What are the key differences between a closed system and an open system in thermodynamics?

Short Answer:

In thermodynamics, a closed system is a system where mass cannot enter or leave, but energy can be transferred in the form of heat or work. A good example is a pressure cooker, where no matter goes in or out, but heat can be added or removed.

An open system, on the other hand, allows both mass and energy to enter or leave the system. A running engine is an example, where fuel and air enter, and exhaust gases and energy exit. Understanding the difference between these systems is important for analyzing real-life engineering applications like engines, turbines, and refrigerators.

Detailed Explanation:

Key differences between a closed system and an open system

In thermodynamics, systems are studied to understand how energy and mass interact within them. Based on the way mass and energy flow across the system boundary, systems are classified as closed or open. These classifications help in choosing the right analysis methods when designing and studying mechanical and thermal systems like engines, heat exchangers, and cooling units.

Closed System

A closed system (also called a control mass system) is one where no mass enters or leaves the system boundary during the process. However, energy in the form of heat or work can still cross the boundary. This means that while the material inside the system remains the same, its energy can change.

Characteristics of a Closed System:

  • Mass is constant inside the system.
  • Heat and work can be exchanged with the surroundings.
  • The boundary may be fixed or movable (like a piston-cylinder).
  • Examples:
    • A gas-filled balloon.
    • A pressure cooker.
    • A piston engine without intake/exhaust valves working.

How it works:

Suppose we heat a sealed gas cylinder. The heat enters the system and increases the internal energy or pressure. But no gas escapes from or enters the cylinder. This is a typical closed system where energy flows but mass stays constant.

Open System

An open system (also called a control volume) is one where both mass and energy can cross the system boundary. This is the most common type of system in practical engineering applications, such as turbines, pumps, boilers, and engines.

Characteristics of an Open System:

  • Mass can enter or leave the system freely.
  • Energy can also enter or leave.
  • Used in systems involving flow (like liquids or gases).
  • Examples:
    • A car engine.
    • A water pump.
    • A turbine in a power plant.

How it works:

Take the example of an internal combustion engine. Air and fuel enter the combustion chamber, energy is released, and exhaust gases exit. This continuous mass flow along with energy transformation makes it an open system.

Key Differences Summary

Although we are not using a table as per the instruction, here are the main differences explained in simple points:

  • Mass Transfer:
    • Closed system → No mass enters or exits.
    • Open system → Mass enters and leaves.
  • Energy Transfer:
    • Both systems allow energy (heat or work) to be exchanged.
  • Example Uses:
    • Closed systems are often used in simplified calculations or sealed environments.
    • Open systems represent real-world machines like engines and pumps.
  • Application Area:
    • Closed systems help in studying internal energy and pressure changes without mass flow.
    • Open systems are more suitable for continuous operations in machines.

Real-Life Relevance

Understanding the difference between these two types is very important for mechanical engineers. When designing a thermal machine, we must know whether mass is flowing in and out. This helps in energy balance calculations, efficiency estimation, and heat loss tracking.

For example:

  • In a refrigerator, we may consider the cooling coil as a closed system to study internal temperature changes.
  • But the whole refrigeration cycle is an open system, where refrigerant flows continuously.
Conclusion

In thermodynamics, the classification of systems into closed and open helps us understand how mass and energy interact. A closed system allows only energy transfer but no mass transfer. In contrast, an open system allows both mass and energy transfer. These concepts are very important in the design and analysis of thermal systems like engines, turbines, and air conditioners. Proper understanding helps in efficient system design and energy-saving technologies.