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5. 2015 HSC Notes from the Marking Centre – Physics

# 2015 Notes from the Marking Centre – Physics

## Introduction

This document has been produced for the teachers and candidates of the Stage 6 Physics course. It contains comments on candidate responses to the 2015 Higher School Certificate examination, highlighting their strengths in particular parts of the examination and indicating where candidates need to improve.
This document should be read along with:

## Section I

### Part B

Candidates showed strength in these areas:

• accurately plotting the datapoints (Q21a)
• correctly substituting into appropriate equations (Q21b and Q22a)
• clearly showing a process (Q21b)
• determining the magnitude of forces (Q22a)
• stating the function of the split ring commutator (Q22b)
• stating the reversal of current every half turn of the coil (Q22b)
• modelling hole flow and electron flow (Q23a)
• identifying a limitation (Q23b)
• identifying household devices that convert energy (Q25a)
• identifying that if voltage is increased, then current must decrease by the same ratio (Q25b)
• recognising that the values provided are not consistent with the transformer equation (Q25 b)
• recognising the variation of g with altitude (Q26a)
• expressing g as a constant in Model X (Q26a)
• identifying a change in gravitational potential energy as a result of the change in g (Q26b)
• equating Fg and Fc or using Kepler’s Law (Q26c)
• identifying the units of velocity (Q26c)
• converting km to m and substituting the correct units (Q26c)
• outlining how Hertz conducted his experiment involving radio waves and the features of radio waves that he was able to identify (Q27)
• identifying some features of the graph (Q28)
• identifying some relevant advantages (Q29a)
• identifying the effects of special relativity on objects travelling at relativistic speeds (Q29b)
• explaining the slingshot effect using Newton’s Laws of Motion and Universal Gravitation (Q30).

Candidates need to improve in these areas:

• plotting with an ‘x’ rather than a dot (Q21a)
• drawing a line of best fit (Q21a)
• calculating gradient from line of best fit (Q21b)
• calculating gradient from widely spaced points (Q21b)
• showing clear working in equation substitutions (Q21b) and Q26c)
• distinguishing between torque and force (Q22a)
• identifying the direction of the force acting on section AB and the direction of the force acting on section BC (Q22a)
• differentiating between split ring and slip ring (Q22b)
• understanding the meaning of modelling (Q23a)
• recognising the need for a potential difference or energy source for electron or hole movement (Q23a)
• understanding the difference between superconductors and semiconductors (Q23a)
• explaining a limitation of the model (Q23b)
• addressing the question instead of providing a lot of irrelevant information (Q25a)
• not confusing torque and force with forms of energy (Q25a)
• not confusing energy conversion with energy conservation (Q25a)
• understanding the law of conservation of energy (Q25b)
• focusing on the physics of the question rather than the presumed purpose of the appliance (Q25b)
• identifying an assumption for model Y (Q26a)
• labelling the assumption as either for model X or for model Y (Q26a)
• differentiating between gravitational potential energy and gravitational field (Q26a)
• recognising the significance of the small change in altitude to the results (Q6b)
• rearranging equations and setting out working for the problem (Q6c)
• identifying the correct equation for the force of gravity (Q26c)
• not confusing escape velocity and orbital velocity (Q26c)
• describing the process of validating a theory using experimental evidence (Q27)
• relating a changing magnetic flux to the induction of an emf/eddy­currents and the associated magnetic field that opposes the change (Q28)
• understanding the Meissner Effect is not used for large-scale magnetic fields (Q29a)
• discussing the application rather than describing the effects (Q29b)
• explaining the launch and orbiting phases of the mission (Q30).

## Section II

### Question 32 – Medical Physics

Candidates showed strength in these areas:

• identifying that reflection of waves forms the basis of ultrasound (part a i)
• identifying differences (part b i)
• recognising advantages of PET scans and limitations of x­ray imaging (part b ii)
• recalling the functions of the different parts of an endoscope and using diagrams to complement text (part c)
• showing some understanding of protons in magnetic fields being subjected to radio waves (part d ii)
• outlining facts and features of different medical imaging techniques (part e)

Candidates need to improve in these areas:

• recognising that reflection occurs at tissue boundaries and the amount of reflection depends on the difference in acoustic impedance of the tissues (part a i)
• showing all relevant working and not omitting orders of magnitude in Ir/Io calculations (part a ii)
• providing a justification with reference to the scenario (part b ii)
• addressing all aspects of the question (part c)
• relating MRI scanning to a distinguishing feature of grey and white matter (part d i)
• understanding the process of resonance (part d ii)
• using examples to support the assessment of the impact (part e).

### Question 33 – Astrophysics

Candidates showed strength in these areas:

• identifying the relevant equation (part a i)
• identifying two methods, either by name or by general description (part a ii)
• identifying some reasons for the different shapes (part b ii)
• identifying celestial objects (part c)
• matching the correct curve to star W (part d i)
• identifying features of the stars from the H-R diagram (part d ii)
• identifying features of space-based telescopes (part e).

Candidates need to improve in these areas:

• using calculators effectively (part a i)
• addressing the requirements of the question rather than restating the question (part b i)
• providing an explanation rather than just describing characteristics of celestial objects (part c)
•  providing a justification in terms of a feature of W (part d i)
• accounting for differences such as fuel source, size or age (part d ii)
• linking features of space-based telescopes to applications and advances in understanding (part e).

### Question 34 – From Quanta to Quarks

Candidates showed strength in these areas:

• identifying features of the Wilson Cloud Chamber (part a i)
• calculating the mass defect (part a ii)
• identifying the relevant equation (part b i)
• identifying features of neutrons or the neutron beams (part b ii)
• identifying a feature of the spectroscope (part c)
• identifying wave­particle duality (part d i)
• identifying the behaviour of orbiting electrons (part d ii)
• identifying at least one advance (part e).

Candidates need to improve in these areas:

• recognising the distinguishing feature between alpha decay and beta decay (part a i)
• correctly converting units (part a ii and part b i)
• rearranging the equation (part b i)
• linking the features of the beam of neutrons to its usefulness (part b ii)
• understanding stable energy levels in atoms (part c)
• recognising the crystal lattice structure (part d i)
• recognising deBroglie’s contribution to the atomic model (part d ii)
• understanding the significance of the strong nuclear force in the nucleus and its effect on electrons that are in the electron shells (part e).

### Question 35 – The Age of Silicon

Candidates showed strength in these areas:

• identifying the inputs to the truth table (part a i)
• identifying that the low voltage and the high voltage relate to 0 and 1 respectively (part a ii)
• identifying features of an amplifier (part c)
• identifying an advantage of integrated circuits (part d i)
• understanding that there is a limitation to the size of integrated circuits (part d ii)
• identifying applications of input and/or output transducers (part e).

Candidates need to improve in these areas:

• determining the output of the logic circuit (part a i)
• relating a gate to the logic circuit (part a ii)
• applying the voltage divider equation (part b i)
• explaining an application for the circuit (part b ii)
• understanding the functions of amplifiers and feedback loops (part c)
• explaining one advantage of integrated circuits over circuits constructed from discrete components (part d i)
• understanding the physical effects due to the limited size of integrated circuits (part d ii)
• assessing the impacts of transducers on society (part e).