Editing Semi-empirical mass formula (section)

===Volume term===
The term <math>a_\text{V} A</math> is known as the ''volume term''. The volume of the nucleus is proportional to ''A'', so this term is proportional to the volume, hence the name.

The basis for this term is the [[Strong interaction|strong nuclear force]]. The strong force affects both protons and neutrons, and as expected, this term is independent of ''Z''. Because the number of pairs that can be taken from ''A'' particles is <math>A(A - 1)/2</math>, one might expect a term proportional to <math>A^2</math>. However, the strong force has a very limited range, and a given nucleon may only interact strongly with its nearest neighbors and next nearest neighbors. Therefore, the number of pairs of particles that actually interact is roughly proportional to ''A'', giving the volume term its form.

The coefficient <math>a_\text{V}</math> is smaller than the binding energy possessed by the nucleons with respect to their neighbors (<math>E_\text{b}</math>), which is of order of 40&nbsp;[[MeV]]. This is because the larger the number of [[nucleon]]s in the nucleus, the larger their kinetic energy is, due to the [[Pauli exclusion principle]]. If one treats the nucleus as a [[Fermi ball]] of <math>A</math> [[nucleon]]s, with equal numbers of protons and neutrons, then the total kinetic energy is <math>\tfrac{3}{5} A \varepsilon_\text{F}</math>, with <math>\varepsilon_\text{F}</math> the [[Fermi energy]], which is [[Fermi energy#Nucleus|estimated]] as 38&nbsp;[[MeV]]. Thus the expected value of <math>a_\text{V}</math> in this model is <math>E_\text{b} - \tfrac{3}{5} \varepsilon_\text{F} \sim 17~\mathrm{MeV},</math> not far from the measured value.