9

9.8 Option — From Quanta to Quarks

Contextual Outline

In the early part of the twentieth century, many experimental and theoretical problems remained unresolved. Attempts to explain the behaviour of matter on the atomic level with the laws of classical physics were not successful. Phenomena, such as black-body radiation, the photoelectric effect, the emission of sharp spectral lines by atoms in a gas discharge tube, could not be understood within the framework of classical physics.

Between 1900 and 1930, a revolution took place and a new more generalised formulation called quantum mechanics was developed. This new approach was highly successful in explaining the behaviour of atoms, molecules and nuclei. As with relativity, quantum theory requires a modification of ideas about the physical world.

Outcomes

This module contributes to the following course outcomes:

A student:

H1 evaluates how major advances in scientific understanding and technology have changed the direction or nature of scientific thinking

H2 analyses the ways in which models, theories and laws in physics have been tested and validated

H5 identifies possible future directions of research in physics

H6 explains events in terms of Newton‘s Laws, Law of Conservation of Momentum and relativity

H7 explains the effect of energy transfers and transformations

H8 analyses wave interactions and explains the effects of those interactions

H9 explains the effects of electric, magnetic and gravitational fields

H10 describes the nature of electromagnetic radiation and matter in terms of the particles

H11 justifies the appropriateness of a particular investigation plan

H12 evaluates ways in which accuracy and reliability could be improved in investigations

H13 uses terminology and reporting styles appropriately and successfully to communicate information and understanding

H14 assesses the validity of conclusions drawn from gathered data and information

H15 explains why an investigation is best undertaken individually or by a team

H16 justifies positive values about and attitudes towards both the living and non-living components of the environment, ethical behaviour and a desire for critical evaluation of the consequences of the applications of science.

1. Problems with the Rutherford model of the atom led to the search for a model that would better explain the observed phenomena

Students learn to:

• discuss the structure of the Rutherford model of the atom, the existence of the nucleus and electron orbits

• analyse the significance of the hydrogen spectrum in the development of Bohr’s model of the atom

• discuss Planck’s contribution to the concept of quantised energy

• define Bohr’s postulates

• describe how Bohr’s postulates led to the development of a mathematical model to account for the existence of the hydrogen spectrum:

• discuss the limitations of the Bohr model of the hydrogen atom

Students:

• perform a first-hand investigation to observe the hydrogen spectrum

• process and present diagrammatic information to illustrate Bohr’s findings with the Balmer series

• solve problems and analyse information using:

• identify data sources, gather process and analyse secondary information to identify the difficulties with the Rutherford-Bohr model, including its inability to completely explain:

– the spectra of larger atoms

– the relative intensity
of spectral lines

– the existence of hyperfine
spectral lines

– the Zeeman effect

2. The limitations of classical physics gave birth to quantum physics

• describe the impact of De Broglie’s proposal that any kind of particle has both wave and particle properties

• describe the confirmation of De Broglie’s proposal by Davisson and Germer

• explain the stability of the electron orbits in the Bohr atom using De Broglie‘s hypothesis

• solve problems and analyse information using:

• gather, process, analyse and present information and use available evidence to assess the contributions made by Heisenberg and Pauli to the development of atomic theory

3. Today, quantum physics is used in a range of technologies, including electron microscopes

• outline the application of the wave characteristics of electrons in the electron microscope

• discuss the relationships in electron microscopes between the electrons, magnetic lenses and refraction

• process and analyse information to compare the resolving powers of light and electron microscopes and assess the impact of their development

4. The work of Chadwick and Fermi in producing artificial transmutations led to practical applications of radiation

Students learn to:

• identify the importance of conservation laws to Chadwick’s discovery of the neutron

• define the contents of the nucleus (protons and neutrons) as nucleons and contrast their properties

• define the term ‘transmutation’

• describe Fermi’s first experimental observation of nuclear fission and his demonstration of a nuclear chain reaction

• identify that Pauli’s suggestion of the existence of neutrino is related to the need to account for the energy distribution of electrons emitted in ß-decay

• describe nuclear transmutations due to natural radioactivity

• evaluate the relative contributions of electrostatic and gravitational forces between nucleons

• account for the need for the strong nuclear force and describe its properties

• explain the concept of a mass defect using Einstein’s equivalence between mass and energy

• compare requirements for a controlled and uncontrolled nuclear chain reaction

Students:

• gather and analyse data to assess the impact of Pauli’s suggestion of the neutrino on Fermi’s work

• identify data sources, and gather, process, and analyse information to describe the use of a specific named isotope in:

– medicine

– agriculture

– engineering

• solve problems and analyse information to calculate the mass defect and energy released in a fission reaction

• analyse information and use available evidence to assess how Chadwick’s and Fermi’s work changed understanding of the atom

5. An understanding of the nucleus has led to large science projects and many applications

Students learn to:

• explain the basic principles of a fission reactor

• describe some medical and industrial applications of radio-isotopes

• explain why neutron scattering is used as a probe by referring to the properties of neutrons

Students:

• gather, process and analyse information to assess the significance of the Manhattan Project to society

• perform a first-hand investigation to determine
the penetrating power of alpha, beta and gamma radiation on range of materials

6. Our attempts to understand the structure of matter is an ongoing process

• identify the ways by which physicists continue to develop their understanding of matter, including:

– the use of accelerators
as a probe to investigate the structure of matter

      –    the key features and
      components of the
      standard model
      of matter, including
      quarks and leptons

      –    the links between high
      energy particle physics
      and cosmology

• analyse information to assess the impact of advances in the understanding of matter
on the work of physicists