Energy

In physics, you are used to dividing energy into two (or more) parts:

$$\input{equations/Etypes}$$ (06-1)

Typically, in thermodynamics, the body remain in the same inertial frame as the observer, so the velocity and rotation of the body can be taken to be zero. The kinetic energy of the system is typically ignored.¹

It is useful, but not necessary, to divide the potential energy into parts that do not depend on its position in space and those that do, for instance:

$$U = U_{int} + Mg h$$ (06-3)

In the majority of cases in materials thermodynamics, changes in the potential energy due to gravity are not considered. If they are required to account for the balance of internal energy, then they must be included and terms like $h$ become a degree of freedom for the system.²We will not consider gravitational energy any further and what we mean by the internal energy $U$ will be intrinsic to the material system under consideration.

In other words, the internal energy of a body is associated with all the internal degrees of freedom of a body.