Strong Nuclear Force (AQA A Level Physics): Revision Note

Strong nuclear force

• 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

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 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

• 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