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Mechanics & Stress Analysis*
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  • A-Level Physics

Nuclear & Particle Physics

Initially, Thomson proposed the ‘plumb pudding’ model for atoms, saying the atom was made up of a blob of positive charge with negatively charged electrons sprinkled in and about this.

Rutherford’s Scattering experiment disproved this, by firing alpha particles at a gold foil and seeing where they emerged on a fluorescent screen. Most alpha particles passed straight through with very little deflection; however, some scattered a lot and others were reflected right back. 

Rutherford's scattering experiment, gold leaf alpha particle deflection., EngineeringNotes, Engineering Notes

This led to the conclusion that an atom was made up of largely empty space, with a very small, very dense, positively charged nucleus in the centre. This Nuclear Model is now widely adopted:

  • The nucleus is surrounded by orbiting electrons in shells

  • The nucleus is about one 10,000th of the size of the atom, but makes up most the mass

  • The protons and neutrons are around 2000 times more massive than the electron

  • Nuclear density is always ~1017 kg m-³; atomic density is far less

Isotopes are atoms with the same proton number but a different nucleon number – the different number of neutrons does not affect the chemical properties, however can make the atom unstable.

The radius of the nucleus is directly proportional to the cube root of the nucleon number:

R = r(0) x ³√A nuclear radius = r(0) x ³√nucleon number

r(0) can be taken as 1.4 E-15

The Four Forces

There are four forces that govern the laws of physics - the strong and weak nuclear, the electromagnetic, and gravitational.

Nuclear force diagram, electrostatic force diagram, nucleon forces diagram, nucleon interactions., EngineeringNotes, Engineering Notes

The nucleons are held together by the strong nuclear force, but as the nucleus gets bigger and the electrostatic repulsion outweighs it on the outside, the nucleus becomes unstable.

Strong Nuclear Force

  • Very short range: attractive to ~3 fm; repulsive to ~0.5 fm

  • Works equally between all nucleons