Internal Energy of a System: Definition & Measurement

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  • 0:04 What Is the Internal…
  • 0:36 Types of Energy
  • 1:58 Measuring Internal…
  • 3:29 Lesson Summary
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Lesson Transcript
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.

Expert Contributor
Christianlly Cena

Christianlly has taught college physics and facilitated laboratory courses. He has a master's degree in Physics and is pursuing his doctorate study.

We often talk about individual types of energy, but how do you measure total energy? One way is to find the internal energy of a system. Let's learn what it means and how it is measured.

What Is the Internal Energy of a System

A system is a collection of parts that is in some way connected or works together. Systems can be a lot of things. The mechanics of a car is a system and so is the Sun. Systems are also used in thermodynamics to describe areas that allow heat to move freely. It's this latter definition that is most relevant when we're talking about internal energy. That's because internal energy is a term that is used commonly in thermodynamics. It's a way of describing all the energy contained within the particles that make up a system.

Types of Energy

These kinds of systems could be a box full of oxygen gas or the fluid inside the brake line of a bicycle. Whatever it is, the particles contained inside the system have a certain amount of total energy. To be exact, the internal energy of the system is the total of the kinetic or movement energy of the particles and the potential or position energy of the particles.

The particles inside a gas are always moving. You know, that shooting around all over the place like the asteroids and meteors often depicted in sci-fi shows or like billiard balls bouncing off the cushions of a pool table. Anything that moves contains energy, also known as kinetic energy. The faster they move, the more kinetic energy the particles have. But objects and particles also have potential energy. When you lift the ball up in the air and let it go, it drops to the ground. The reason it does that is because a ball that is higher up contains gravitational potential energy. When you let go of it, that gravitational potential energy is released. Gas particles are the same. Based on the position relative to other particles, they have potential energy. The more you separate the particles, the more potential energy they have because of the attraction between them. Add those two numbers up - kinetic energy plus potential energy - for every particle in a system, and you get the total internal energy of that system.

Measuring the Internal Energy of a System

Okay, so that's nice in theory, but how do you actually do it? What does it take to measure the internal energy of a system?

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Additional Activities

Internal Energy Word Search Activity

This activity will help you assess your knowledge regarding the definition and measurement of internal energy, as presented in the lesson.


For this activity, print this page on a blank piece of paper. Search and highlight the word that will complete each of the given clues. Afterward, neatly write them on the appropriate blank space in the clues.


  1. _____ is the measure of the kinetic energy of particles within a system.
  2. The _____ energy of an object is the energy that it possesses due to its motion.
  3. Particles or _____ in air move faster when heated.
  4. The total kinetic and potential energy of all the atoms and molecules in the system is known as _____ energy.
  5. _____ have particles that jiggle fast, slide around each other, and are more separated than solids.
  6. _____ energy is the energy held by an object because of its position relative to other objects.
  7. Matter can exist in four _____: as solids, liquids, gases, or plasma.
  8. _____ is the ability to do work and is present in all things that are either moving or at rest.
  9. _____ is the science of the conversions between heat and other forms of energy in a system.
  10. A set of things working together as parts of a mechanism or an interconnecting network is called a _____.

Answer Key

  10. SYSTEM

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