What are cyclic and non-cyclic processes?

Short Answer:

In thermodynamics, a cyclic process is a process where a system returns to its original state after completing a series of changes. In this case, all properties like pressure, volume, and temperature return to their initial values, and the net change in internal energy is zero. Examples include the working of heat engines and refrigerators.

A non-cyclic process, on the other hand, does not return to the original state. The system ends up in a different condition after the process. These are common in natural and practical situations like heating water or expanding a gas in a piston without compression.

Detailed Explanation:

Cyclic and non-cyclic processes

In thermodynamics, we study how energy and matter interact during physical or chemical changes. These changes are called thermodynamic processes, and they can be either cyclic or non-cyclic depending on whether the system returns to its starting point or not. This classification is important for understanding the behavior of engines, pumps, turbines, and real-life energy systems.

Cyclic Process

A cyclic process is one where the system goes through a series of thermodynamic changes but finally returns to its initial state. This means that at the end of the process, all the system’s properties like pressure, volume, temperature, and internal energy become the same as they were at the beginning.

Since the internal energy depends only on the state of the system (not the path), the net change in internal energy in a cyclic process is zero.

Key Characteristics of a Cyclic Process:

  • The process forms a closed loop on a P-V or T-S diagram.
  • No change in internal energy (ΔU = 0).
  • The net work done by the system equals the net heat added (W = Q).
  • Common in reversible engines, turbines, compressors, and refrigerators.

Examples:

  • Carnot Cycle: Ideal reversible cycle using two isothermal and two adiabatic processes.
  • Rankine Cycle: Used in steam power plants.
  • Otto Cycle: Idealized cycle for petrol engines.
  • Refrigeration Cycle: Working cycle of a refrigerator or AC.

Working Principle:

Let’s take the example of a steam engine. The steam expands to do work, then gets cooled and condensed, and the process repeats. Every time the cycle completes, the engine returns to the same state it started with. This repeatable nature is what defines it as a cyclic process.

Non-Cyclic Process

A non-cyclic process is a thermodynamic process in which the system does not return to its original state. The final state is different from the initial one, meaning that properties like internal energy, pressure, and volume also change permanently.

In a non-cyclic process, there is a net change in the internal energy (ΔU ≠ 0), and the system usually does not operate repeatedly in the same manner.

Key Characteristics of a Non-Cyclic Process:

  • The process is not repeatable in the same path.
  • System moves from one state to another.
  • Net change in internal energy is not zero.
  • Often represents one-time or real-life energy transfers.

Examples:

  • Heating water in an open container.
  • Expanding gas into a vacuum.
  • Melting of ice into water.
  • Burning of fuel in open air.

Working Principle:
Suppose we heat a gas in a piston. The gas expands, does work, and finally reaches a new volume and pressure. If we stop there without compressing it back, this becomes a non-cyclic process because the system has changed permanently.

Importance of the Classification

This classification helps in applying first law of thermodynamics correctly:

  • For cyclic process: ΔU = 0 → Q = W.
  • For non-cyclic process: ΔU = Q – W.

This also helps engineers decide whether they are dealing with a system that can be reused cyclically (like in engines), or one that changes permanently (like burning fuel).

Real-world relevance:

  • Engines and turbines rely on cyclic processes for repeated operation.
  • Heating, melting, and expansion examples are non-cyclic and happen in natural or practical uses.
Conclusion

Cyclic and non-cyclic processes are two main types of thermodynamic operations. A cyclic process returns the system to its original state, with no change in internal energy, making it useful for engines and machines that repeat actions. A non-cyclic process leads to a new state with permanent changes. Understanding this difference is important for applying thermodynamic laws and analyzing energy systems in mechanical engineering.