Physics 2009-2014

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Frequently Asked Questions

Unit 1 Area of Study 1: Nuclear physics and radioactivity

1. How should students use radiation weighting factors and tissue weighting factors?
   Students should convert absorbed dose to equivalent dose by multiplying the absorbed dose by the radiation weighting factor/s. (Note: 'Radiation weighting factor' is of similar meaning to the superseded term 'quality factor'.)
   Students should convert equivalent dose to effective dose by multiplying the equivalent dose by the tissue weighting factor/s.

2. Where can students find up-to-date values for radiation weighting factors and tissue weighting factors?
   The Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) is the Federal Government agency which is responsible for setting Australian radiation standards.  The Australian standards set by ARPANSA can be found by reading the pdf document Radiation Protection Series No. 1 (RPS 1) at http://www.arpansa.gov.au/Publications/codes/rps1.cfm Students should note that although RPS 1 (which is based on ICRP publication 60*) is still the basis of legislation in Victoria, ICRP publication 60 has been replaced by ICRP publication 103.  The latter publication contains different radiation weighting factors and tissue weighting factors from the former publication.  Victoria’s regulations “define” these two factors without specifying a source for the factors, as they are considered, legally, “terms of art”.  As such, the radiation and tissue weighting factors that apply in Victoria are the most up-to-date ones i.e. the ones contained in ICRP publication 103 (despite the fact that RPS 1 is based on ICRP publication 60).  *ICRP publication 60 contains the main recommendations of the International Commission on Radiological Protection, including radiation protection philosophy, radiation weighting factors, tissue weighting factors and dose limits.

 

Unit 3 Detailed study 3.1: Einstein's Special Relativity

1. How much detail is required in relation to the prediction from Maxwell’s equations about the speed of light?
   No mathematical detail is required.
   Students should understand that Maxwell used the laws of electromagnetism to deduce that electromagnetic waves would propagate through space at a speed related only to the electric and magnetic constants. This speed was the speed of light.
   As with other waves, it was expected that light would require a medium through which to travel. However, as light can travel through a vacuum this was an unknown medium (referred to as the aether) which would have to permeate the whole of space. This leads to a clash with the classical principle of relativity (see Frequently Asked Question 2).
   The purpose of Michelson and Morley’s famous experiment was to show whether or not there was an aether.

2. Which comparisons should students make between the predictions from Maxwell’s equations and the classical principle of relativity?
   Students should be aware that Maxwell’s equations implied that light should have an absolute velocity independent of the frame of reference and that this was in complete contradiction to the classical principle of relativity (also known as Galilean relativity). That principle implies that the measured speed of light must be dependent on the velocity of the observer through the medium in which the light is travelling.

3. In what way was Einstein’s first postulate an extension of the classical principle of relativity?
   According to classical relativity, the laws of physics were independent of the (inertial) frame of reference chosen. While this applied to mechanical phenomena, it did not appear to apply to electromagnetic phenomena when they were discovered. Einstein’s first postulate is not any different to that of classical relativity but he intended that it should apply to electromagnetic radiation, including light.
   Applying the principle to electromagnetic phenomena led Einstein to revise how inertial frames are related.  This led to the effects of special relativity, such as length contraction.

4. How did Einstein’s second postulate compare with classical physics?
   The second postulate was in complete contradiction to classical physics which held that the velocity of anything depended on the frame of reference in which it was measured.

5. To what extent should students know the details of the Michelson-Morley experiment?
   Knowledge of the details of the experimental setup is not required for the purposes of this Detailed study.
   It is sufficient that students know that the experiment attempted to measure the difference in the speed of light in two directions perpendicular to each other. They should understand that as Earth was considered to be moving through the hypothetical aether it was expected that there would be a small difference in the measured speed of light in the direction of Earth’s motion as compared to the direction perpendicular to the motion.
   The result of importance for students is that within the experimental errors no difference in the speed of light in the two different directions was found. This implied no support for the aether hypothesis but supported Einstein’s postulate that the speed of light would not depend on the motion of the observer.

6. What type of ‘thought experiments’ should students consider?
   Since it is not possible in the classroom to perform experiments with normal objects travelling at relative speeds close to the speed of light, students should understand that Einstein (and many people since) made extensive use of  ‘thought experiments’ to illustrate the implications of special relativity.
   These ‘thought experiments’ are often set in everyday situations such as trains travelling at impractically high speeds where we are able to observe (in our mind) and calculate aspects of phenomena such as time dilation and length contraction occurring to a significant degree.
   Examples of selected thought experiments will be available in the Physics section of the ‘VCE Studies and Resources’ link on the VCAA website. 

7. Are students required to mathematically derive time dilation and length contraction equations?
   No.
   While teachers will often use some form of mathematical derivation to illustrate the meaning of these equations, the mathematical details of the derivation is not required for the purposes of this Detailed study. Students should, however, understand that these expressions follow from Einstein’s two postulates.

8. Are students required to establish the total mass-energy relation from first principles?
   No.
   It is sufficient that students realise that Einstein showed that this expression was a consequence of his postulates. Students should understand that the two expressions E_tot = E_k + E_rest = mc² and E_k = (γ − 1)m_oc² are equivalent, as demonstrated through the manipulation E_k = (γ − 1)m_oc² = m_oγc² – m_oc² = mc² – m_oc² = E_tot – E_rest . In the expression m = m_oγ, students should refer to the m as the 'relativistic mass' (rather than just the 'mass').  The term 'mass' on its own usually refers to the 'rest mass'. At velocities significantly less than c the expression E_k = (γ − 1)m_oc² reduces to E_k = ½mv².

9. In what sense are mass and energy equivalent?
   Mass is another form of energy, and is part of the forms of energy listed in the eighth dot point of Unit 3, Area of Study 1, Motion in one and two dimensions, on page 28 of the Study design.
   In nuclear reactions, for example, energy can be transformed from one form of energy (e.g. ‘mass’ energy) to another form of energy (e.g. kinetic energy).

10. Why can’t an object with non-zero rest mass be accelerated to travel faster than the speed of light?
    Another consequence of Einstein’s postulates is that it is not possible to accelerate an object with non-zero rest mass to a speed greater than the speed of light. Conversely, every object with zero rest mass – such as a photon – is always travelling at the speed of light.
    Students should understand that if we apply a force to accelerate an object the energy we provide is transformed into the kinetic energy of the object. The kinetic energy of an object is given by E_k = (γ-1)m₀c², where as v approaches c, γ approaches infinity. So we would have to provide an infinite amount of energy to accelerate an object to the speed of light.

 

Unit 4 Detailed study 3.1: Synchrotron and its applications

1. What do students need to know about how a synchrotron produces electromagnetic radiation?
   Whenever an electric charge (in this case an electron) is accelerated, electromagnetic radiation is emitted. The greater the acceleration the more energetic is the emitted radiation.  Acceleration is the change of VELOCITY with time.  Thus if the electron changes speed or direction, radiation results.  In the case of synchrotron radiation, the radiation results from a change of direction due to the bending magnets, or the magnetic insertion devices.

2. Do students need to know Maxwell’s equations?
   No.

3. How does the production of synchrotron radiation differ from other methods of producing electromagnetic radiation?
   Electromagnetic radiation is produced in a number of ways.  These include the acceleration of charged particles by an electric and/or magnetic field, the transition of electrons from one energy level to another within an atom, the annihilation of a particle with its anti-particle. Synchrotron radiation is electromagnetic radiation produced by the acceleration of very fast moving electrons in magnetic fields (see FAQ 1). When electromagnetic radiation is produced by accelerating slow moving electrons (e.g. electrons in a radio transmitter) electromagnetic waves are emitted in ‘all directions’. However, relativistic effects result in synchrotron radiation being shaped into a narrow beam (a cone) along a tangent to the curved path of the electron.

4. What do students need to know about electric fields?Electric field (E) is a vector quantity. The direction of an electric field is given by the direction in which a positive charge (q) would move as a result of a force F=qE.
   The unit for electric field strength is Newton/coulomb, and an equivalent unit is Volt/metre (compare with N/kg and m/s² for gravitational field strength).
   For the special case of parallel plates separated by a distance (d), with a voltage (V) across them, a uniform electric field (E) is produced.  The strength of the electric field is given by the formula E = ΔV/d).

5. Which equations should be used to analyse electron acceleration?
   The acceleration of an electron results from the force given by F = qE. Alternative expressions for the force on a charge between parallel plates (F = qV/d), and for the gain in kinetic energy (ΔE_k = Vq) may be used in analysing electron acceleration specifically in an electron gun.

6. Why does the Study Design not include equations associated with the design of the Australian Synchrotron and the general purpose of the nominated components?
   Since the electrons are travelling at very close to the speed of light, a more complete description of the behaviour of electrons in the various electric and magnetic fields within the actual linac, booster and storage rings would require competence with the equations of the theory of Special Relativity. This is not required in this Detailed study.

7. Is radiation produced by the synchrotron always more suitable for experimental work than other radiation sources?Not necessarily.
   While for many experiments synchrotron radiation has characteristics that make it more suited than other radiation sources, this is not always true. In addition, synchrotrons are expensive to use and access can be limited so that many ‘everyday experiments’ are carried out using cheaper, more accessible sources.
   For the purposes of this Detailed study students are expected to limit the comparison of synchrotron radiation to radiation from conventional, readily available sources (i.e. laser light and X-ray tubes).

8. How is the radiation ‘tuned’ in a beamline?
   Synchrotron radiation has a very broad spectrum. While some regions of the spectrum can be filtered out, the end user usually wants a single wavelength with a very precisely known value. This selection process is called ‘tuning’.
   In the case of a beamline, this is done using a device called a ‘monochromator’, which selects a single X-ray wavelength.
   For the purposes of this Detailed study, students only need to know that this selection process can be achieved by diffraction.  The monochromator uses a precisely-cut crystal (often of silicon) and Bragg’s Law to select a particular wavelength (which emerges at a particular angle.

9. How does the process of the photoelectric effect differ for visible light and X-rays?
   As described by Einstein, the actual process of the photoelectric effect is identical for both UV light and X-rays. However, the wavelength of X-rays is significantly smaller than that of visible light. This means that long wavelength (lower energy) electromagnetic radiation (i.e. UV light) is suited to probe the outer shell electronic structure of atoms, while shorter wavelength (higher energy) electromagnetic radiation (i.e. X-rays) can be used to probe the inner shell electrons.