From the Universe to the Atom | HSC Physics Study Guide Part 4
The last module in Year 12 Physics brings everything from the previous modules together and covers everything from, well…the Universe to the Atom. This module expands further into modern physics and builds on quantum mechanics and relativity, introducing nuclear physics, astrophysics and cosmology. It’s a fascinating module, but it’s also important to understand the details of these new complex ideas clearly.
The flowcharts below show how these ideas are clearly structured in the Matrix Year 12 Physics Theory Books.
1. The Discovery and Characterisation of the Electron
One of the most important advances students learn about in this module is the discovery and characterisation of the electron. Following its discovery by J. J. Thompson, Robert Millikan and Harvey Fletcher determined the charge and mass of the electron. The process is outlined below:
2. The Development of the Nuclear Model of the Atom
Following the discovery of the electron Thompson proposed them plum pudding model of the atom. However, this failed to explain how alpha particles scattered from thin gold foil.Ernest Rutherford developed the nuclear model of the atom to explain these experimental results:
3. Atomic Absorption and Emission Spectra
An unexplained experimental observation at the time were atomic absorption and emission line spectra: each element absorbed or emitted only specific frequencies of light. This phenomenon was eventually explained with the Bohr model of the atom, which proposed that electrons exist only in certain stableorbits. Bohr used this model to explain the spectrum of hydrogen.
4. The Wave Nature of Matter
Following Einstein’s proposal of the photon model and the dual wave-particle nature of light, Louis de Broglie suggested that all matter has wave-like properties. These properties explained the stability of the Bohr model, and were extended upon by Schrödinger in the quantum model of the atom:
5. Atomic Fusion and Atomic Fission
Fusion describes two nuclei fusing together to form a larger nucleus.Fission is the opposite process – a large nucleus splits into two or more daughter nuclei and other decay products. Interestingly, both of these processes may release energy, depending on the binding energy of the reactants and products. Typically, the fusion of small nuclei (atomic number less than iron) will release energy, as will the fission of large nuclei (atomic number greater than iron).
6. Element Formation in Stars
All elements in the universe (except hydrogen and some helium) were formed through fusion reactions inside stars. Elements lighter than iron can be formed through fusion reactions during the life of a star, as fusion of these elements will release energy. As the fusion of heavier elements requires a net energy input, elements heavier than iron can only be formed in supernovae, when larger stars explode. These incredibly energetic explosions provide the energy required for heavier nuclei to fuse. The flowchart below outlines how elements are formed in such a large star:
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