Closed & Open Systems: Definition & Examples

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  • 0:01 What Is a System?
  • 1:10 Open vs. Closed Systems
  • 2:20 Example
  • 3:16 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

In this lesson, you'll learn the difference between closed and open physical systems. Explore examples of when each kind of system occurs, and learn why scientists sometimes choose one over the other.

What Is a System?

Systems are extremely important in science—especially physical systems. They're a way to focus in on the exact thing you're really interested in and to help simplify the situation. But before we get into the details of how that works, we should first ask the question…What exactly is a physical system?

A physical system, in general, is a set of connected parts that together form a cohesive whole. They're the parts that work together to do a job or the parts that are simply connected together because they happen to be in contact with one another. A physical system is the part of the world you're focusing on and trying to explain.

For example, let's say that you're studying the movement of the pendulum in a grandfather clock. Are you looking at the motion of the pendulum bob itself? Are you interested in the string as well? What about the place where the string connects to the clock? Exactly what part are you studying? Deciding that might make a big difference as to how complicated the results will be. The bigger the system, the more complex it tends to be. When you make the decision on what to focus on, we say that you are defining the system.

Open vs. Closed Systems

Systems can be either open or closed. A closed system is one where a quantity or series of quantities cannot enter or leave the system. For example, a system might be closed to energy, meaning energy might not be able to enter or leave the system. A vacuum thermos flask does a really good job of stopping energy from leaving the system to keep your drink warm. So it might make sense to treat it as a closed system - but no system in the real world is ever perfectly closed, so it will only be an approximation.

The opposite of a closed system is an open system. An open system is one where a quantity or series of quantities can enter or leave the system to a significant degree. If you pour your hot drink into a mug instead of a vacuum thermos flask, the heat will escape relatively quickly into its surroundings. So a mug is most certainly an open system! Open systems are a lot more complicated to understand than closed systems, and so scientists prefer to work with closed systems when possible. It makes things much simpler to explain and can be a good starting point before trying to explain open systems, too.


One of the places where you see scientists analyze closed systems the most is when working on thermodynamics, or the study of the movement of heat energy. One of the laws of thermodynamics says that heat can only travel from hot places to cold places unless you do some kind of 'work' to stop it. So, if you put a hot object in contact with a cold one, heat will transfer from the hot one to the cold one.

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