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Daily Dose 014 | Thermodynamics
How do you determine whether a thermodynamic system is open or closed?
How do you know if a thermodynamic system is open or closed?
I know that as engineers, we tend to prefer working with things we can see and touch.
But what about those intangibles?
The pressure, density, temperature and entropy?
Are you good on this level?
Thermodynamics is the study that deals with the relationships between heat and these other properties.
It focuses in on systems undergoing thermodynamic processes and specifically how the heat transfer relates to the various energy changes that typically result in work being done.
In this lesson, we will dive in to a practice problem in the subject THERMODYNAMICS, specifically, we will be working on determining whether or not a thermodynamic system is open or closed.
Key Definition
What is an OPEN & CLOSED SYSTEM?
There are two types of thermodynamic systems that we will want to become familiar with on the FE Exam: OPEN SYSTEMS and CLOSED SYSTEMS.
The OPEN SYSTEM is one that INTERACTS with their internal elements and the environment.
CLOSED SYSTEMS, on the other hand, are held to be ISOLATED from their environment. Equilibrium thermodynamics, for example, is a field of study that applies to closed systems.
The Process
How to know if a system is open or closed?
In the world of Thermodynamics, systems play a vital role.
A thermodynamic system can be thought of as an imaginary boundary where certain processes occur, separating it from its surroundings.
Here’s how to think about it.
Imagine a kettle boiling water.
Does it keep all its steam? Or does some escape into your kitchen?
It is such characteristics that help us classify systems as open or closed.
If the steam is maintained, then it would be considered a closed system. However, we all know the steam escapes into the surrounding kitchen – therefore, it’s an example of an open system.
Being asked to determine whether or not a thermodynamic system is open or closed may appear complicated at first, but it’s not – let’s break it down into a simple, streamlined, progression of questions that you can ask yourself.
- Understand the Definition of each system
- Observe the System
- Study the Process
- Consider Real-world Examples
- Ask Critical Questions
- Note Exceptions
An Open System allows both mass and energy (heat and work) to cross its boundaries.
A Closed System allows only energy to cross its boundaries, not mass.
Does the system have an inlet or outlet for mass (like a vent, valve, or opening)? If yes, it could be an open system.
However, if the system is sealed off from its surroundings, preventing any mass exchange, it leans towards being a closed system.
If during a process, the amount of substance inside the system changes (due to entry or exit of mass), it’s an open system.
If on the other hand, the mass inside the system remains constant throughout the process, it’s a closed system.
A boiling kettle with a spout letting out steam is an open system (both steam (mass) and heat are transferring).
A sealed thermos bottle, on the other hand, is a closed system (heat may transfer, but the coffee inside doesn’t exit the thermos).
Does the process involve refilling, emptying, or continuous supply? If yes, it’s more likely to be open.
Is the system sealed during the entire process, with no mass entering or leaving? Then, it’s closed.
Some systems can behave as both open and closed under different conditions. For instance, a pressure cooker is a closed system until its pressure valve releases steam, making it behave momentarily as an open system.
Before jumping into our practice problem, let’s run through a simple scenario.
Imagine a refrigerator. You can add or remove food items (mass), and it also exchanges heat with its surroundings.
Using the progession of questioning laid out above, we determine that:
- It has an inlet/outlet for mass (door)
- The amount of substance (food) can change.
- Energy (in the form of cold air) can transfer when the door is open.
Therefore, a refrigerator behaves as a thermodynamic open system when the door is opened, but once closed, it behaves predominantly as a closed system since no mass is exchanged, only energy.
Systems and their behaviors lie at the heart of many engineering processes. So, whenever you see an apparatus, a system, let yourself run through the questioning: Is this thermodynamic system is open or closed?
And with all of that being stated, check out the video and see how we can go about solving this type of problem in the most efficient manner.
As always, we are here to help, Prepineer
