HSC Physics Question Thread

[Baylsskool]

So normally a moving conductor inside or being affected by a magnetic field,(key word moving), has induced current into it(not perfect), however in this case, a superconductor apparently only needs to be affected by a magnetic field to have a perfect induced current, am I reading it wrong, or is it that the superconductor doesn't need to move? And if so my question is why wouldn't it need to move to do this, as this contradicts Lenz law??
Use this link for the photo of why I'm asking https://m.imgur.com/a/QL5F2

[jamonwindeyer]

wait so does the mri measure the change in Magnetic vector or the re-emitted radio waves? how do they make the image? so the radio wave returns in the specific larmor frequency set up by the gradient coils and what information do they carry? oh wait is the magnetic vector just a way of explaining the time taken for the relaxation to happen?

The radio waves, but because the magnetic field is different everywhere, this affects the radio waves that are emitted in that location!

Hi, could I please get an explanation on why this is C?

Thanks.

The radial magnetic field removes the cosine term from the torque equation by ensuring the force on the conductor lengths is always perpendicular to the coil, but it actually doesn't do anything to the magnitude of the force! That is always just \(F=BIl\), it just isn't oriented optimally in a non-radial magnetic field so force is always constant (swapping direction every half turn in a DC motor), which matches C

Isnt the induced emf equal to negative change in flux over change in time, meaning the graph should be flipped?

You are right, but the flipped version isn't presented! Which means we are just measuring EMF the opposite way, and that's okay

[jamonwindeyer]

So normally a moving conductor inside or being affected by a magnetic field,(key word moving), has induced current into it(not perfect), however in this case, a superconductor apparently only needs to be affected by a magnetic field to have a perfect induced current, am I reading it wrong, or is it that the superconductor doesn't need to move? And if so my question is why wouldn't it need to move to do this, as this contradicts Lenz law??
Use this link for the photo of why I'm asking https://m.imgur.com/a/QL5F2

This highlights the issue with explaining the Meisner Effect in this way, the textbook is incorrect! The Meisner Effect is distinct from the idea of eddy currents, so yes, a magnet would need to move for eddy currents to form! And the Meisner Effect is separate, it happens for other reasons  that you don't really need to know

[eternalconflux]

Hi guys. I have a question related to photoelectric effect that i need help with.
Here's the question.

When a zinc plate was exposed to ultraviolet light, electrons were released. One electron had kinetic energy of 3.0 x 10^-19 J

a) If a retarding voltage of 0.80V was applied, calculate the kinetic energy of the electron when it reached the anode.
b) Find the minimum retarding voltage that would inhibit the electron from reaching the anode.

Since my inability to solve this stems from my lack of understanding on the relationship between work function, retarding/stopping voltage and threshold frequency, an explanation would be much appreciated on how to solve these questions! I get the general gist of what photoelectric effect is (planck proposed light was packets of discrete energy, and Einstein, from Planck's observations, made further discoveries and derived that light was instead quantised into photons, and that there was a threshold frequency required before photoelectric effect could take place, and then intensity would take its part after threshold frequency was reached.

[beau77bro]

hey so why do we use positron emitters rather than just gamma emitters?

[sidzeman]

Does anyone know where I can get sample answers for the option medical phys from hsc  2001 - 2007? The sample answers of Andrew Harvey only include the option Quanta to Quarks

For part b) i - what would you discuss. I'm not quite sure why its given 3 marks

[Baylsskool]

This highlights the issue with explaining the Meisner Effect in this way, the textbook is incorrect! The Meisner Effect is distinct from the idea of eddy currents, so yes, a magnet would need to move for eddy currents to form! And the Meisner Effect is separate, it happens for other reasons  that you don't really need to know

Yeah alright cheers jamon
Could you explain the exclusion principle as electrons cannot be in the same place at the same time at the same spin, so what did Heisenberg then tell us when he said that the momentum and displacement of a certain electron could not be determined simultaneously ? That both combined meant, it's uncertain where electrons are but they are never in the same spot as each other basically??

[aryak]

Can someone explain to me 2014, question 26 (b). How do I find the x -intercept? Help would be appreciated.

https://www.boardofstudies.nsw.edu.au/hsc_exams/2014/pdf_doc/2014-hsc-physics.pdf

[winstondarmawan]

Hello! Can someone please explain why the answer is A.
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/23022130_1363865803738939_1475372061_n.png?oh=4cf36567bde7eb0f93a759853295f976&oe=59F77BE6

[beau77bro]

Does anyone know where I can get sample answers for the option medical phys from hsc  2001 - 2007? The sample answers of Andrew Harvey only include the option Quanta to Quarks

For part b) i - what would you discuss. I'm not quite sure why its given 3 marks

for the sample answers - you most likely need to buy a success one or coroneos book.

in terms of reliability you can say:
- you checked if the author was credible, and whether their sources were reputable.
- you compared this info with other sources to see if it was consistent
- and you checked with your teacher (you can also say that the principles used clearly show the logic/reasoning to support the concepts - but hard to say with how complex mri is)

[sidzeman]

for the sample answers - you most likely need to buy a success one or coroneos book.

in terms of reliability you can say:
- you checked if the author was credible, and whether their sources were reputable.
- you compared this info with other sources to see if it was consistent
- and you checked with your teacher (you can also say that the principles used clearly show the logic/reasoning to support the concepts - but hard to say with how complex mri is)

Wouldn't the authors being credible relate to accuracy, and not reliability?

[beau77bro]

Hi guys. I have a question related to photoelectric effect that i need help with.
Here's the question.

When a zinc plate was exposed to ultraviolet light, electrons were released. One electron had kinetic energy of 3.0 x 10^-19 J

a) If a retarding voltage of 0.80V was applied, calculate the kinetic energy of the electron when it reached the anode.
b) Find the minimum retarding voltage that would inhibit the electron from reaching the anode.

Since my inability to solve this stems from my lack of understanding on the relationship between work function, retarding/stopping voltage and threshold frequency, an explanation would be much appreciated on how to solve these questions! I get the general gist of what photoelectric effect is (planck proposed light was packets of discrete energy, and Einstein, from Planck's observations, made further discoveries and derived that light was instead quantised into photons, and that there was a threshold frequency required before photoelectric effect could take place, and then intensity would take its part after threshold frequency was reached.

If these answers are right I can explain it. I'm not sure though, so if they aren't I can't help and actually need help too hahah

[beau77bro]

Can someone explain to me 2014, question 26 (b). How do I find the x -intercept? Help would be appreciated.

https://www.boardofstudies.nsw.edu.au/hsc_exams/2014/pdf_doc/2014-hsc-physics.pdf

volts are equal to eV since the units are 1:1 (im pretty sure - jamon can help here), so what you can do is create a linear y = mx + b, with y = eV, x = frequency, b = - 4.1 eV and M = the gradient from the line. then u can sub in the eV = 1.2 and rearrange to find the frequency, or just do it by graphing the new line by making eV = 0 and finding the threshold frequency whilst keeping the gradient the same as the first graph.

[beau77bro]

Wouldn't the authors being credible relate to accuracy, and not reliability?

yes, but i also think being credible means their information is reliable (aka consistent or trustworthy - since it's hard to repeat a research task without just comparing this is a good measure of reliability). also reliability is about repeatability - so if the source isn't credible/is faulty then it's unlikely you will find a common answer from an esteemed professor or whomever.

[sidzeman]

yes, but i also think being credible means their information is reliable (aka consistent or trustworthy - since it's hard to repeat a research task without just comparing this is a good measure of reliability). also reliability is about repeatability - so if the source isn't credible/is faulty then it's unlikely you will find a common answer from an esteemed professor or whomever.

Ahhh I see thank you very much!

Could someone also help me with part d of question 20 from the 2006 hsc?

i attempted it by multiplying the mass of the rod by 9.8 to give to force that should act on the balance. Then using a given current and the mass at that force i calculated the actual force that was acting on the balance (e.g. at I = 2.8, reading = 0.5485 - times that by 9.8 to give force on balance). Subtract the original weight from this = force due to parallel conductors = equate to lkII/d to find d - however this does not work