Thermodynamic Processes and Types

We know that if we have to take a thermodynamic system from the initial to the final state, we have several paths that can be taken. In this article, we will be discussing those thermodynamic processes. Before that, we will see what a quasi-static process is. It has been discussed that state variables are defined only when the thermodynamic system is in equilibrium with the surrounding. So a process in which the system is in thermodynamic equilibrium with the surrounding is known as a quasi-static process at each moment.

The Thermodynamic Processes

Isothermal Process:

It is a thermodynamic process in which temperature remains constant. We know,

According to Gas law,

Using this value of P in work done, we get,

Key Points

• W = +ve (VB > VA)
• W = -ve (VA > VB)
• dU = 0 
• Internal energy only depends on temperature
• If temperature = Constant
• Internal Energy = Constant.
• From first law of thermodynamics,
• Q = W + dU
• Q = W (dU = 0)

Read More: First Law of Thermodynamics

Adiabatic Process:

It is a thermodynamic process in which no heat is exchanged between the system and the surrounding. So, Q = 0. Mathematically this process is represented as

Substituting P, we get,

For adiabatic process,

Key Points

• According to the 1st law of thermodynamic process 
• Q = W + dU
• If Q= 0
• dU = -W

So if work done is negative internal energy increases and vice versa.

Isochoric Process:

In isochoric process the change in volume of thermodynamic system is zero. A volume change is zero, so the work done is zero.

• Volume of the system = Constant
• Change in volume = 0
• If, change in volume = 0, then work done is zero.
• According to the 1st law of thermodynamic law
• Q = W + dU
• If W = 0
• Q = dU

Read More: Work Done

Isobaric Process:

The pressure remains constant during this process. So,

So if volume increases, work done is positive, else negative.

Key Points

• Pressure = Constant during this process
• W = P∆V
• So if volume increases, work done is positive, else negative.
• According to thermodynamic 1st law lof thermodynamics
• Q = W + dU
• Q = P∆V + dU

Cyclic Process:

It is a process in which the final state of the system is equal to the initial state. As we know, change in internal energy is a state function, so, in this case, ∆U = 0.

Thermodynamics Process Examples

1. Following is a PV curve showing two isothermal processes for two different temperatures. Identify the process that has a higher temperature.

Solution:

To identify the process with higher temperature: 

• First, a horizontal line must be drawn parallel to the x-axis. 

• This horizontal line represents the constant pressure line.

• Let V₁ and V₂ be the volumes that belong to the same pressure as the vertical lines such that they meet the constant pressure line.

• We know that, at constant pressure, as the volume of the gas increases, the temperature also increases. 
• From the above graph, we can say that V₁ > V₂, therefore, T₁ > T₂. 
• Also, an easier way to determine the temperature is that the curve close to the origin will have a lower temperature.

2. Following is a V-T graph for isobaric processes at two different pressures. Determine the curve that occurs at higher pressure.

Sol: From ideal gas equation, we get

A volume-temperature graph is a straight line passing through the origin.

Also, the slope of the volume-temperature graph is 

The slope of the graph is inversely proportional to the pressure. Therefore, if the slope is greater, the pressure will be lesser.

From the graph, it is clear that P₁ has a larger slope than P₂. Therefore, P₂ > P₁.

The video compares different thermodynamic processes graphically

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