Nuclear Force

In the context of nuclear fusion energy, the nuclear force refers to the short range force that binds the nucleons in atomic nucleus together. This allows the "like" charged protons (positive charge) to be tightly bound to the atomic nuclei, overcoming the electromagnetic Coulomb forces that would act to repel these particles.

The strength of this residual force varies with the separating distance between nucleons. It is repulsive at very low separation distances between nucleons (same as the Coulomb force), but attractive at higher distances (opposite direction to the Coulomb force). The magnitude of the force decays to a negligible amount as the distance increases, and Coulomb forces dominate at longer distances. This also contributes to the stability of large nuclei being poorer than smaller nuclei, but there are nuances to this, as covered in the section on Binding energy.

In short, the Coulomb forces dominate over the long range compared to the Nuclear force and vice versa.

Origin of the nuclear force:

While this level of abstraction may be sufficient for studying the physics of nuclear fusion with the aim of commercial energy generation, the "nuclear force" is in reality a manifestation of more fundamental forces and subatomic particle interactions.

The protons and neutrons of a nucleus are, in turn, composed of more fundamental particles called "quarks". The neutron is made of one "Up" quark and two "Down" quarks. The proton is made of two "Up" quarks and one "Down" quark. The force, which helps bind quarks together, is one of the four fundamental forces in physics, and is called the “strong force”. This is within the domain of "Quantum Chromodynamics" (or "QCD"), which provides a framework to model gluons, the force particles mediating the strong force.

This strong force interaction between the quarks within a nucleon is dissimilar to the more intuitive forces we are used to (like electromagnetism or gravity), in that it acts a bit like a rubber band (called a quark "gluon flux tube"), with the force staying strong, roughly constant, as the quarks move further apart, so the energy stored in the tube keeps growing. However, when it gets too far apart, the stored energy becomes enough to create a new quark & anti-quark pair, and the tube "snaps" into two. This is why a lone quark is never observed. A bound quark & anti-quark pair is called a meson. Separately, the attraction between nucleons (proton or neutron) can be modelled as the exchange of virtual mesons, mainly pions, between them. This explains the phenomenon of nucleons in an atom being bound together (overwhelming the Coulomb force, where there are multiple protons). This residual effect of the underlying fundamental force (strong force) is the nuclear force. Therefore, the nuclear force is also called the "residual strong force" or the "strong nuclear force".

Some sources [3] have remarked this to be analogous to a "residual colour Van der Waals force", similar to the "Van der Waals force" that describes the residual Coulomb forces of charge-neutral molecules. The nucleon-nucleon interactions are still an area of active research as they have not been calculated in an exact form (especially limited with unstable nuclei) and instead approximate approaches are used, which rely on comparisons to data for validation [4].

This is a crude simplification, and QCD currently provides the most comprehensive framework that describes these interactions. See the videos from PBS below for an overview of QCD and the nuclear force.

Relevant external content:

The PBS Space Time channel on YouTube provides a great overview of QCD and the nuclear force.