U as internal energy?

[Rebekah_Park_2C]

Why is U internal energy? Can't it also be referred to as potential energy?

[Chem_Mod]

U is the sum of all energy, potential and kinetic. In ideal gases however there is no potential energy

[Chem_Mod]

In physics, internal energy and potential energy share the same symbol because they effectively represent the same thing in different systems. Internal energy is the energy that can be used (or converted or changed) into other forms of energy (such as chemical or kinetic energy - think of the energy stored within the microscopic environment), and thus represents the potential for a system to be able to release energy. A liquid/gas may not seem to be an energetic system, but the random and chaotic motion of the molecules within the liquid suggest that there is some amount of internal energy. The change in internal energy results from applying heat or changes in work (you can change number of a gas molecules, temperature or even pressure/volume).

Potential energy is the energy that is stored in a physical system, and is often converted into kinetic energy. We have many examples of this in our macroscopic world (such as a ball falling off a cliff, in which potential energy is converted into kinetic energy).