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5. 2011 HSC Notes from the Marking Centre — Physics

2011 HSC Notes from the Marking Centre – Physics

Contents

• Introduction.
• Section I – Core
• Section II – Options

Introduction

This document has been produced for the teachers and candidates of the Stage 6 course in Physics. It contains comments on candidate responses to the 2011 Higher School Certificate examination, indicating the quality of the responses and highlighting their relative strengths and weaknesses.

This document should be read along with the relevant syllabus, the 2011 Higher School Certificate examination, the marking guidelines and other support documents developed by the Board of Studies to assist in the teaching and learning of Physics.

General comments

Teachers and candidates should be aware that examiners may ask questions that address the syllabus outcomes in a manner that requires candidates to respond by integrating their knowledge, understanding and skills developed through studying the course and including the prescribed focus areas. It is important to understand that the Preliminary course is assumed knowledge for the HSC course.

Candidates need to be aware that the marks allocated to the question and the answer space (where this is provided on the examination paper) are guides to the length of the required response. A longer response will not in itself lead to higher marks. Writing in excess of the space allocated may reduce the time available for answering other questions.

Candidates need to be familiar with the Board’s Glossary of Key Words, which contains some terms commonly used in examination questions. However, candidates should also be aware that not all questions will start with or contain one of the key words from the glossary. Questions such as ‘how?’, ‘why?’ or ‘to what extent?’ may be asked, or verbs may be used that are not included in the glossary, such as ‘design’, ‘translate’ or ‘list’.

Teachers and candidates are reminded that mandatory skills content in Module 9.1 is examinable in both the core and option questions and that all objectives and outcomes, including the Prescribed Focus Areas, are integral to the Stage 6 Physics Course.

Candidates are required to attempt only one question in Section II, and are strongly advised to answer the option they have studied in class.

Section I – Core

Part B

Question 21

1. Although most candidates correctly plotted the missing points on the graph, in better responses, candidates correctly drew a straight line of best fit which minimised the distance of all points from the line. Many candidates incorrectly assumed the line went through the point where the axes met. Most candidates correctly read the value of resistance at 24ºC from their line of best fit.
   
2. In better responses, candidates realised that the resistance was a straight line only near room temperature. These candidates correctly assessed the data as invalid. In weaker responses, candidates often incorrectly used superconductivity and critical temperatures in their reasoning.

Question 22

1. In correct responses, candidates identified the reason for the experiment was to show the existence of the ‘aether’.
2. In better responses, candidates labelled the components of the apparatus used in the experiment and drew a clear diagram. Better responses identified that the apparatus was turned (or rotated) to perform the experiment.

Question 23

1. Most candidates identified the mass of the satellite as constant and gravitational-field strength as varying with altitude.
2. In better responses, candidates recognised that gravitational force provided the centripetal force required for circular motion of orbit, derived an expression for orbital velocity, then substituted correctly. There were a number of candidates who failed to use correct SI units.
3. In better responses, candidates provided two effects on motion with related explanations based either on mathematical relationships, or with reference to specific details of cause and effect.

Question 24

In better responses, candidates recognised this as a relativity-of-simultaneity concept and accounted for the observations of both observers. Candidates communicated their understanding of this concept, using concise language and clearly linking the constancy of the speed of light to each set of observations.

Question 25

Most candidates identified the magnet that left the tube first. Many candidates identified the importance of eddy currents and braking. Some went on to relate braking to Lenz's law. In better responses, candidates related braking to magnetic flux.

Question 26

1. Two common errors were to leave out the high-tension cables and the arrows needed in a flow chart.
2. In better responses, candidates described the much broader impact of AC generators. They focused on several of the major environmental effects that have resulted from AC power generation. In particular, these responses linked CO2 emissions from coal-fired power stations to global warming, as well as discussing positive and negative environmental impacts of hydroelectricity and nuclear power generation.

Question 27

1. Most candidates selected the correct formula, but many incorrectly assumed that the side AB was not perpendicular to the magnetic field and incorrectly applied an angle of 45º in the calculation.
2. Although many candidates recognised that the forces on either side of the coil were opposing, many did not relate this to the change in direction of the current around the coil. Some candidates confused the torque on the coil with the net force on the sides of the coil.

Question 28

1. Some candidates cited the Maltese cross experiment in which the observations could be attributed to either particle or wave behaviour.
2. In better responses, candidates clearly linked the components of an electron gun to their specific role in producing an electron beam. In weaker responses, candidates confused the potential difference supplied to the filament of the cathode with the potential difference applied across the electrodes to accelerate electrons.

Question 29

1. Most candidates successfully determined appropriate equations but some did not substitute all relevant data correctly. Many candidates completed their response with reference to the energy of a single photon. Some candidates could not correctly convert the supplied wavelength for substitution.
2. In better responses, candidates referred to individual photon energy and number rather than wave properties, such as wavelength or frequency. Many candidates confused particle theory and wave theory.

Question 30

1. In better responses, candidates associated the higher temperature with increased lattice vibrations in the sample and related this to an increase in collision rate of free electrons with the lattice ions. In weaker responses, candidates attempted explanations based on BCS theory.
2. Most candidates identified the existence of Cooper pairs below the critical temperature. However, many did not clearly explain the role of lattice distortions in Cooper pair formation.

Section II – Options

Question 31 – Geophysics

1. Most candidates recognised a natural hazard and correctly identified the monitoring instrument used. In better responses, candidates included reference to warning time.
2. 1. Most candidates correctly outlined magnetic-field reversals but some did not include methods of radiometric dating.
   2. In better responses, candidates correctly used the scale data.
3. 1. Most candidates recalled and outlined a relevant investigation.
   2. Most candidates correctly identified relevant techniques. In better responses, candidates linked these techniques to regrowth of vegetation.
4. 1. Most candidates correctly identified the importance of changing radius.
   2. In better responses, candidates provided an outline of gravity surveys and related density anomalies to resource exploration.
5. In better responses, candidates provided details relating to wave properties and related these properties to the structure of the Earth.

Question 32 – Medical Physics

1. 1. Most candidates correctly identified the type of scan and the information that can be        obtained by such a scan.
   2. In better responses, candidates explained the difference in signal strength at X and Y as due to the acoustic impedance differences of the different mediums at X and Y.
   3. In better responses, candidates wrote the correct formula and showed all working out.
2. 1. Some candidates gave a general description of how X-rays are produced by describing an X-ray tube, rather than describing the two mechanisms by which X-rays are produced. 
   2. In better responses, candidates compared the information provided by a conventional X-ray image with those provided by a transverse slice produced by a CAT scan. Candidates had problems in identifying image B, thinking it was either a PET scan or an MRI scan.  
3. Many candidates gave a description of the structure AND use of both coherent and incoherent bundles of optic fibres. Some candidates confused refraction with reflection.
4. In better responses, candidates linked the measurement of hydrogen density to increased concentration of hydrogen in brain tumours.

Question 33 — Astrophysics

1. 1. In better responses, candidates defined one parsec with an accurately labelled diagram.
   2. In better responses, candidates recognised the difference between sensitivity and resolution, and linked this to Galileo’s observations.
   3. In weaker responses, candidates seemed to be unaware that active or adaptive optics cannot be used with lenses.
2. 1. In better responses, candidates recognised the correct nuclear process in each star and compared the characteristics of the two stars.
   2. A significant number of candidates confused M with m.
   3. Many candidates followed the longer way of determining the value of m first, instead of simply substituting into the applicable formula.
3. Most candidates distinguished between intrinsic and extrinsic variables. In the better responses, candidates named Cepheids as intrinsic variables and eclipsing binaries as extrinsic variables. Some candidates explained their answer by describing the periodic variations in brightness of each star in terms of the processes that caused them.
4. Most candidates described how spectroscopy provides information about celestial objects. Some candidates also described how photometry provides information about celestial objects. They showed how properties obtained from spectroscopy could be combined with properties obtained from photometry to produce information that could not be obtained from spectroscopy alone.

Question 34 — From Quanta to Quarks

1. Most candidates correctly named an isotope and its mass number. In better responses, candidates linked the named isotope with a correct use in agriculture.
2. 1. Most candidates linked the connection between control rods, neutron number and reaction rate. Better responses included the moderator as also affecting reaction rate.
   2. In better responses, candidates explained the mass–energy equivalence.
3. In better responses, candidates recognised the correct relative strengths of the forces and clearly stated their direction and range.
4. Most candidates identified the correct equation then correctly substituted the variables into the equation.
5. In better responses, candidates correctly tallied the type and number of quarks and leptons.
6. In better responses, candidates clearly stated two contributions of each physicist and outlined how these contributions assisted in our understanding of the atom.
7. In better responses, candidates described the mathematical models of the atom developed by Bohr and De Broglie and identified experimental evidence that supported those models.

Question 35 — The Age of Silicon

1. Most candidates identified the gates and their operation, and constructed a suitable truth table with only minor errors or omissions.
2. Many candidates correctly identified op amps being used as open-loop comparators with better responses describing the lack of feedback connections.
3. Most candidates identified a simple voltage divider. In weaker responses, candidates used inappropriate feedback equations.
4. In better responses, candidates used the information in parts (a)–(c) to derive a meaningful relationship between the input V batt and the output O0 or the LED.
5. Most candidates produced a recognisable diagram of a LED. Some candidates omitted or incorrectly labelled salient features for their operation. The better responses provided valid disadvantages.
6. In better responses, candidates outlined valid properties of silica for optical properties, with occasional explanation of why these were required.