THE NUCLEUS

First discovered by Ernest Rutherford and his group of very young physicists. They bombarded very thin gold with alpha particles from the newly discovered radium source. Huge surprise when occasional alpha particles returned towards them. Rutherford showed that the pattern of deflections could be matched by assuming Coulomb's Law and a positive central charge on a tiny but massive centre.

Previous models of the atom were less than convincing with JJ Thomson's model of an atom the "best" of the lot. This model had the atom as a positive matrix with little negative electrons embedded in it. ( The "Plum Pudding" model. )

Rutherford's model technically shouldn't have worked according to Maxwell's Equations! Electrons are accelerating when in orbit hence should have been radiating! ( According to Maxwell's work, all electromagnetic radiation is a result of accelerating charges. Charges moving in circles DO radiate - called synchronous radiation - and is a by product of the big circular accelerators - and used in special XRay analysis. )

The size of a nucleus is a few femtometres fm ( 10^-15 m ) in diameter depending on the atom. At a simple level, it consists of merely protons and neutrons (discovered in 1932). At a more complex level, it seems to be a horrendous mess! Its density is about 10¹⁶ kgm^-3 which is huge.

It is POSITIVE so electrostatically it is unstable! Protons should blow apart! What is holding in the neutrons?

( Force of repulsion electrostatically - using Coulomb's Law F = k Q1Q2 /r² , dump in r ~ 10^-14m, k = 9 x 10⁹, Q = 1.6 x 10^-19 C

gives F ~ 2 N repulsion which is HUGE on things that size!

STRONG NUCLEAR FORCE ( STRONG INTERACTION )- necessary new force required to hold together nuclei. ( Note - some nuclei cannot be held together by this force - they are "unstable" and decay to something smaller. )

Properties

• Very short range ~ a few femtometres

• Acts equally on protons and neutrons, does not act on electrons

• Is ATTRACTIVE only

• Carried by VIRTUAL particles between nucleons.

• VERY STRONG - approximately 100 x electrostatic forces over size of nucleus ( this is the reason why the largest stable nuclei are about 100 protons)

It is now understood that this force is a manifestation of the deeper colour force carried by gluons between quarks.

Looking at the graph above of an approaching, very high energy proton, towards another proton, the proton slows as it approaches losing kinetic energy and gaining electrostatic potential energy. However, at about 3 ~ 4 fm, the strong nuclear force grabs the proton and drags it to the other proton. This immense force acting over a short distance does work and energy is released usually as gamma photons. ( Definitely not as sound! )

The Strong Nuclear Force reduces as the protons "strike" each other to give balanced forces with the electrostatic repulsion.

These photons are released at the expense of MASS of the nucleus! ( the old E = mc² trick! ).

Note, this is the process taking place in stars where the extraordinary high temperature and pressures allow protons to get near enough to each other for this fusion process to take place.