as a Fermi ball of nucleons, with equal numbers of protons and neutrons, then the total kinetic energy is , with the Fermi energy, which is estimated as 38 MeV. Thus the expected value of in this model is not far from the measured value. Surface term The term is known as the surface term. This term, also based on the strong force, is a correction to the volume term. The volume term suggests that each nucleon interacts with a constant number of nucleons, independent of A. While this is very nearly true for nucleons deep within the nucleus, those nucleons on the surface of the nucleus have fewer nearest neighbors, justifying this correction. This can also be thought of as a surface-tension term, and indeed a similar mechanism creates surface tension in liquids. If the volume of the nucleus is proportional to A, then the radius should be proportional to and the surface area to . This explains why the surface term is proportional to . It can also be deduced that should have a similar order of magnitude to . Coulomb term The term or is known as the Coulomb or electrostatic term. The basis for this term is the electrostatic repulsion between protons. To a very rough approximation, the nucleus can be considered a sphere of uniform charge density. The potential energy of such a charge distribution can be shown to be where Q is the total charge, and R is the radius of the sphere. The value of can be approximately calculated by using this equation to calculate the potential energy, using an empirical nuclear radius of and Q = Ze. However, because electrostatic repulsion will only exist for more than one proton, becomes : where now the electrostatic Coulomb constant is Using the fine-structure constant, we can rewrite the value of as where is the fine-structure constant, and is the radius of a nucleus, giving to be approximately 1.25 femtometers. is the proton reduced Compton wavelength, and is the proton mass. This gives an approximate theoretical value of 0.691 MeV, not far from the measured value. Asymmetry term The term is known as the asymmetry term (or Pauli term). The theoretical justification for this term is more complex. The Pauli exclusion principle states that no two identical fermions can occupy exactly the same quantum state in an atom. At a given energy level, there are