The strong nuclear force is a short-range attractive force between nucleons namely, protons and neutrons, that binds the atomic nucleus together. In IB Physics E.3, this concept is required at both SL and HL; evidence for the force is an Additional Higher Level extension.
How Does It Hold a Nucleus Together?
Protons have positive electric charge, so the electrostatic force makes them repel one another. A stable nucleus is possible because, at the very small separations between nucleons, the strong nuclear force is sufficiently strong to overcome this electrostatic repulsion.
The force acts between proton-proton, proton-neutron, and neutron-neutron pairs. Unlike the electrostatic force, it has a very short range, comparable to nuclear dimensions, and becomes negligible beyond a few femtometres.
| Interaction | Effect inside the nucleus | Range |
|---|---|---|
| Strong nuclear force | Attracts nucleons and binds the nucleus | Very short |
| Electrostatic force | Repels protons | Long range |
| Gravitational force | Attracts nucleons but is negligible at nuclear scales | Long range |
Binding the nucleons produces a nucleus with less mass than the total mass of its separate nucleons. This mass defect corresponds to the nuclear binding energy through . Energy must therefore be supplied to separate a nucleus completely into its nucleons.
