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

Properties 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)

Comparison of Electrostatic and Strong Forces

• The graph below shows how the strength of the electrostatic and strong forces between two nucleons vary with the separation between them
  • The red curve represents the strong nuclear force between nucleons
  • The blue curve represents the electrostatic repulsion between protons

At separations between 0.5 and 3.0 fm, the attraction of the strong force is far more powerful than the repulsion of the electrostatic force

• The repulsive electrostatic force between protons has a much larger range than the strong nuclear force
  • However, it only becomes significant when the proton separation is more than around 2.5 fm

• The electrostatic force is influenced by charge, whereas the strong nuclear force is not
• This means the strength of the strong nuclear force is roughly the same between all types of nucleon (i.e. proton-proton, neutron-neutron and proton-neutron)
  • This only applies for separations between 0.5 and 3.0 fm (where the electrostatic force between protons is insignificant)

• The equilibrium position for protons, where the electrostatic repulsive and strong attractive forces are equal, occurs at a separation of around 0.7 fm