HSC Physics Question Thread

[Cindy2k16]

Hey guys, for the medical physics dot point about bone scans vs x rays,  are bone scans CAT scans or PET scans?
I'm getting some conflicting information.
Thanks.

Its neither! Bone scans involve injecting a radiopharmaceutical into the body- but the radioisotope used is not a positron emitting one therefore its not PET- Technetium-99m is the radioisotope commonly used for bones scans. Tc-99m emits gamma radiation which is detected with gamma cameras outside the body. It works similar to PET though, as the radioisotope is taken up into the bones and accumulates in areas of high metabolism in the bones and then multiple detections of the radiation from the body allows for the scan to show areas of high concentration of the radioisotope -> shows the presence of bone tumours. Which x-rays can't detect well.
Hope this helps

[MarkThor]

You estimate the peak weavelength ~7 micrometres from the graph and calculate the energy with E=hf. You'll find the value you get is closest to the band gap of B.

Thank you 

[Cindy2k16]

Hi when a spacecraft is travelling through space, theoretically (just in terms of what we need to know in the HSC) does it not need to fire any rockets to continue moving? If yes, is this because theres practically no friction in space and unless the spacecraft is passing close to a planet, its not experiencing any (significant) forces and so by Newton's 1st law it'll just continue on its straight line path?

Also should we be able to describe thought experiments for time dilation, mass dilation or length contraction? Currently I can only describe a thought experiment for the relativity of simultaneity.
TIA

[annabelkaren]

Explain what happens to a rocket's chemical energy, kinetic energy, and gravitational potential energy when it is being launched from the surface of Earth. - 3 Marks

[Cindy2k16]

Explain what happens to a rocket's chemical energy, kinetic energy, and gravitational potential energy when it is being launched from the surface of Earth. - 3 Marks

When a rocket burns its fuel during rocket launch, it is converting the chemical energy of the fuel into kinetic energy (the gases released from the combustion of fuel eject out, causing the rocket to move upwards). Thus the rocket's chemical energy decreases and its kinetic energy increases.
When the rocket is launching from the surface of the earth, it is doing work against the gravitational field. Thus, its gravitational potential energy increases.
Hope this helps

[Mei2016]

Hi, I have a few questions:

So Pauli predicted the existence of the neutrino after studying beta decays and suggested the existence of them to satisfy the laws of conservation of momentum and energy. However, beta decay (ie beta minus) is due to emission of neutrons, where the neutrons decay into a proton, an electron and an anti-neutrino. So an anti-neutrino is produced by beta decay, so why did Pauli suggest the  neutrino?

-When Pauli suggested the neutrino, this covers the three types right? (the electron neutrion, tau and muon neutrinos?)

-Also, is it advised to remember the nuclear equation for what was actually happening in Fermi's Uranium Problem(the neutron + 235 U --> 2 daughter products and 3 neutrons)?

-In 2006 paper, in Q9, the answer is B and do they mean clockwise or anticlockwise?
(is it anticlockwise because charges would move clockwise Y to X, however, it's an induced current so due to Lenz's law it would flow the opposite direction, so anticlockwise?)

And for 2013, Q16, why is the answer D and not C?, because isn't it in metals that Valence Band and Conduction Band are both full at the same time. The question doesn't specify what band it is 'electrons in a fully filled band' so I wasn't quite sure on this one.

-Also, for 2011 Q34 c) for forces in the atomic nucleus, is electrostatic only for protons to protons or electrons to electrons? (so it's direction is always 'repulsive'?) or could it be between protons and electrons (then the direction would be 'attractive')
-and isn't the direction for SNF attractive at certain distances and then repulsive if the distances are less than 1x10^-15m? ( the answers for the table in Q34c) only had 'attractive' as the direction...)

[Cindy2k16]

Hi, I have a few questions:

So Pauli predicted the existence of the neutrino after studying beta decays and suggested the existence of them to satisfy the laws of conservation of momentum and energy. However, beta decay (ie beta minus) is due to emission of neutrons, where the neutrons decay into a proton, an electron and an anti-neutrino. So an anti-neutrino is produced by beta decay, so why did Pauli suggest the  neutrino?

-When Pauli suggested the neutrino, this covers the three types right? (the electron neutrion, tau and muon neutrinos?)

-Also, is it advised to remember the nuclear equation for what was actually happening in Fermi's Uranium Problem(the neutron + 235 U --> 2 daughter products and 3 neutrons)?

-In 2006 paper, in Q9, the answer is B and do they mean clockwise or anticlockwise?
(is it anticlockwise because charges would move clockwise Y to X, however, it's an induced current so due to Lenz's law it would flow the opposite direction, so anticlockwise?)

And for 2013, Q16, why is the answer D and not C?, because isn't it in metals that Valence Band and Conduction Band are both full at the same time. The question doesn't specify what band it is 'electrons in a fully filled band' so I wasn't quite sure on this one.

-Also, for 2011 Q34 c) for forces in the atomic nucleus, is electrostatic only for protons to protons or electrons to electrons? (so it's direction is always 'repulsive'?) or could it be between protons and electrons (then the direction would be 'attractive')
-and isn't the direction for SNF attractive at certain distances and then repulsive if the distances are less than 1x10^-15m? ( the answers for the table in Q34c) only had 'attractive' as the direction...)

Hi i can help with some of these questions (i dont do quanta to quarks)

-In 2006 paper, in Q9, the answer is B and do they mean clockwise or anticlockwise?
(is it anticlockwise because charges would move clockwise Y to X, however, it's an induced current so due to Lenz's law it would flow the opposite direction, so anticlockwise?)

It is clockwise from Y to X. When you apply the right hand palm rule, you point the thumb in the direction that the wheel is turning through the magnetic field since that is the direction of movement of positive charge. The movement of the wheel through the magnetic field is basically the movement of the 'positive charge' in the metals as they are being moved with the wheel. Does this make sense? And then you point ur fingers in the direction of the magnetic field and you will get a downwards force on the positive charges. Thus current flows down out of the wheel through Brush Y, through the external circuit, to X.

And for 2013, Q16, why is the answer D and not C?, because isn't it in metals that Valence Band and Conduction Band are both full at the same time. The question doesn't specify what band it is 'electrons in a fully filled band' so I wasn't quite sure on this one.

It isn't true that in conductors both the valence band and conduction band are both full at the same time. In fact too many electrons in the conduction band make it too crowded and in fact makes it harder for current to flow, I think. Sorry I dont know a very scientific explanation.. But in conductors the conduction band is partially filled. So its C.
Hope this helps

[jamonwindeyer]

Hi when a spacecraft is travelling through space, theoretically (just in terms of what we need to know in the HSC) does it not need to fire any rockets to continue moving? If yes, is this because theres practically no friction in space and unless the spacecraft is passing close to a planet, its not experiencing any (significant) forces and so by Newton's 1st law it'll just continue on its straight line path?

Also should we be able to describe thought experiments for time dilation, mass dilation or length contraction? Currently I can only describe a thought experiment for the relativity of simultaneity.
TIA

First point: Yep! You don't need to do any work to keep a craft moving in space (theoretically)

Second: The experiments you know are fine, the syllabus only asks you to know about the 'train experiments' and be able to discuss the benefits/drawbacks of thought experiments in general

Ps - You are answering so many questions, you are awesome!! Thank you!

[jamonwindeyer]

Explain what happens to a rocket's chemical energy, kinetic energy, and gravitational potential energy when it is being launched from the surface of Earth. - 3 Marks

Welcome to the forums Anna! Let me know if you need a hand finding anything!

[pels]

Hi all,

What do we need to know about transistors and diodes?

[Cindy2k16]

First point: Yep! You don't need to do any work to keep a craft moving in space (theoretically)

Second: The experiments you know are fine, the syllabus only asks you to know about the 'train experiments' and be able to discuss the benefits/drawbacks of thought experiments in general

Ps - You are answering so many questions, you are awesome!! Thank you!

Thanks for clarifying Jamon! And its no problem

[annabelkaren]

When a rocket burns its fuel during rocket launch, it is converting the chemical energy of the fuel into kinetic energy (the gases released from the combustion of fuel eject out, causing the rocket to move upwards). Thus the rocket's chemical energy decreases and its kinetic energy increases.
When the rocket is launching from the surface of the earth, it is doing work against the gravitational field. Thus, its gravitational potential energy increases.
Hope this helps

Thank you so much!!!

[Brenda0708]

Hi I'm confused about the angle used in the equation F=qvBsintheta here. If theta is the angle between the velocity vector and the direction of the MF lines, how does that look for this question?

[Cindy2k16]

(Image removed from quote.)

Hi I'm confused about the angle used in the equation F=qvBsintheta here. If theta is the angle between the velocity vector and the direction of the MF lines, how does that look for this question?

Hi the way this question is set up is kind of a trick. They give you an angle, but its not needed.. This is because the angle used in F=qvBsintheta is the angle between the velocity vector of the electron and the direction of the magnetic field. If you look at the diagram carefully, you will realise that the electron is in fact travelling perpendicular to the magnetic field and so theta is actually 90 degrees not 40 thus sin90=1. I believe everything else you're doing is fine- just use 90 degrees instead (or just use F=qvB, its the same thing as F=qvBsin90)
I've seen a few questions with this trick so always be careful with these questions where they provide an angle!
Hope this helps

[imtrying]

With the dotpoint about Hertzs observations with radio waves producing a photoelectric effect, would I be correct in saying that the fact that the spark stopped when UV was blocked by glass showed that it was preventing the photoelectric effect from happening because no UV light was hitting the receiver meaning it was no longer emitting electrons which therefore meant there was no spark?