HSC Physics Question Thread

[bsdfjnlkasn]

Boron doping means it is p type so extra holes.

Positive flow goes left in the silicon block hence holes move left, so therefore electrons move right in the valence band as they causing the apparent hole movement, thus A.

Hey there,

Wouldn't the negative electrons move towards the positive terminal, so the left? Against conventional current i.e. positive hole flow?

Not sure, let me know if you can please clarify

[blasonduo]

Hey there,

Wouldn't the negative electrons move towards the positive terminal, so the left? Against conventional current i.e. positive hole flow?

Not sure, let me know if you can please clarify

Yep! He accidentally thought the positive terminal was on the right all good!

[bsdfjnlkasn]

Hey there,

I'm not sure how to calculate this, legit done it 3 times and am still getting B? I believe I've calculated the period and mass and converted it... but still confused

With the M-M question, all the answers seem wrong to be honest

With the coil, doesn't the CW current produce a magnetic field going into the page between the two rings? So that causes the inner ring to experience an increase in flux there meaning it will produce a current such that the magnetic field is out of the page (to nullify the increase going into the page?) This current will have to be CW. Answer is D?

Thanks!

[blasonduo]

Hey there,

I'm not sure how to calculate this, legit done it 3 times and am still getting B? I believe I've calculated the period and mass and converted it... but still confused

With the M-M question, all the answers seem wrong to be honest

With the coil, doesn't the CW current produce a magnetic field going into the page between the two rings? So that causes the inner ring to experience an increase in flux there meaning it will produce a current such that the magnetic field is out of the page (to nullify the increase going into the page?) This current will have to be CW. Answer is D?

Thanks!

Hey! Lets see what I can do

6) Straight out of the bat, we know that Michelson and Morley had a very valid experiment, so C and D are wrong. If we look at A, it says the experiment tested the theory of relativity, which is very wrong, the theory of relativity was not tested, however the test did strengthen that theory.

5) This question requires us to use the formula:















which is closest to D

11) Your theory is all right! However, if the second ring were to go clockwise the magnetic field in the middle of the rings would be into the page, and would help increase the magnetic field. If it were to go anti-clockwise, we would get that magentic field out of the page, opposing the change in flux, so it HAS to be anti-clockwise.

Hope this helps

[maevecouch]

thanks cyber scope, the explanation made it a little clearer. but im struggling to understand how the time of the radio pulses allows them to measure relaxation time. like how are we measuring these and what is the distinct difference which we are observing and quantifying? all the explanations just seem inadequate. like so the magnetic field is in the transcerse plane when the hydrogen nuclei move to anti parallel - but then what, why does that matter and what does that have to do with the relation time? is it to do with that magnetic vector or the hydrogen atoms precessions im just very confused. im trying to work out analogies and stuff to simply and explain to my friends and class mates but im struggling

Ok so I'm doing medical physics too and I have to agree that the "Relaxtion" section of the syllabus is vague around the complexity. If I'm right (which I'm most likely not), I believe that when the radio waves are applied to nuclei, the parallel aligned nuclei will absorb the energy and move into their higher anti parallel alignments. When the radio waves being applied are switched off, in accordance to the Law of Conservation of Energy, the nuclei 'relax' or return from their anti parralel to parallel alignments and release the radio wave energy they absorbed. The time it takes to do such is measured by MRI scanners which pick up these radio waves to enhance tissue contrast (or ehance the areas where this relaxtion is occuring) and use Echo Delay Time or Repetition time depending on whether they want to measure T1 or T2 relaxtion.

[winstondarmawan]

The Excel book has simplified the scenario a tad - I know what you are thinking, but the eddy currents in both P and R are both negligible. P is an insulator so they don't flow, and the break in R has the same effect. So assuming both are negligible, they'll hit the bottom at the same time and then Q will follow

Thank you for your response!
I'm just wondering why the eddy currents on R are negligible. I've always learnt that they had an impact on these types of questions, because even thought they are significantly smaller than the looped current produced in Q, they are still there.

[bsdfjnlkasn]

11) Your theory is all right! However, if the second ring were to go clockwise the magnetic field in the middle of the rings would be into the page, and would help increase the magnetic field. If it were to go anti-clockwise, we would get that magentic field out of the page, opposing the change in flux, so it HAS to be anti-clockwise.

Hope this helps

Hey there,

Still not so sure about this last explanation..

If the current in the inner coil is going ACW, then between the two rings, there will be a very large B field going into the page instead of there being nothing, which the CW current in the innermost coil would achieve.

Maybe if you could draw the magnetic fields or something? Really struggling with visualising it.. Thank you!

[blasonduo]

Hey there,

Still not so sure about this last explanation..

If the current in the inner coil is going ACW, then between the two rings, there will be a very large B field going into the page instead of there being nothing, which the CW current in the innermost coil would achieve.

Maybe if you could draw the magnetic fields or something? Really struggling with visualising it.. Thank you!

Hey I've attempted a drawing down below XD

I think there are 2 keys components here;

1) Clockwise produces a SOUTH pole, Anti-clockwise a NORTH pole
2) Look at the bigger picture, yeah, maybe the area between them might contradict, but look at what would have a bigger impact (which is the area in the middle) The overall net force here will decrease

Still unsure? Keep asking away!

[beau77bro]

Ok so I'm doing medical physics too and I have to agree that the "Relaxtion" section of the syllabus is vague around the complexity. If I'm right (which I'm most likely not), I believe that when the radio waves are applied to nuclei, the parallel aligned nuclei will absorb the energy and move into their higher anti parallel alignments. When the radio waves being applied are switched off, in accordance to the Law of Conservation of Energy, the nuclei 'relax' or return from their anti parralel to parallel alignments and release the radio wave energy they absorbed. The time it takes to do such is measured by MRI scanners which pick up these radio waves to enhance tissue contrast (or ehance the areas where this relaxtion is occuring) and use Echo Delay Time or Repetition time depending on whether they want to measure T1 or T2 relaxtion.

see i understood u right up until the end - how do they pick up this relaxtion time - i understand they simply measure the time to return, or for the magnetic vector to return, but how is that localised (is that with the gradient coil - but how does a big as radio coil which is measuring a large change identify just one localised area - ik this is the complicated part so ill ignore that) and also echo delay and repetition time - i feel uve said everything i need to know - i just need to grasp it. thankyou soo much! could u just explain that last bit?

[Mymy409]

In how much detail do we need to know forward-biased and reverse-biased?

[CyberScopes]

Sort of a general question, in many questions and answers ive seen that when the value of 'q' of an electron is taken, its taken as a positive rather than a negative as stated in the data sheet. My question is what does the negative actually mean, and why do we take a positive for the value of the charge?
TIA

[bsdfjnlkasn]

Hey I've attempted a drawing down below XD

I think there are 2 keys components here;

1) Clockwise produces a SOUTH pole, Anti-clockwise a NORTH pole
2) Look at the bigger picture, yeah, maybe the area between them might contradict, but look at what would have a bigger impact (which is the area in the middle) The overall net force here will decrease

Still unsure? Keep asking away!

Thanks! Makes heaps of sense now - it was the bigger picture I should have been looking at

[Zainbow]

What's the hardest hsc physics exam (arguably)?

[Zainbow]

Sort of a general question, in many questions and answers ive seen that when the value of 'q' of an electron is taken, its taken as a positive rather than a negative as stated in the data sheet. My question is what does the negative actually mean, and why do we take a positive for the value of the charge?
TIA

Usually, the questions would require you to calculate the magnitude of a force and its direction, so to calculate the magnitude you only need the value of the charge. The negative is only there to indicate direction, or that it behaves opposite to a positively charged particle with the same value for charge.

[maevecouch]

see i understood u right up until the end - how do they pick up this relaxtion time - i understand they simply measure the time to return, or for the magnetic vector to return, but how is that localised (is that with the gradient coil - but how does a big as radio coil which is measuring a large change identify just one localised area - ik this is the complicated part so ill ignore that) and also echo delay and repetition time - i feel uve said everything i need to know - i just need to grasp it. thankyou soo much! could u just explain that last bit?

It's pretty convultued hey? By the last part I'm guessing you mean Repition Time and Echo Delay. For T1 Relaxtion they use a short Repition Time to match the short T1 they use. The repition time is the time elapsed between consecutive  pulse of input radio waves. The relaxtion of the M vector happens very quickly, leaving some 'free' radio waves to be absorbed. So, the extra radio waves will make the image appear brighter as it is a more 'active' area. Whereas T2 use a long Echo Delay Time to match the long T2 in use. Echo time delay is the time delay betwee the emission of radio waves and measurement of those which return first. So in T2 images, the signals from the T2 relaxtion are measured after the initial radio waves are sent out which results in tissues with only long T2 contributing to the returning signals. Also, in T1 images, the Echo Delay Time is short so as to suppress any T2 and likewise, a long Repition Time is used in T2 images to suppress any T1. Hope that makes sense?