What is internal energy?

Internal energy is the total energy stored within a system due to the motion and positions of its particles.

In more detail, internal energy is a key concept in thermodynamics and refers to the sum of all the kinetic and potential energies of the particles within a substance. Kinetic energy comes from the movement of particles, such as atoms and molecules, while potential energy arises from the forces between these particles. For example, in a gas, the particles move rapidly and collide with each other, contributing to the system's kinetic energy. In a solid, particles vibrate around fixed positions, and the forces holding them together contribute to the potential energy.

When we heat a substance, we increase its internal energy. This can cause the particles to move faster (increasing kinetic energy) or to move further apart (increasing potential energy). Conversely, cooling a substance reduces its internal energy. Changes in internal energy can also occur during phase changes, such as melting or boiling, where energy is used to break the bonds between particles without necessarily increasing their temperature.

Internal energy is an important concept because it helps us understand how energy is transferred and transformed within a system. For instance, when you boil water, the heat energy from the stove is transferred to the water, increasing its internal energy until it changes from a liquid to a gas. Understanding internal energy also helps explain why different materials have different capacities to store and transfer heat, which is crucial in many practical applications, from designing engines to insulating homes.

In summary, internal energy is the total energy contained within a system due to the motion and interactions of its particles, and it plays a vital role in understanding how energy behaves in physical processes.