Nuclear Forces

Student Expectation

The student is expected to describe evidence for and effects of the strong and weak nuclear forces in nature.

Key Concepts

• • Nuclear forces only matter inside atomic particles and/or within the atom. Their effect becomes weak very quickly as the distance increases.The effect of nuclear forces is always attractive.

  • The weak nuclear force is responsible for changing particles into other particles as evidenced by beta decay.

  • The strong nuclear force builds particles and holds atom nuclei together. This is evidenced by the protons in the nucleus which repel each other and yet stay together because of the strong nuclear force.

NUCLEAR FORCES

• • • • THE STRONG NUCLEAR FORCE

        • Evidence for the Strong Nuclear Force in Nature: Nuclear forces, the strongest of the four fundamental forces, only occur within very short distances and their effect becomes weak very quickly as the distance increases. For instance, if the distances are larger than 0.7fm (femtometer), the forces between nucleons are attractive. If the distance keeps increasing, the nuclear forces would decrease dramatically. Furthermore, if the distance between the nucleons is more than 2.0 fm, the forces can be considered as zero since they are really small. The picture beneath illustrates the relation between the nuclear forces and the related distances.

          • The dashed line indicates that the distance between nucleons approaches zero, but could never be exactly zero. (The picture above is not drawn to the scale.) Strong nuclear forces build particles and hold atom nuclei together. The strong nuclear force is evidenced by the protons in the nucleus which repel each other and yet stay together.

          • Other evidence of the strong nuclear force is called the mass defect. The mass of a stable nucleus is always less than the sum of the masses of the protons and the neutrons that make up the nucleus. By this, we mean the mass that each proton or neutron would have if it existed separately, outside a nucleus. The difference between the mass of the assembled and unassembled nucleus is the mass defect. Some of the mass of the protons and neutrons was converted into energy.

          • The Effects of the Strong Nuclear Force: We know that the nucleus is formed by protons and neutrons. The neutrons carry no electrical charge and thus are neutral. The protons have positive electric charges. The protons and neutrons are not the fundamental particles. These nucleons are made up of quarks. The fundamental force, which is called strong nuclear force, is ‘carried’ by particles called gluons; that is, when two particles interact through the strong force, they do so by exchanging gluons. Thus, the quarks inside of the protons and neutrons are bound together by the exchange of the strong nuclear force. The residual effect of the strong nuclear force yields the stronger attractive force to overcome those electromagnetic repulsions between the protons and keeps the nucleus together. Thus, the final effects of nuclear forces are always attractive, although nuclear forces only occur inside atomic particles and/or within the atom.

        • THE WEAK NUCLEAR FORCE

          • Evidence for the Weak Nuclear Force: Weak nuclear forces are responsible for changing particles into other particles as evidenced by beta decay (β decay). Beta decay is a kind of radioactive decay. During the process of this radioactive decay, an electron or a positron would be emitted from the nucleus resulting in two kinds of beta decay by the weak nuclear forces.

            • • Beta Plus Decay: When a proton is converted into a neutron while releasing an electron, neutrino, and a positron emission, this radioactive decay is called beta plus (β+).

              • Beta Minus Decay: Beta decay that produces an electron emission is referred to as beta minus (β-). For instance, let’s take a look at how carbon-14 decays into nitrogen-14 through β minus decay:

          • During the carbon-14 decay, the mass number is not changed, but the atomic number will increase by one, to nitrogen-14. Thus, the particle element will be changed to another type. Meanwhile, an electron antineutrino will be emitted as well.

        • Effects of the Weak Nuclear Force

          • Calculations have shown that the weak nuclear and electromagnetic forces are two versions of the same force, which become identical at extremely high temperatures. For this reason, these two forces are sometimes referred to jointly as the electroweak force. The effects of the weak nuclear force is the radioactive decay of certain elements that have been harnessed for use in science and medicine. Carbon-14 is used in dating materials. Other radioactive isotopes are used in cancer treatments.