Nuclear Force

Proton – Nucleon and Neutron – Nucleon

Interactions

Nuclear forceBut why does this happen? All particles are surrounded by force fields. A proton exerts both electrostatic force and strong force (nuclear force), but a neutron exerts only strong force and does not feel any electrostatic interaction, because it is not carrying charge.

The nuclear force is a force far stronger than electromagnetic forces, but it has a very short range of action. In fact, it acts only on distances of the order of the radius of an atomic nucleus (about 10-14 m = fm).

Neutron

Nuclear force field

Electric field

Proton

Nuclear Potential EnergySo there will be a potential well very near the centre of a nucleon and when another nucleon approaches that well it will fall in it (if the energetic conditions are right).

It’s a bit like playing golf:

• if a proton approaches another proton the hole is at the top of a hill and a golf ball shot with too little velocity will lose KE and gain PE as it climbs up and fall back down.

Proton – Proton interaction

Nuclear Potential Energy• if the KE of the ball is sufficient, the ball will reach the top of the hill and fall in the hole.

• In other words, if the proton has enough KE to enter the nuclear force field, it will be trapped by the nuclear force and fall in the nuclear potential well.

Proton – Proton interaction

Nuclear Potential Energy• if the approaching nucleon is a neutron, things are a lot easier, because the potential well will be in a “flat surface”. It doesn’t matter if it is approaching a proton, because it will not feel its electrostatic field.

• However, if the neutron is too fast (high KE), it will go past the potential hole. This is normally the case, because nucleons outside their nuclei have usually very high KE as it takes a lot of energy to separate them from the nucleus in the first place.

Neutron – Proton, or Neutron - Neutron interaction

Nuclear Potential Energy• However, it is possible to slow the neutron down, so that the nuclear force will grab it and cause it to fall in the potential well. This method is used in nuclear power stations and nuclear bombs to start a chain reaction.

Neutron – Proton, or Neutron - Neutron interaction