Types of Processes

A transition from one equilibrium macrostate to another is defined as a thermodynamic process. The process’s distinguishing elements are the initial and final states. A system begins from an initial state 1, defined by a pressure P1, a volume V1, and a temperature T1. It progresses through various quasi-static stages to a final state 2, represented by a pressure P2, a volume V2, and a temperature T2. This process involves the movement of energy from or into the system and the performance of work by or on the system. Increasing gas pressure while maintaining a constant temperature is an example of a thermodynamic process.

Let’s dive into detail in the types of processes study material.

Types of Process

• Adiabatic Process

Thermodynamically, an adiabatic process is one in which no heat is transferred into or out of the system (Q = 0). Because heat flow requires a certain amount of time, a process that is completed quickly is adiabatic. When the first law is applied to an adiabatic process, we get 

ΔU = U2−U1 = − Δ W

For an ideal gas’s adiabatic process at constant pressure and volume, PVy= constant, where ‘y’ is the ratio of specific heats (ordinary or molar).

              ‘y’=Cp/Cv

As a result, if an ideal gas changes state adiabatically from (P1, V1) to (P2, V2):

              P1V1y=P2V2y

As Q = 0 for an adiabatic process, we get ΔU = -W from the first law of thermodynamics (Q = ΔU + W). As a result, if the work done is negative, internal energy will rise, and vice versa.

• Isochoric process 

The system’s volume stays unchanged during an isochoric process, i.e., V = 0. The system accomplishes no work (since ΔV = 0, PΔV, or W is also zero). Because of the first law of thermodynamics (Q = ΔU + W), the change in internal energy for an isochoric process equals the heat transferred (ΔU = Q). Because the process is isochoric, dV = 0 means zero pressure-volume work. The internal energy may be computed using the ideal gas model by:

∆U = mCv∆T

Where the property Cv (J/mol K) is known as specific heat (or heat capacity) at a constant volume because it relates the temperature change of a system to the amount of energy added by heat transfer under certain specified conditions (constant volume). The first law of thermodynamics demands that ∆U = ∆Q since no work is done by or on the system. Therefore,

Q = mCv∆T

1. Isothermal Process

An isothermal process is one in which the temperature of a system remains constant (T = const). Heat transfer into or out of the system is normally done at such a slow rate that the reservoir temperature is constantly adjusted through heat exchange. The thermal equilibrium is maintained in each of these states.

For an ideal gas, the circumstance ‘n = 1’ is for both – isothermal (constant-temperature) as well as for polytropic process. 

In contrast to the adiabatic process, in which n = κ, where ‘k’ is any constant for the equation (written below) and the system exchanges no heat with its surroundings (Q = 0; ∆T0), in an isothermal process. 

There is no change in the internal energy, due to ∆T=0. Hence ΔU= 0 (for ideal gases) and Q ≠ 0 are both true. 

Therefore, W = nRT ln (VB/VA) is the amount of work done in an isothermal process.

If VB is greater than VA, the work done will be positive. Otherwise, it will be negative. As internal energy is temperature-dependent, ΔU = 0 because the temperature is constant, and hence Q = W is obtained by using the first law of thermodynamics (Q = ΔU + W).

• Isobaric Process

A thermodynamic process in which the system’s pressure remains constant 

(p = const) is known as an isobaric process. The system’s volume changes because the pressure (P) remains constant in this process. W = P (Vfinal – Vinitial) can be used to determine the amount of work done (W). The work done is positive if ΔV is positive (expansion). The work done is negative for negative ΔV (contraction).

For an ideal gas, the situation n = 0 corresponds to isobaric (constant-pressure) and polytropic processes. An isobaric process involves a change in internal energy (due to ∆T≠0), and henceΔU ≠ 0 (for ideal gases) and Q ≠ 0. When the external pressure, Pext of the surroundings on the system is equal to P, which is the system’s pressure, the maximum work is done. If V is the volume of the system, W12 is the maximum work performed as the system goes from state 1 to 2 during an isobaric thermodynamic process is given by

W12 = 12 P dV  = P 12 dV = P (V2 -V1)

Conclusion

A system starts from an initial state i which is defined by a pressure Pi, a volume Vi, and a temperature Ti. It then moves through various quasi-static states to a final state f, which is defined by a pressure Pf, a volume Vf, and a temperature Tf. Increasing the gas pressure while maintaining a constant temperature is an example of a thermodynamic process. The importance of thermodynamic processes is seen in the engineering of heat engines.