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 separating distance between nuclei. It is repulsive at very low separation distances between nuclei (same as 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 atoms being poorer than smaller atoms, 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 holding quarks together is possible through the mediation of force particles called gluons. This is within the domain of Quantum Chromodynamics (or QCD), which provides a framework to model the gluons mediating the strong force. This force, which binds quarks together, is one of the four fundamental forces in physics, and is called the strong force.

This strong force interaction between the quarks within a nucleon is dissimilar to 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 being stronger when the quarks mover further apart. However, when it gets too far apart (due to quantum fluctuations, especially when nucleons are very close together), a virtual particle (like a quark & anti-quark pair, but there are other carriers too) can be created, which is called a meson. The exchange of mesons between nucleons (proton or neutron) can be used to explain the phenomenon of nucleons in an atom being bound together (overwhelming the Coulomb force, where there are multiple protons). This this is residual effect of the underlying fundamental force (strong force). Therefore, the nuclear force is also called residual strong force or as 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 nuclear force.