Internal energy

Everything in our world holds a hidden kind of energy. Scientists call this internal energy. It is not the energy from an object moving through space. It is also not the energy from its position in a gravity field. Instead, it is the energy found deep inside a system. This energy is what we use to describe the state of a thing. It matters because it helps us understand how matter changes.

How does this energy work on a tiny scale? We can look at it in two main ways. First, there is microscopic kinetic energy. This comes from the tiny parts, like atoms, moving, spinning, or shaking. This movement is what we feel as temperature. Second, there is microscopic potential energy. This energy comes from the forces between those tiny parts. It includes the chemical bonds that hold particles together.

People have studied these ideas for a long time. A scientist named Clausius introduced this idea. He used it to help explain the first law of thermodynamics. This law is a very important rule in science. It says that energy cannot be created or destroyed. In an isolated system, the total internal energy stays the same. This is known as the law of conservation of energy.

There are many important facts to remember about this energy. We measure energy using a unit called the joule (J). If we look at energy for a specific mass, we call it specific internal energy. If we look at it for an amount of substance, we call it molar internal energy. Internal energy is an extensive property. This means the amount of energy depends on how much stuff is in the system.

Internal energy changes when energy moves in or out of a system. This can happen through heat or through work. For example, a gas might expand to push a piston. This is a way the system does work on its surroundings. If a system is not closed, matter can move in or out too. This also changes the internal energy. You can think of it like a bank account for energy. Adding heat or matter is like making a deposit.