The Strong Nuclear Force (OCR A Level Physics): Revision Note

The Strong Nuclear Force

• In the nucleus, there are electrostatic forces between the protons due to their electric charge and gravitational forces due to their mass

• Comparatively, gravity is a very weak force and the electrostatic repulsion between protons is therefore much stronger than their gravitational attraction

• If these were the only forces, the nucleus wouldn’t hold together

• Therefore, the force that does hold the nucleus together is called the strong nuclear force

• The strong nuclear force keeps the nucleus stable since it holds quarks together

• Since protons and neutrons are made up of quarks, the strong force keeps them bound within a nucleus

Whilst the electrostatic force is a repulsive force in the nucleus, the strong nuclear force holds the nucleus together

Range of the Strong Nuclear Force

• The strength of the strong nuclear force between two nucleons varies with the separation between them

• This can be plotted on a graph which shows how the force changes with separation

The strong nuclear force is repulsive before a separation of ~ 0.5 fm and attractive up till ~ 3.0 fm

• The key features of this graph are that the strong nuclear force is:

  • Repulsive closer than around 0.5 fm

  • Attractive up to around 3.0 fm

  • Reaches a maximum attractive value at around 1.0 fm (the typical nuclear separation)

  • Becomes zero after 3.0 fm

• In comparison to other fundamental forces, the strong force therefore has a very small range (only up to 3.0 fm)