Strong Nuclear Force (AQA A Level Physics)

• In a nucleus, there are
  • Repulsive electrostatic forces between protons due to their positive charge
  • Attractive gravitational forces due to the mass of the nucleons

• Gravity is the weakest of the fundamental forces, so it has a negligible effect compared to the electrostatic repulsion between protons
• If these were the only forces acting, the nucleus would not hold together
• Therefore, there must be an attractive force acting between all nucleons which is stronger than the electrostatic force
  • This is known as the strong nuclear force

• The strong nuclear force acts between particles called quarks
• Protons and neutrons are made up of quarks, so the interaction between the quarks in the nucleons 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 below a separation of ~0.5 fm and attractive up to ~3.0 fm

• The key features of the graph are:
  • The strong force is highly repulsive at separations below 0.5 fm
  • The strong force is very attractive up to a nuclear separation of 3.0 fm
  • The maximum attractive value occurs at around 1.0 fm, which is a typical value for nucleon separation
  • The equilibrium position, where the resultant force is zero, occurs at a separation of about 0.5 fm

• In comparison to other fundamental forces, the strong nuclear force has a very small range (from 0.5 to 3.0 fm)