2012 HSC Physics Sample Answers

When examination committees develop questions for the examination, they may write sample answers or, in the case of some questions, answers could include. The committees do this to ensure that the questions will effectively assess students knowledge and skills.

This material is also provided to the Supervisor of Marking, to give some guidance about the nature and scope of the responses the committee expected students would produce. How sample answers are used at marking centres varies. Sample answers may be used extensively and even modified at the marking centre OR they may be considered only briefly at the beginning of marking. In a few cases, the sample answers may not be used at all at marking.

The Board publishes this information to assist in understanding how the marking guidelines were implemented.

The sample answers or similar advice contained in this document are not intended to be exemplary or even complete answers or responses. As they are part of the examination committees working document, they may contain typographical errors, omissions, or only some of the possible correct answers.

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2012 HSC Physics Sample Answers

Section I, Part B

Question 21 (a)

Sample answer:

[Investigation could use different methods, including being based on a pendulum or timing a falling mass.]

A computer-based timing system should be set up using a sensor to measure how long it takes for a mass to fall to the ground from several heights between 0.5 m and 3.0 m. To increase reliability several readings for each height should be recorded. The results should be plotted on a time2 vs height graph. The acceleration due to gravity is equal to 2 x the reciprocal of the slope of the line of best fit.

Question 21 (b)

Sample answer:

The known value (measured using more accurate equipment) should be looked up for my location, as published, for example, on the National Measurement Institute website and compared to the measured value. The closer the measured value is to the reference value, the more accurate it is.

Question 21 (c)

Sample answer:

The same measurement (using the same procedure) should be repeated at each height several more times. Statistically this would reduce the uncertainty in the average of all the results for each height.

Question 21 (d)

Sample answer:

The difference between each of the measurements and the average reading for each height the object was dropped from should be compared. If there is a large variation in the readings (relative to the average value) then the data is not very reliable.

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2012 HSC Physics Sample Answers

Question 22 (a)

Sample answer:

The magnetic field around the bar magnet passes through the turns of the coil, resulting in a certain enclosed magnetic flux. An emf is induced when the coil is moved because the enclosed magnetic flux decreases as the coil is moved away from the magnet. Since the coil terminals are short-circuited, this emf will cause a current to flow.

Question 22 (b)

Sample answer:

As the coil is moved away, initially the reading on the balance becomes less and then it goes back to 42.42 g. The induced current in the coil that results from the change in flux produces a magnetic field that tends to maintain the flux through the solenoid. The field due to the induced current in the coil has a south pole at the bottom of the coil, resulting in a force of attraction on the magnet causing the force and, therefore the reading on the balance due to the bar magnet to decrease. As the coil is moved further away, the interaction between the magnet and the coil effectively disappears and the reading on the balance returns to 42.42 g.

Question 23

Sample answer:

The mass at the top of tower B is in geostationary orbit since it is at the correct height above Earth and travelling at the same rotational speed as Earth. Therefore, when it is released it will not fall and will not move relative to tower B.

In contrast, the mass at the top of tower A is not in geostationary orbit since it is sitting on the rotation axis of Earth and therefore has zero orbital velocity. This means that it will fall directly towards Earth. Since the mass starts out at such a high altitude, the acceleration due to gravity starts off lower and increases as the mass falls (approaching 9.8 m s-2 as it nears Earths surface). This means that this mass falls with an increasing acceleration until it reaches the atmosphere.

Question 24 (a)

Sample answer:

Conduction band

Conduction band Conduction band

E Valence band

Valence band Zero Small Large gap energy energyValence band gap gap

Conductor Semiconductor Insulator

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2012 HSC Physics Sample Answers

Question 24 (b)

Sample answer:

At any temperature above absolute zero, some electrons in the valence band of a semiconductor are excited into the conduction band, creating a corresponding hole in the valence band.

An applied potential causes electrons in the conduction band to move in the opposite direction to the applied electric field and holes in the valence band to move in the direction of the electric field. This is a current.

Question 25 (a)

Sample answer:

Transistor circuits are smaller than thermionic devices and are less fragile as they are not made of glass.

Question 25 (b)

Sample answer:

Transistors have had a significant positive effect on society. The development of transistors led to the development of microchips that are used in a wide range of communication devices and technologies, such as satellites.

Microchips have led to the development of cheap and compact consumer products, such as mobile phones. This has made these devices accessible to large members of the population and enabled people to communicate flexibly throughout the day and from any location.

Microchips have been incorporated into computers to provide fast processing speeds and have revolutionised society through the near-instantaneous transfer of information via the internet. This has transformed the business world, allowing financial trading to be instantaneous, and has led to new and increased social interactions between people in society such as Facebook.

Satellites enable weather to be monitored and allow us to predict natural disasters so that people and property can be protected. The use of transistors in electronic devices has radically shaped society.

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2012 HSC Physics Sample Answers

Question 26

Sample answer:

Design A Power loss is (120 100) MW = 20 MW over the low-voltage transmission line

Design B Voltage drop along power line, V = 8 kV Power loss along power line, P = V2/R = 80002/40 = 1.6 MW

Design C Power use along transmission line = 100 km 30 kW/km = 3 MW

Design B is the best way of transmitting power from the power station to the substation because it results in the least total power loss/use of the three processes.

Question 27

Sample answer:

u u = u cos 60 = x 2

3u u = usin 60y = 2

45 90 45 = ( u ) t t = x =u u x y = ( 1u t at2 34 x ) =2

3 90 1 90 2 34 = u (9.8) 2 u 2 u

39690 34 = 45 3

u2

39690 u = 45 3 34

= 30m / s

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2012 HSC Physics Sample Answers

Question 28 (a)

Sample answer:

The special theory of relativity predicted that a clock would run slower if it were moving relative to an observer. The GPS navigation system corrects for this slowing of the atomic clocks on satellites used in the system. This evidence supports the theory of relativity because if the theory were not correct, its application in this context would result in a system that did not work.

Question 28 (b)

Sample answer:

A theory is validated when it agrees with observations and is consistent with the results of scientific/controlled experiments designed to test specific aspects of the theory. Validation of a theory is complete when it can be used to make predictions that can be tested and demonstrated to be correct.

Question 28 (c)

Sample answer:

l = l0 multiplied by the square root of (1-v2/c2) = 40 cm 0.994987 = 39.8 cm

Question 29

Sample answer:

When a current passes through the wire in the coil, a force is exerted on it because the current has a component perpendicular to the magnetic field direction this is the motor effect. By passing AC through the coil, the force produced on the coil causes it to oscillate, moving the speaker cone correspondingly to produce sound waves. The pitch of the sound can be altered by changing the AC frequency and the volume of the sound can be increased by increasing the current through the coil.

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2012 HSC Physics Sample Answers

Question 30 (a)

Sample answer:

The two charged particles have opposite charges.

Question 30 (b)

Sample answer:

The paths are circular because the force produced by the magnetic field on the moving charge is constant in magnitude and always perpendicular to the particles velocities, and these are the conditions necessary for circular motion to be produced by a force.

Question 30 (c)

Sample answer:

The greater the mass of a particle travelling through a magnetic field, the greater the radius of curvature of its path, other variables being unchanged. The greater the charge of the particle, the smaller the radius of curvature of its path.

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2012 HSC Physics Sample Answers

Section I