Thermal energy

Thermal energy is a term often used in science and engineering, but it can mean a few different things. Sometimes it refers to the energy stored inside objects or radiation, known as internal energy. This is different from the energy that keeps everything in one place, called potential energy.

Another meaning of thermal energy is heat. Heat is the energy that moves between objects and their surroundings, but not through work or by moving matter from one place to another.

Thermal energy can also describe a special kind of energy linked to temperature. This energy depends on how hot or cold something is and a constant called the Boltzmann constant.

Because the term "thermal energy" can be confusing, some scientists, like Mark Zemansky, suggest using clearer words such as "internal energy" and "heat" instead. Even so, some books still use the term thermal energy.

Relation between heat and internal energy

In thermodynamics, heat is a type of energy that moves between things. This movement can happen in different ways, like when you feel warmth from the sun or when a metal spoon gets hot in a pot of soup. Heat is not something a single object "has" — it’s more about energy moving from one place to another.

On the other hand, internal energy is something that belongs to just one object or system. It’s a property of that thing’s current state, like how much energy its atoms have right now, no matter how that energy got there. Both heat and work are ways energy can move, but internal energy is about where the energy is at any moment.

Macroscopic thermal energy

Thermal energy is a part of the total energy inside an object. It includes the energy from the tiny movements of molecules and the energy stored in the bonds between molecules. When we talk about thermal energy, we are often talking about how much energy is stored inside something because of its temperature.

Chemical reactions can change the energy inside an object. For example, when a reaction happens, the energy stored in the bonds of molecules can turn into other kinds of energy. We might describe this by saying the chemical energy has turned into thermal energy, like when we talk about the heat given off by a chemical reaction.

Microscopic thermal energy

In simple terms, thermal energy is the energy inside tiny parts of matter, like the tiny bits that make up a gas. When these bits move around, they have kinetic energy, which is the energy of motion. This moving around is what we call heat when it moves from one place to another.

In physics, thermal energy can also mean something called "kT," which is a way to measure energy using a special constant and the temperature of the material. This helps scientists understand how temperature affects the energy of tiny particles.

Thermal current density

When there is no movement of matter, the term "thermal energy" can also mean the energy moved by heat flow.