1. Introduction

A thermodynamic process is a transformation in which a thermodynamic system changes from one equilibrium state to another due to changes in pressure, temperature, volume, or heat transfer.

During a thermodynamic process, energy may be transferred in the form of:

• Heat (Q)
• Work (W)

The relationship between heat and work is governed by the First Law of Thermodynamics:$$Q = \Delta U + W$$Q=ΔU+W

Where:

• Q = Heat supplied to the system
• ΔU = Change in internal energy
• W = Work done by the system

Thermodynamic processes are usually represented on a P–V diagram (Pressure–Volume diagram).

Types of Thermodynamic Processes

1. Isothermal Process

Definition

An isothermal process is a thermodynamic process in which the temperature remains constant throughout the process.$$T = constant$$T=constant

Characteristics

• Heat is exchanged with surroundings.
• Internal energy remains constant for an ideal gas.
• Pressure and volume change according to Boyle’s Law.

$$PV = constant$$PV=constant

Work Done

$$W = nRT \ln \left(\frac{V_2}{V_1}\right)$$

Where

• n = number of moles
• R = gas constant
• T = absolute temperature

Example

• Slow expansion of gas in a piston-cylinder arrangement in contact with a heat reservoir.

2. Isobaric Process

Definition

An isobaric process is a thermodynamic process that occurs at constant pressure.$$P = constant$$P=constant

Characteristics

• Volume and temperature change.
• Heat supplied increases internal energy and does work.

Work Done

$$W = P(V_2 – V_1)$$

Heat Transfer

$$Q = m C_p (T_2 – T_1)$$

Where

• Cp = Specific heat at constant pressure

Example

• Heating water in an open container.

3. Isochoric Process (Constant Volume Process)

Definition

An isochoric process occurs at constant volume.$$V = constant$$

Characteristics

• No boundary work is done.
• Heat supplied only increases internal energy.

Work Done

$$W = 0$$

Heat Transfer

$$Q = m C_v (T_2 – T_1)$$

Where

• Cv = Specific heat at constant volume

Example

• Heating gas in a rigid sealed container.

4. Adiabatic Process

4

Definition

An adiabatic process is a process in which no heat is exchanged with the surroundings.$$Q = 0$$Q=0

Characteristics

• Temperature changes significantly.
• Work is done at the expense of internal energy.

Relation

$$PV^\gamma = constant$$

Where$$\gamma = \frac{C_p}{C_v}$$

Work Done

$$W = \frac{P_1V_1 – P_2V_2}{\gamma – 1}$$

Example

• Compression stroke of an internal combustion engine.

5. Polytropic Process

Definition

A polytropic process follows the relation:$$PV^n = constant$$PVn=constant

Where n is the polytropic index.

Special Cases

Process	Value of n
Isothermal	n = 1
Isobaric	n = 0
Isochoric	n = ∞
Adiabatic	n = γ

Example

• Gas compression in compressors and turbines.

Comparison of Thermodynamic Processes

Process	Constant Parameter	Heat Transfer	Work Done
Isothermal	Temperature	Yes	Yes
Isobaric	Pressure	Yes	Yes
Isochoric	Volume	Yes	No
Adiabatic	No heat transfer	No	Yes

Applications of Thermodynamic Processes

1. Internal combustion engines
2. Refrigeration and air conditioning
3. Gas turbines
4. Steam power plants
5. Compressors and pumps

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