HSC Physics Question Thread

[12carpim]

Hey there!

Got a question why does a three-phase generator use electricity to generate electricity? Makes no sense if someone could help.

Thank you very much!

[julies]

Hey guys, are we supposed to explain how a magnet levitates over a superconductor by Lenz's Law? I've heard that this is technically incorrect?

Also would a really heavy magnet be able to hover above the superconductor, or is the opposing magnetic force unable to cancel out its weight force?

If somebody could please clarify, it would be greatly appreciated! : )

[khadeeja_]

Hey could someone explain how transistors work? don't get the whole current flowing through emitter and base and collector thing..

[austv99]

Would appreciate if someone could help me clarify these questions.
Braggs question: I'm not too sure how the interference pattern produced in the experiment exactly led them to conclude the crystal lattic structure. Is it because it showed a geometric/periodic structure? Is there any significance with the contructive and deconstructive interferences produced?

Quanta to Quarks: Would I includes Chadwicks discovery of the neutron and the corresponding explanation by conservation laws, then fermis demonstration of nuclear fission, increasing the understanding of the ability to break up the atomic structure for energy production. Also, would i include that he used paulis proposal of the neutrino to comprehensively explain beta decay?

TIA

[CyberScopes]

Hey guys, are we supposed to explain how a magnet levitates over a superconductor by Lenz's Law? I've heard that this is technically incorrect?

Also would a really heavy magnet be able to hover above the superconductor, or is the opposing magnetic force unable to cancel out its weight force?

If somebody could please clarify, it would be greatly appreciated! : )

This depends on the question, if the question asks how the magnet levitates if the magnet is dropped on the superconductor then yes, the Lenz's Law method should be correct. But if the magnet is initially left stationary on the superconductor, and say helium was poured so it reached its critical temperature and attained its superconducting qualities, then Lenz's Law cannot be used since there is no relative motion.

For this case, it can be explained through one of the vital properties of superconductors: that magnetic fields cannot penetrate through them. Because of this, if a magnet is left on the superconductor, its magnetic field lines are cut off at the boundaries of the superconductor. Now, the magnetic field lines need to somehow reconnect back to the magnet (or just go somewhere for a matter of fact), but since the superconductor cannot be penetrated, this cannot happen. As a result, this creates a diamagnetic surface at the superconductor. In order for the magnetic field lines to enter the opposite pole of the magnet, the magnet is then pushed upwards, thus causing it to appear as if its levitating.

Pretty much: Superconductors dont take in magnetic field lines -> Magnets field lines have to re-enter the opposite pole of the magnet -> Forces magnet to lift up.

[julia9102]

Hey there,

could anyone please explain what eddy currents are and how we can determine its direction?

[hinakamishiro]

Hey guys can I please have some help with these questions?
BTW i was also hoping someone could explain the difference between magnetic flux and magnetic flux density?
Thanks!

[Jayden Nguyen]

This question so hard. From qanta to qwarks can someone help me with an example of how they would answer it? TIA

[Baylsskool]

Hey so just to give you a quick explanation the initial curve as indicated by classical theorty suggested that as wavelength decreased radiance/intensity would increase to infinity now classical theory was wrong as firstly evidence showed otherwise and secondly thus increase in energy would violate the principle of consevation of energy as energy is supposedly being created without anthing causing it !
Also from my understanding there is no actual section of energy missing in the beginning .
I get that your confused about why there can't be an infinite source of energy for an infinitly small wavelength but the reason is radiation emitted and absorbed by the walls of a black body cavity is quantised meaning the energy of photons is related to the frequency and thus in accordance to Einstein's contribution there is a threshold frquency , even if a light is very bright and carries large energy .
So whilst E=hf (which equals E= hc/λ)suggests that there can be an infinite source of energy for an infinitly small wavelength Einstein said nope actually there is a threshold frquency and this theory was discovered based on experimental data thus the reason for the peak .
Hope it makes sense

Yeah I guess so, so it safe to say that it doesn't increase infinittly due to the threshold frequency of the sun, so there's a certain point of frequency where a hot star emits high frequency but then due to the threshold, it stops it from going higher ?

[sidzeman]

For the PE effect of this question, I would discuss how Einsteins ability to explain the PE effect with the quantisation of energy which classical physics could not - evidence for quantum theory and dual wave particle model for light. However I'm not sure what I would discuss for the special relativity theory - what did it change about the model of light, besides the fact that it is constant in all frames of references?

[hobocop]

Hi, do all the radius values in satellite orbit equations take into account the objects altitude above earth and radius of earth?

For instance, the radius value in the orbital velocity equation v= (Gm/r)^0.5  because the solutions in this past paper did not account for radius of earth.

[beau77bro]

can someone explain photovoltaic cells please? i dont get the directions and attractions -the photons strike the n-p junction and the electrons travel where and why? how does it create a current from this?

i get the electrons are emitted by the photoelectric effect and travel towards the n-type since it is positively charged by the diode's depletion zone. but why does that create a current throuhgout? its only migrating a short distance? or does it go around the whole circuit? confused

[CyberScopes]

Hey guys can I please have some help with these questions?
BTW i was also hoping someone could explain the difference between magnetic flux and magnetic flux density?
Thanks!

Ill do what I can:

19. An electron is in an electric field, so its attracted to the positive plate. Since electric fields go from +ve to -ve, the image indicates that the bottom plate is positive and the top is negative. Therefore, the electron is attracted to the bottom plate, and will have force (D) pulling it down.

20. If GPE is half its original value, then its distance between Earth and the satellite has decreased. Assuming this, you can say that d goes from 1 to 1/2. Now assuming the other variables are constant, you can make Newtons Gravitational Equation become F = 1/d². If d = 1, then F = 1. Since d is now 1/2, F becomes 1/(1/2)² = 4. So the answer is C
(Please check this not sure if i did it correct, there may be another method.)

20. Im not sure I might get back to you if i find out

15. A stronger magnetic field means that the circle will have a smaller radius since it will turn quicker, so it cannot be C or D. Knowing F = qvB, F is proportional to B, so the force should double. In this case, the radius should be halved, so its B. (Sorry theres probably a better way to explain this by equating centripetal force with magnetic force but im too lazy )

Correct me if im wrong on any of them

[CyberScopes]

Hi, do all the radius values in satellite orbit equations take into account the objects altitude above earth and radius of earth?

For instance, the radius value in the orbital velocity equation v= (Gm/r)^0.5  because the solutions in this past paper did not account for radius of earth.

They should account for the distance from the centre of both masses, which includes the radius of the Earth. Weird that you say one of the questions didnt, maybe the radius given includes the radius of the Earth aswell?

[hobocop]

They should account for the distance from the centre of both masses, which includes the radius of the Earth. Weird that you say one of the questions didnt, maybe the radius given includes the radius of the Earth aswell?

It might just be a mistake. This is from the sample answers for the 2015 HSC paper, question 26c).