not indicate the direction of the heat transfer.
  2. First law of thermodynamics does not explain the phenomena why the process is reversible or irreversible.
  3. The first law off thermodynamics does not give to what extent the mechanical energy is obtained from the heat energy.

If a substance undergoes a process in which the volume remains constant, the process is called isochoric process.

If the working substance is taken in an expanding chamber in which the pressure is kept constant, the process is called isobaric process.

A reversible process is one which can be retraced in the reverse order, so that it passes exactly through the same state as in the direct process.

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