Nuclear force

The nuclear force is a powerful force that holds particles called nucleons together inside the tiny centers of atoms, known as atomic nuclei. Nucleons are made up of protons and neutrons. Even though protons all have a positive charge and normally push each other away, the nuclear force is so strong at very close distances that it can overcome this push and hold the nucleus together.

This force works best when nucleons are about 0.8 femtometres apart, which is a distance one million times smaller than the width of a hair. If the nucleons get any closer than 0.7 femtometres, the force suddenly pushes them apart instead, which helps decide how big atomic nuclei can get. Because of this, atoms are much larger than their nuclei — about five times bigger!

The nuclear force is very important for energy. When protons and neutrons come together to form a nucleus, some of their mass is turned into energy. This energy can be released in nuclear power plants or used in nuclear weapons. Scientists describe this force using special equations that match what they see in experiments, helping us understand how tiny particles behave inside atoms.

Description

The nuclear force is a special power that works between tiny particles called hadrons, especially between protons and neutrons in atoms. These particles are influenced almost equally by this force. Even though protons push each other away because they have a positive charge, the nuclear force can pull them together when they are very close.

At very short distances, the nuclear force pushes the particles apart to keep them a certain distance from each other. But when the particles are a little farther apart, the force pulls them together strongly. This pulling force is what helps keep the nucleus of an atom together, even though the protons want to push each other away. The strength of this force changes depending on how the particles spin and how close they are to each other.

History

The nuclear force has been important in nuclear physics since 1932, when the neutron was discovered. Scientists have worked to understand how protons and neutrons stick together in the nucleus.

Soon after the neutron's discovery, scientists like Werner Heisenberg suggested models to explain how protons and neutrons work together. In the 1930s, Hideki Yukawa tried to explain the nuclear force using tiny particles called mesons. Later, scientists made more detailed models to describe how the nuclear force works.

As a residual of strong force

The nuclear force is a weaker effect of a more powerful force called the strong force. The strong force is what holds tiny particles named quarks together to make up protons and neutrons. This strong force is one of the basic forces of nature and works through particles called gluons. Gluons keep quarks together using something called colour charge, which is like electric charge but much stronger. While quarks and gluons stay inside protons and neutrons, some of their effects reach just beyond, creating the nuclear force.

This nuclear force between protons and neutrons is similar to forces between neutral atoms in chemistry, known as London dispersion forces. These forces are weaker and work over shorter distances than the forces that hold atoms together. In the same way, the nuclear force is much weaker than the strong force inside protons and neutrons, and it only works over very short distances. However, it is strong enough to hold neutrons and protons together in the nucleus, even though protons would normally push each other apart because of their similar electric charge.

Nucleon–nucleon potentials

Two-nucleon systems such as the deuteron, the nucleus of a deuterium atom, as well as proton–proton or neutron–proton scattering are great for studying the special force between nucleons. Scientists describe these systems by giving a special "potential" to the nucleons and using these potentials in a math equation. The shape of the potential comes from experiments, and for longer-range actions, theories about meson-exchange help build the potential. The details of the potential are found by matching them to real experiment results.

The most used potentials for nucleons are the Paris potential, the Argonne AV18 potential, the CD-Bonn potential, and the Nijmegen potentials. A newer way is to use special math theories to describe the forces between nucleons and groups of three nucleons. One of these theories, called Quantum hadrodynamics, works like other theories that describe different forces in nature. Another theory looks at how nucleons interact using particles called pions.

From nucleons to nuclei

The big dream in nuclear physics is to explain all the forces inside nuclei by starting with the basic forces between nucleons. This is called the "microscopic" way of studying nuclear physics. There are two big challenges: doing very hard math for many particles at once, and including forces that involve three nucleons together.

This area is always changing, with new math methods helping scientists understand the structure of nuclei better. Scientists have used these methods for small nuclei with up to 12 nucleons.

Nuclear potentials

Another way to understand nuclear forces is to make one potential for the whole nucleus instead of each nucleon. This is called the "macroscopic" way. For example, when neutrons bounce off a nucleus, scientists can think of the nucleus as having a certain potential, like how light behaves when it hits a glass ball.

Nuclear potentials can be "local" or "global." Local potentials work only for a small range of energies or nucleus sizes, while global potentials can be used for more different situations but are usually less exact.