AQA A Level Physics

Revision Notes

2.1.3 Strong Nuclear Force

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

2.1.3Electrostatic-vs-Strong-Nuclear

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

2.1.3Strong-Nuclear-Force-Graph

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)

Exam Tip

  • You may see the strong nuclear force also referred to as the strong interaction
  • Remember to write that after 3 fm, the strong force becomes ‘zero’ or ‘has no effect’ rather than it is ‘negligible’.
  • Recall that 1 fm = 1 × 10–15 m

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