HSC Physics: Suggested Answers and Discussion

[jamonwindeyer]

We'll be posting suggested answers throughout the afternoon! Spot a mistake or want something clarified? Sound off below and help us make sure these are as accurate/helpful as they can be!!

Just click the spoilers below to see our suggested answers!

Multiple Choice

Q1: D - Transformers don't convert AC to DC; they are called rectifiers. Decrease the voltage to prevent damaging the phone.
Q2: C - Westinghouse was AC, and lower currents reduce energy loss
Q3: C - Magnetic fields only exert a force on MOVING particles
Q4: A - Alloys were discovered first, then ceramic compounds
Q5: D - \(E=\frac{V}{d}=\frac{15}{0.005}=3000Vm^{-1}\)
Q6: D - Speed of light is constant in all frames of reference (special relativity)
Q7: B - Lenz's Law opposes the motion of the magnet; pushes it away when it is coming in, pulls it back when it is going away
Q8: D - D is exposed to the greatest rate of change of magnetic flux, and so experiences the largest EMF (and thus current).
Q9: B - Back EMF will oppose Supply EMF until an equilibrium is reached, thus limiting the speed of the motor
Q10: C - The observer is in the same frame of reference as the light and doors, so length contraction does not affect observations.
Q11: B - \(\lambda=\frac{hc}{E}=\frac{6.626\times10^{34}\times3\times10^8}{3.5\times1.602\times10^{-19}}=3.5\times10^{-7}\)
Q12: B - By the right hand grip rule or its variants
Q13: C - More intensity equals more photons, but they have the same energy
Q14: B - By Kepler's 3rd Law
Q15: A - The light causes formation of electrons and holes which both carry current through the external circuit.
Q16: A - This is a loudspeaker being used in reverse, and so acts as a generator. Use of the right hand grip rule and Lenz's Law gives direction
Q17: C - The lift slowing down generates a slight upwards acceleration on the ball. This increases flight time.
Q18: C - \(v=\sqrt{ar}=\sqrt{3.6\times9.8}=5.9\)
Q19: A - In the Muon's reference frame, classical physics works. Earth views an increase in lifetime due to time dilation.
Q20: B - The changes in polarity of the magnetic field and the current ensure the force torque is always clockwise, while the commutator ensures it does not reverse with every half turn (very similar to a universal motor, if anyone learned that)

Short Answer (Brief Solutions)

Question 21A
- Additional ozone particles
- Friction with satellite causes deaccleration -> Orbital Decay

Question 21B
\(F_g=\frac{GMm}{d^2}=\frac{6.67\times10^{-11}\times50\times6\times10^24}{8000000^2}=312.66N\) (2dp)

Question 22A
Moving the bar magnet near Coil B to expose it to a changing magnetic flux. Brief explanation of this and Faraday's Law for 2 marks.

Question 22B
Initially, velocity increases (negatively) due to gravity. Strong eddy currents then apply an upwards acceleration which forces the magnet back upwards. Good explanation of Eddy Currents, why they are formed, and why they oppose the magnets motion, would be needed for the full 4 marks.

Question 23A
Horizontal deflection plates cause the cathode ray to sweep across the screen. Vertical deflection plates are connected to an input source such that vertical axis becomes representative of the amplitude of the input signal. Both come together to create the typical CRO graph.

Question 23B
\(Bqv=Eq \implies B=\frac{E}{v}=\frac{10}{5.2\times10^4}=0.2mT\)
The field is applied UP the page.

Question 24
The following material is relevant:
- Resistance is related to the ease at which electrons can flow through the lattice
- Normally, electrons require energy to move through the lattice
- Superconductors below critical temperature have low enough lattice vibration to form phonons around moving electrons (regions of positive change)
- This allows formation of cooper pairs that flow through the lattice experiencing zero effective resistance

Question 25A
Team A shows an inverse-square relationship over a wide range of data points. Team B took data over a narrow range, and thus shows only a linear relationship.

Question 25B
Team B's data was not taken over a wide range of distances, and thus misappropriates the nature of the relationship between force and distance. Answers that say this in varying ways (with good explanation) would be worth 3 marks.

Question 26
Typical societal impact question. Discuss the ability to move energy production out of regional centres (because we can now distribute), discuss improved energy efficiency and thus lower costs of power, discuss the improvements to quality of life due to appliances that can now operate on different input voltages (household transformers), and other useful information. Clear and well rounded answers that show clear links to SOCIETY and don't just explain what a transformer does will score well.

Question 27A
Equipment to Mention: "Spark gaps", ultraviolet light sources, large voltage sources (induction coils could blend the voltage/gap equipment). Outline the observations that ultraviolet light increased the sparking in the typical experiment. 1 mark for equipment, 1 mark for experiment and observations made with ultraviolet light, one mark for link to photoelectric effect.

Question 27B
You could discuss a few things here; Hertz experiment would work, as would Einstein's formulation. Einstein was likely the more common answer. Discuss the photon model of light and how Einstein used it to explain the photoelectric effect for full marks.

Question 28
Some things to cover (not exhaustive)
- Law of Conservation of Momentum: Momentum in closed system must be conserved
- Momentum of rocket is always equal and opposite to momentum of fuel (which is assumed a constant)
- As mass of rocket decreases, but momentum is constant, velocity must increase (acceleration)
- Decrease in mass also causes increase in acceleration by Newton's 2nd Law (seen in diagram)

Question 29
Far out, what the hell is this. Super broad; pick three technologies and explain the discoveries that lead to them. Things from I2I are what would likely work best in this question, since discovery/application is the idea of the topic to begin with. CRO's, Photo Cells, Solar Cells (of course the electric generator needs to be one of them, with Faraday's Law being the discovery that spurred it).

Question 30A

Calculate change in GPE of mass:



This is the energy that would be converted to electrical energy to the light bulb (ignoring losses), so this is the maximum energy.

Question 30B
When the switch is closed, the DC generator will in actuality have Back EMF which opposes the fall of the mass. It falls more slowly as a result. When the switch is open, those oppositional 'eddy currents'/'back emf' cannot flow, and so the mass falls under the normal effects of gravity on Mars. Some explanation to this effect (with a tad more detail perhaps) would earn the 3 marks.

Medical Physics

A (i) - Hard X-rays have a higher frequency than soft X-rays. Explain that hard X-rays are most useful for medical imaging.
A (ii) - Cheap, easily accessible, less radiation exposure than other methods while still providing excellent detail of skeletal structures. Expanding on these ideas would yield 3 marks.

B (i): Piezoelectric materials consist of dipoles which align themselves to electric fields. Applying an AC current causes the material to vibrate due to the alignment of these dipoles. Exposing it to sound waves results in an AC in the material. This allows for production and receiving of ultrasound.
B (ii) At the first boundary, \(Z_1=Z_2\), so there will be no reflected ultrasound. At the second boundary:



So 37% of the pulse is reflected. This will be registered by the ultrasound receiver and indicate the location of the bone.

C: Doppler ultrasound is regular ultrasound, but the frequency of reflected pulses is measured to check for Doppler Effect. This allows movement in the body (EG - detecting blood flow in the heart) to be detected. CAT instead combines X-ray images from multiple angles into a 3D image. This allows better visualisation of size/position (EG - size of fluid buildups in the lungs). An answer to this effect, with the mechanics of each expanded and more detail on the use, would get 4 marks.

D: This question requires a breakdown of how PET works; a positron emitting pharmaceutical such as Fluorine 18 is attached to glucose (usually) to form FDG molecules, which are injected into the patient. The location of the molecules is indicated by the release of positrons, which annihilate with electrons to form gamma ray pairs which are detected by gamma cameras. Etc etc. This would get 3/4 marks, but you would probably then need to provide an example of how it is used in diagnosis, and why it is useful in certain scenarios, to get all the way there. Scans of brain activity would be an option.

E: Full explanation of MRI. It should cover:
- The Fundamentals: Spin, precession and parallel-anti parallel alignments, and the Larmor Frequency (2? marks)
- The effect of exposing precessing nuclei to the Larmor Frequency and the idea of absorbed energy (1 mark)
- Explanation of relaxation of these nuclei back into parallel alignments, including reference to T1/T2 (2? marks)
- Emitted radio wave detection and combination into an image (1? mark)
Mark values are indicative, there is more than one way to skin a cat

Quanta to Quarks (courtesy of ParallelPlatypus)

These solutions are courtesy of the user, ParallelPlatypus, absolute legend!

Ai) Neutrons and protons. Protons are positive, neutrons neutral.
ii)
•   Electrostatic force is a stronger repulsive force than gravity is attractive at these distances, hence another force is required.
•   The strong force, mediated by quark anti-quark gluons, binds protons and neutrons together, and is essentially a residual colour force
•   Hence, the strong force balances the electrostatic force and allows the nucleus to be stable.
Bi)
•   Relative intensity of spectral lines
•   Hyperfine spectral lines
•   Zeeman effect
ii)n=4
c) Pauli = Pauli’s exclusion principle which used the four quantum numbers to provide an explanation of the unique states of electrons in their orbits (e.g. 2,8,18 etc.) Pauli explained the quantum arrangement of electrons in the hydrogen atom, making Bohr’s model more credible and justified.
Heisenberg = Matrix mechanics which supported Schrodinger’s probabilistic wave function- lead to the conceptualisation of the quantum probability cloud adaptation of the Bohr model. The Heisenberg Uncertainty principle implies an inaccuracy in measurement which lends further support to an uncertain interpretation of orbital positions and momentums.
d) This one is a bit vague to tackle- I would include an explanation of what synchrotrons, cyclotrons and linear accelerators are, what they achieve and why this is important. I.e. Accelerators make charged particles reach near light speeds and then causes extremely high energy collisions. In the process, exotic sub-atomic particles are revealed from decomposition processes. These can be observed using electromagnetic and magnetic detection devices which record extensive amounts of data from within the extremely short duration of the collision. This data when analysed, can reveal the particles of the standard model. I included a table of the leptons, quarks and force bosons just to make sure. I also mentioned the discovery of the Higgs Boson which lent great support to the standard model.
e) Pretty standard question- you should mention his work in neutron bombardment experiments in which he discovered that slower thermal neutrons were more effective in producing fission. He worked in the Manhattan Project and created the first working nuclear reactor. This involved pioneering moderators like graphite and control rods made of cadmium or boron. This is reflected in modern nuclear reactors which have similar components. You can then talk about the impacts of nuclear reactors on society. E.g. clean energy and the production of isotopes (where I dumped my prepared isotopes, I did industrial and medical, and related this to their societal impact). You could also relate the discussion of the rate of nuclear reaction and controlled/uncontrolled reactions to nuclear weaponry + plutonium 239 production from nuclear reactors. Then you can discuss the pros and cons of nuclear weapons which are well documented. This was my take on the question

[proficles]

i found it an alright test, wasnt too difficult but not easy either. some multiple choice questions were not clear in my opinion, but they have never been so i wasnt surprised.  the Q2Q questions were  quite generic. overall i feel like tis test was quite similar to previous years so i wasnt too surprised

[Zyzyzyzy]

I thought the one about team A and team B was left field

[imtrying]

Overall, was pretty much as expected. Q30 with the light on mars thing threw me a bit though! Other than that I felt like it wasn't too difficult.

[alexander.chu1]

OK

[bluealder]

16 is wrong, since mechanical energy is creating electrical energy ie generator. Then using Lenz Law current is Y -X so B

[Tom McMahon]

Correct me if wrong, but I though 16 was A because mechanical movement creating electrical energy (generator) and the right side of coil would be north to attract to south and reduce motion (Lenz's Law), and by right hand grip rule would be A?

[pels]

16 is wrong, since mechanical energy is creating electrical energy ie generator. Then using Lenz Law current is Y -X so B

Actually, the loudspeaker is an application of the motor effect.

[noonedoesnt]

Elevator is also incorrect

[bluealder]

Actually, the loudspeaker is an application of the motor effect.

But it's not asking that, it's asking what the speaker is acting as, in this case mechanical energy is converted to electrical. Read the top bit of the question.

[thomasdo1]

Actually, the loudspeaker is an application of the motor effect.

No, there was no power supply (amplifier)

[ParallelPlatypus]

I agree, there was no power supply, and the cone is pushed providing a current, therefore it is a generator. I disagree with the current direction though- we are looking at conventional current, so the thumb should point to the left, and the fields go from the north to south, therefore towards the center, therefore current should be going down on the front side, and travel from X to Y (conventional). So I would say A is the correct answer. (Hope it is anyway). I think the initial answer C is correct for the elevator- we experience weightlessness as the elevator slows, which implies a lower downwards acceleration, therefore it should not fall as fast.

[jakesilove]

Just as a side note, how hard were the multis aha

[EjonMP]

Can someone explain to me why the answer for 8 is not A? I thought that having a stronger external magnetic field would increase the induced current.

[ParallelPlatypus]

If the magnets were in the same direction, it would be stronger, but the magnets have their poles near each other- so the fields would be very small and only go from N to S in small contained areas- hence very minimal interaction with coil.