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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.

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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?Well, we would have to measure kinetic energy and potential energy separately.

The kinetic energy of particles is something you can identify in your everyday life. It manifests itself as a property that we hear whenever we watch a weather forecast: temperature. When the air is hotter, the air molecules are moving faster.

That’s what temperature is. So temperature is a measure of the kinetic energy of particles. The higher the temperature, the more internal energy the system must have.Potential energy is a little harder to measure. But one thing that relates to potential energy is the phase or state of the substance. Solids, like ice or wood, contain tightly packed particles that are close to each other. This means that they have less potential energy, like when the ball is close to the earth.

Liquids have particles that slide around each other and are more separated than solids. They have a little more potential energy. And gases have particles that are spread apart, giving them lots of potential energy, like when the ball is lifted a long way above the earth. When we cause a solid to melt into a liquid, or liquid to boil into a gas, we’ve added energy to the system through heat. This is where the potential energy of the molecules comes from.

So when you look at the temperature and state of a system, those are two clues that tell you about the internal energy that it contains.

Lesson Summary

A system is a set of connected parts. Those might be the parts of an engine, but in thermodynamics we’re usually talking about particles that are near to each other so that energy can transfer between them. The internal energy of such a system is equal to the sum total of all the kinetic energies and potential energies of all the particles inside it. Kinetic energy is movement energy, and potential energy is position or separation energy.Temperature is a measure of the kinetic or movement energy of the particles.

To be exact, temperature is the average kinetic energy of the particles in a substance. State or phase is a measure of the potential energy of the particles. It’s about the separation between them. More separation means more potential energy.

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