Quick The internal energy, usually substituted by the letter U, may be considered a thermodynamic property. Typically bulk kinetic and potential energy are considered separately and then all other energy of a control mass is lumped into the internal energy.
Nomenclature
U
internal energy of a given mass of a substance
u
internal energy per unit mass, or specific internal energy (NOT to be confused with velocity)
Details Internal energy may be considered a thermodynamic property. Internal energy is an extensive property because it depends on the mass of the system. Similarly, kinetic and potential energies are extensive properties. Internal energy may at times refer to both internal energy per unit mass and the total energy.
From the lesson Conservation of Energy, raising the temperature of a body increases its internal energy; lowering the body's temperature decreases its internal energy. Therefore, change in internal energy can often be associated with a change in temperature.
In the absence of motion, gravity, surface effects, electricity, or other effects, the state of a pure substance is specified by two independent properties. It is very significant that, with these restrictions, the internal energy may be one of the independent properties of a pure substance. This means, for example, that if the pressure and internal energy are specified (with reference to an arbitrary base) of superheated steam, the temperature is also specified.
Thus, in a table of thermodynamic properties such as the steam tables, the value of internal energy can be tabulated along with other thermodynamic properties. Most thermodynamic tables should list the internal energy for saturated states. Included should be the internal energy of saturated liquid uf and the internal energy of saturated vapor ufg. The values are given in relation to an arbitrarily assumed reference state, which, for water in the steam tables, is taken as zero for saturated liquid at the triple-point temperature, 0.01°C. All values of internal energy in the steam tables are then calculated relative to this reference (note that the reference state cancels out when finding a difference in u between any two states). Values for internal energy are found in the steam tables in the same manner as for specific volume. In the liquid–vapor saturation region:
U = Uliq + Uvap
or:
mu = mliquf + mvapug
Dividing by m and introducing the quality x gives:
u = (1 − x)uf + xug
u = uf + xufg
As an exmaple, the specific internal energy of saturated steam having a pressure of 0.6 MPa and a quality of 95% can be calculated as:
In thermostatics, the only energy in a substance is that stored in a system by molecular activity and molecular bonding forces. This is commonly denoted as internal energy. A commonly accepted adjustment to this static situation for fluid flow is to add two more energy terms that arise from newtonian mechanics: potential energy and kinetic energy.
In the study of thermodynamics, it is convenient to consider the bulk kinetic and potential energy separately and then to consider all the other energy of the control mass in a single property that we call the internal energy. The kinetic and potential energy of the control mass are associated with the coordinate frame that is selected and can be specified by the macroscopic parameters of mass, velocity, and elevation. The internal energy U includes all other forms of energy of the control mass and is associated with the thermodynamic state of the system.