Nuclear Forces | Nuclear Forces Definition

 

Nuclear Forces | Nuclear Forces Definition :- In previous article Binding Energy Per Nucleon we have seen that for average mass nuclei the binding energy per nucleon is approximately 8 MeV, which is very large. Therefore, a question came to the minds of scientists that what force holds the nucleons inside the nucleus of very small size (~ 10^–15 m).

This force cannot be gravitational force, because calculations showed that coulomb’s repulsive force between two protons is 10³⁶ times greater than the attractive gravitational force between them. So there must be a strong attractive force of a totally different kind.

Then to account for the stability of a nucleus, a new force called nuclear force was postulated by a Japanese physicist Yukawa. This nuclear force was assumed to be strong attractive force and its magnitude is greater than the coulomb’s repulsive force. Yukawa predicted that the nuclear forces arise due to the exchange of particles known as π-mesons between the nucleons.

Nuclear Forces Definition

Nuclear forces are the strong forces of attraction which holds together the nucleons in the tiny nucleus of an atom, inspite of strong electrostatic forces of repulsion between protons.

 

Properties of Nuclear Forces

1. Nuclear force is charge independent :- The interaction between two nucleons is independent of whether one or both nucleons have charge on them. In other words, the nuclear force between proton-proton (p–p), proton-neutron (p–n) and neutron–neutron (n–n) is same, so these force are charge independent.
2. Nuclear force is the strongest force in nature :- The magnitude of nuclear force is 100 times the electrostatic repulsive force and 10³⁸ times the gravitational attractive forces. Hence nuclear forces are strongest forces in nature.
3. Nuclear force is short range force :- The nuclear forces between two nucleon exist only when the distance between nucleons is comparable to the size of nucleus(~ 10^–15 m). These forces cease to act as the distance between two nucleons exceeds 10^–15 m. Moreover, a nucleon can interact with only its neighboring nucleons just as an atom in solid form bonds only with the surrounding atoms. Thus, these forces are short range forces and act upto distances of the order of a few fermi.
4. Nuclear force is a saturated force :- A nucleon can attract only the nearest neighbors and has no influence on other nucleons. Thus, nuclear forces are saturated forces.
5. Nuclear force is non-central force :- The force between two nucleons does not act along the line joining their centres and therefore called non-central force.
6. Nuclear force is an exchange force :- Nuclear forces are due to the exchange of π-mesons between the nucleons, so they are called exchange forces.
7. Nuclear force is spin dependent :- It has been observed that the nuclear force between nucleons having parallel spins is greater than the force between nucleons having anti-parallel spins. Thus they are spin dependent.
8. Nuclear force do not obey inverse square law :- Unlike Coulomb’s law or the Newton’s law of gravitation there is no simple mathematical form of the nuclear force.
9. Nuclear force has a small component of repulsive force :- The variation of nuclear force with distance (r) between the nucleons is shown in figure below :

From the above graph we note that :-

• Nuclear forces are negligible, when distance between nucleons is more than 10 fermi.
• When nucleons are brought closer, nuclear force goes on increasing rapidly with decreasing distance.
• When distance between nucleons become less than 0.8 fermi, the nuclear force becomes strongly repulsive. Hence nuclear force has a small component of repulsive force. The variation of potential energy between a pair of nucleons with distance (r) is shown below :-

In the above graph :-

1.  At r = 0.8 fermi, Nuclear force is zero and P.E. is minimum.
2. For r > 0.8 fermi, Nuclear force is negative(attractive) and Negative P.E. goes on decreasing.
3. For r < 0.8 fermi, Nuclear force is positive(repulsive) and Negative P.E. decreases to zero and becomes positive.