Short Answer

A cyclic process is a thermodynamic process in which a system undergoes a series of changes and finally returns to its initial state. This means all state variables such as pressure, volume, and temperature come back to their original values after completing the cycle.

In a cyclic process, the system may exchange heat and work with the surroundings during the cycle, but the net change in internal energy is zero because the system ends where it started. Heat engines and refrigerators work based on cyclic processes.

Detailed Explanation :

Cyclic Process

A cyclic process is a sequence of thermodynamic processes that begins and ends at the same state. The system undergoes several individual processes—such as isothermal, adiabatic, isobaric, or isochoric—but after completing all of them, the system returns to its initial condition. Because the initial and final states are identical, the system’s properties remain the same at the end of the cycle.

A cyclic process is commonly represented by a closed curve on a pressure–volume (P–V) diagram. This closed loop shows that the system has returned to the same state after completing the cycle.

Key Features of a Cyclic Process

A cyclic process has several important characteristics:

1. Returns to Initial State

The most important feature is that the final state is exactly the same as the starting state.
So,

• Final pressure = Initial pressure
• Final temperature = Initial temperature
• Final volume = Initial volume

2. Change in Internal Energy is Zero

According to the first law of thermodynamics:

ΔU = Q – W

For a complete cycle,
ΔU = 0, because internal energy depends only on state.

Thus,
Q = W

This means the net heat added equals the net work done over one cycle.

3. Heat and Work May Vary in Each Step

During individual steps of the cycle:

• Heat may enter or leave the system
• Work may be done on or by the system

But these changes balance out over the whole cycle.

4. Represented by a Closed Curve

On a P–V diagram, a cyclic process is always shown as a closed loop, meaning the path comes back to the starting point.

Working of a Cyclic Process

A cyclic process generally includes:

1. Expansion

The gas expands, doing work on surroundings.
Heat may be absorbed.

2. Compression

The gas is compressed.
Work is done on the system.
Heat may be released.

3. Heat Addition and Rejection

In some steps, heat is added; in others, it is removed.

By combining these steps, the full cycle allows the system to return to its starting state.

Types of Cyclic Processes

Different machines use different types of cycles. Some common types include:

1. Carnot Cycle

Includes two isothermal and two adiabatic processes.
Used for theoretical analysis.

2. Otto Cycle

Used in petrol engines.
Consists of isochoric and adiabatic processes.

3. Diesel Cycle

Used in diesel engines.
Consists of isobaric, adiabatic, and isochoric processes.

4. Refrigeration Cycle

In refrigerators and air conditioners.
Involves compression, expansion, and heat exchange.

All these cycles operate based on the principle of returning to the initial state.

Graphical Representation

On a P–V diagram, a cyclic process is drawn as a closed loop.
Two important points:

1. Clockwise cycle
   • Area of loop = work done by the system
   • Found in heat engines
2. Anticlockwise cycle
   • Area of loop = work done on the system
   • Found in refrigerators and heat pumps

The area enclosed by the loop represents the net work done over the entire cycle.

Examples of Cyclic Process

Cyclic processes are widely seen in machines and natural systems:

1. Heat Engines

Steam engines, petrol engines, and diesel engines operate using cyclic processes to convert heat into work.

2. Refrigerators and Air Conditioners

These appliances use cyclic processes to remove heat from inside and release it outside.

3. Power Plants

Thermal and nuclear power plants use turbine cycles that return to their initial state repeatedly.

4. Atmospheric Cycle

Air parcels in large convection loops undergo cyclic changes in temperature and pressure.

5. Human Breathing (Approximate Cycle)

Breathing in and breathing out forms a repeating cycle of gas exchange.

Importance of Cyclic Processes

Cyclic processes are important because:

• They allow continuous operation of engines
• They help measure efficiency of machines
• They are essential in power generation
• They make refrigeration and cooling possible
• They help understand thermodynamic behavior in repeating cycles

Without cyclic processes, it would be impossible for machines to operate continuously.

Conclusion

A cyclic process is a thermodynamic process in which the system undergoes a series of changes and finally returns to its starting state. During the cycle, heat and work are exchanged, but the net change in internal energy is zero. Cyclic processes form the basis of engines, power plants, refrigerators, and many natural phenomena. They are essential for systems that operate continuously and repeatedly.