Topic 4: Quanta to Quarks

1.1.1 Discuss the structure of the Rutherford model of the atom, the existence of the nucleus and electron orbits

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Unexplained Phenomena: According to Maxwell, accelerating electrons emit electromagnetic radiation so the electrons should lose energy and spiral into the nucleus.

1.1.2 Analyse the significance of the hydrogen spectrum in the development of Bohr’s model of the atom

If gases at low pressure are excited by either intense heat or by applying a high voltage to a ‘gas discharge tube, then radiation is emitted only at discrete frequencies, producing a line spectrum rather than a continuous one.

If light with a continuous spectrum is passed through a gas, an absorption spectrum is produced, with inverted colour scheme

With Hydrogen’s spectrum, Rutherford’s model could not explain the spectral lines so another model (Bohr’s) was sought.

1.1.3 Define Bohr’s postulates

  1. Electrons exist in fixed non-radiating orbits
  2. Electrons only emit energy by ‘quantum jumps’ from one stationary state to another (producing discrete line emission spectra)
  1. The radiated photon is equal to the difference in energy between one stationary state and the next (E2 - E1 = Change in E = hf)
  1. Angular momentum (L = mvr) of electrons is quantised and can only take values of n(h/2pi) where n is an integer called quantum number.

1.1.4 Discuss Planck’s contribution to the concept of quantised energy

1.1.5 Describe how Bohr’s postulates led to the development of a mathematical model to account for the existence of the hydrogen spectrum.

 

1.1.6 Discuss the limitations of the Bohr model of the hydrogen atom

and

1.2.4 Analyse secondary information to identify the difficulties with the Rutherford-Bohr model

Failure to explain:

1.2.1 Perform a first hand investigation to observe the visible components of the hydrogen spectrum

1.2.2 Process and present diagrammatic information to illustrate Bohr’s explanation of the Balmer series

https://upload.wikimedia.org/wikipedia/commons/thumb/b/b2/Hydrogen_transitions.svg/400px-Hydrogen_transitions.svg.png

Balmer series refers to transitions from the 2nd energy level. (n = 2)

1.2.3 Solve problems using the Rydberg equation

2.1.1 Describe the impact of de Broglie’s proposal that any kind of particle has both wave and particle properties

2.1.2 Define diffraction and identify that interference occurs between waves that have been diffracted

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2.1.3 Describe the confirmation of de Broglie’s proposal by Davisson and Germer

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2.1.4 Explain the stability of the electron orbits in the Bohr atom using de Broglie’s hypothesis

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2.2.1 Solve problems and analyse information using

2.2.2 Assess the contribution made by Heisenberg and Pauli to the development of atomic theory

And

3.1.6 Discuss Pauli’s suggestion of the existence of neutrino and relate it to the need to account for the energy distribution of electrons emitted in beta decay

Heisenberg:

Pauli:

3.1.1 Define the components of the nucleus (protons and neutrons) as nucleons and contrast their properties

Nucleons: Particles that make up the nucleus (i.e. protons and neutrons)

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3.1.2 Discuss the importance of conservation laws to Chadwick’s discovery of the neutron

neutrons equation

3.1.3 Define the term ‘transmutation’

Transmutation: Any transformation of the nucleus (any nuclear reaction)

3.1.4 Describe nuclear transmutations due to natural radioactivity

Alpha: Large number of protons and neutrons emits an alpha particle

Beta +: Neutron decays to create a proton and an electron and a neutrino

Beta -:

Gamma:  Transmutation Nucleus can be in an excited state and emits a photon and drops to a lower E state.

3.1.5 Describe Fermi’s initial experimental observation of nuclear fission

When Fermi bombarded Uranium -235 with neutrons he was puzzled when he found many unidentified products were produced (before he had only seen nuclear fusion which produced transuranic elements).

nuclear fission

3.1.6 Discuss Pauli’s suggestion of the existence of the neutrino and relate it to the need to account for the energy distribution of electrons emitted in beta decay

Neutrino existence was suggested because in beta decay nuclear reactions:

To explain this missing energy Pauli proposed the existence of a new particle that

3.1.7 Evaluate the relative contributions of electrostatic and gravitational forces between nucleons

and

3.1.8 Account for the need for the strong nuclear force and describe its properties

The electrostatic force is way stronger than the gravitational force inside the nucleus so there must be a stronger force.

Properties:

Force                                        Strength                                Range

Gravitational

Weak attractive force between nucleons

Long

Electrostatic

Strong repulsive force between protons

Long

Nuclear Force

Strong attractive

Very short

3.1.9 Explain the concept of a mass defect using Einstein’s equivalence between mass and energy

Nuclear Binding Energy: Energy required to disassemble a nucleus into free unbounded neutrons and protons.

E=MC2 states that an energy has a mass

Because the nucleons in a nucleus are attracted to each other they are at a lower potential energy together than when they are apart.

The mass of any nucleus is smaller than the sum of the masses of its constituent protons and neutrons

Mass Defect: Sum of parts of proton and neutrons - measured mass of nucleus

Binding Energy = Mass defect x c2

3.1.10 Describe Fermi’s demonstration of a controlled nuclear chain reaction in 1942

3.1.11 Compare requirements for controlled and uncontrolled nuclear chain reactions

Criticality Factor (K) - Reproduction Factor

Control rods suck out all the neutrons to control the reaction - typically made out of cadmium or boron.

3.2.1 Perform a first-hand investigation to observe radiation emitted from a nucleus using Wilson’s Cloud Chamber
Wilson’s Cloud Chamber: An early detector used in particle physics

The radiation ionises the vapour and creates a seed upon which vapour can condense.

3.2.2 Solve problems to calculate the mass defect and energy released in natural transmutation and fission reactions

Mass defect: Mass of atom - mass of nucleons
Binding Energy: = Mass defect * c2 (via Einstein’s E = MC2). If in amu look at data sheet.

4.1.1 Explain the basic principles of a fission reactor

Parts:

4.1.3 Describe how neutron scattering is used as a probe by referring to the properties of neutrons.

Neutron Scattering utilises the wave characteristics of neutrons to study the internal structure and properties of matter.

4.1.4 Identify ways by which physicists continue to develop their understanding of matter, using accelerators as a probe to investigate the structure of matter

Particle Accelerators collide high energy particles to:

This can further our knowledge of the subatomic (forces and composition) in order to predict new particles and make predictions about cosmological theories.

Cyclotron:

 

Linear Accelerator:

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

Standard Model: Everything can be broken down into matter and forces

Fermions

Protons (2 up 1 down)

Neutrons (2 down 1 up)

Bosons

4.2.1 Analyse information to assess the significance of the Manhattan Project to Society

Advantages

Disadvantages

4.1.2 Describe some medical and industrial applications of radioisotopes

and

4.2.2 Describe the use of isotopes in:

Medicine: Cobalt-60 a gamma emitter so it can kill cells in a tumor leading to a reduction in cancer. In addition it can sterilize surgical equipment to prevent infection.

Engineering: Cobalt-60 a gamma emitter which has high penetrating power and so can be used for thickness control and also for metal fatigue inspection when used with photo film.

Agriculture: Cobalt-60 is a gamma emitter and so can sterilize food to increase shelf life. Its long half life of 5.3 years means it doesn’t need replacement often and the same emitter can be used for multiple sterilisations.

Medicine: Tc-99m: Technetium-99m is a very commonly used radioisotope in medicine. This radioisotope has a short half-life of about six hours which minimises harm to the body. It decays from Te-99m to Te-99 by emitting a gamma ray, which can be detected by PET and other medical scanners. Te-99m is injected into patients’ bodies as a liquid solution where it will flow through the bloodstream, releasing gamma radiation as it does. This radiation gives a clear picture of the tissue structures inside the patient’s body.

Agriculture: P-32: a phosphate solution containing radioactive P-32 is injected into the root system of a plant. Chemically it behaves identically to P-31 that plants normally use in their biological processes, and its movement can be detected by a Geiger counter. By observing the movement of P-32 through the plant, scientists can determine the metabolic rate of plants and determine whether certain factors can affect this rate. E.g. this may be useful in researching the effect of certain fertilisers on certain crops. P-32 has a half life of 14 days and emits beta rays which means that it can be easily detected but disappears relatively quickly.
Engineering: Co-60: Cobalt-60 is used to detect stress fractures in metals, particularly in aircraft. Stress fractures occur when metals are repeatedly exposed to strong forces, such as those experienced by the wings of an aircraft. Small fractures can form which can eventually result in catastrophic failure. These fractures are extremely hard to detect, because they can occur inside a solid piece of metal, and are often extremely small. By placing cobalt-60 on one side of the metal, and a gamma detector on the other side (often photographic film), the cracks can be identified easily and non-destructively