HSC Physics Question Thread

[imda.beast]

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.

hi, thank you for your help, but the correct answer was B from 2008 HSC Q15.
any idea why?

[CyberScopes]

could someone please explain T1 and T2 relaxation times simply, as well as in detail/explanation hard out. im seriously struggling with this and it seems both overly complicated and lackingly explained all at once. thanks and pliz.

As far as ive seen, every explanation either seems beyond the scope of the syllabus or too shallow to understand the concept, but i can try from what I know.

Assume that the radio waves have already been projected, and the nuclei of the atoms have already absorbed the energy, etc.
When the radio waves are switched off, different tissues with different densities release energy at different time periods.

When pulses of radio waves are quickly switched on and off, only tissues with short T1 values release their energy quickly and can be detected by the scanning procedure. These tissues are tissues like fat, liver, spleen, etc. This allows the MRI image to contrast between these tissues and watery tissues.

If the MRI wants to the opposite and instead highlight the watery tissues, it uses delayed radio pulses that are slow and only allow tissues with long T2 values are detected (like the brain), so the fatty tissues mentioned earlier will not appear as contrasted as the watery tissues.

This isnt the best explanation but its all I have 

[Bubbly_bluey]

Could Someone please help explain this to me
What I understand is that it's a DC source which means that instead of trying to minimise the Voltage as we would if it were an AC source in order to maximise the current - we are instead trying to maximise the voltage as this corresponds to an increase in emf which corresponds to an increase in current? I don't understand why in this case a high voltage in the primary coil corresponds to a high current.
This is from the 2015 hsc q18. The answer is A.
please help

Many Thanks

I think the best way to work this out is to use a bit of maths. Using Np/Ns=Vp/Vs. So q ask for greatest deflect infering that we want the greatest volatage in secondary coil. Make Vs the subject and you get Vs= (Vp x Ns)/ Np. So if you decrease Np (number of coils in primary you have greater secondary volatage) Therefore A

[blasonduo]

hi, thank you for your help, but the correct answer was B from 2008 HSC Q15.
any idea why?

I think there might've been a mixup!

The way I interpreted this was just to follow the positive terminal, and you see current flows left to right but as electrons move in the opposite direction and thus RIGHT to LEFT (going left) so B

[beau77bro]

As far as ive seen, every explanation either seems beyond the scope of the syllabus or too shallow to understand the concept, but i can try from what I know.

Assume that the radio waves have already been projected, and the nuclei of the atoms have already absorbed the energy, etc.
When the radio waves are switched off, different tissues with different densities release energy at different time periods.

When pulses of radio waves are quickly switched on and off, only tissues with short T1 values release their energy quickly and can be detected by the scanning procedure. These tissues are tissues like fat, liver, spleen, etc. This allows the MRI image to contrast between these tissues and watery tissues.

If the MRI wants to the opposite and instead highlight the watery tissues, it uses delayed radio pulses that are slow and only allow tissues with long T2 values are detected (like the brain), so the fatty tissues mentioned earlier will not appear as contrasted as the watery tissues.

This isnt the best explanation but its all I have 

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

[CyberScopes]

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

I honestly wish I knew, I feel like this entire topic is lacking. The content that we need to know for a strong understanding isnt a part of the syllabus, and its difficult to wrap our heads around whats actually going on if we dont understand this missing content. If you ever find anything please let me know too, Im as lost as you are haha

[beau77bro]

I honestly wish I knew, I feel like this entire topic is lacking. The content that we need to know for a strong understanding isnt a part of the syllabus, and its difficult to wrap our heads around whats actually going on if we dont understand this missing content. If you ever find anything please let me know too, Im as lost as you are haha

u will be the first to know - i have high hopes that atarGODS will save us in our time of need.
btw which avenger u reckon each atarGOD is...? lmao. thats a q for another time doe.

[statues]

I think there might've been a mixup!

The way I interpreted this was just to follow the positive terminal, and you see current flows left to right but as electrons move in the opposite direction and thus RIGHT to LEFT (going left) so B

Hey thanks for responding - It's attached to a galvanometer which measures current so a large current would induce a large deflection not as a direct result of voltage. However voltage apparently causes a large current which is what I don't understand.

[blasonduo]

Hey thanks for responding - It's attached to a galvanometer which measures current so a large current would induce a large deflection not as a direct result of voltage. However voltage apparently causes a large current which is what I don't understand.

Sorry, where's the galvanometer?

[statues]

Sorry, where's the galvanometer?

It's attached to the secondary coil

[blasonduo]

It's attached to the secondary coil

Oh I see! We were talking about different questions! My apologies!

[statues]

Oh I see! We were talking about different questions! My apologies!

oh jeez I quoted the wrong person, my bad! It's evidently been a long day of study haha

[statues]

I think the best way to work this out is to use a bit of maths. Using Np/Ns=Vp/Vs. So q ask for greatest deflect infering that we want the greatest volatage in secondary coil. Make Vs the subject and you get Vs= (Vp x Ns)/ Np. So if you decrease Np (number of coils in primary you have greater secondary volatage) Therefore A

Hey thanks for responding - It's attached to a galvanometer which measures current so a large current would induce a large deflection not as a direct result of voltage. However voltage apparently causes a large current which is what I don't understand.

[pikachu975]

hi, thank you for your help, but the correct answer was B from 2008 HSC Q15.
any idea why?

Oops my bad I thought the right one was positive for some reason!

Hey thanks for responding - It's attached to a galvanometer which measures current so a large current would induce a large deflection not as a direct result of voltage. However voltage apparently causes a large current which is what I don't understand.

As stated before, the reason why isn't in the HSC course. It has to do with the fact that the input voltage is DC which is why the formula np/ns = Is/Ip does not work. It's dumb that BOSTES put that question in.

If it was an AC input the answer should be what you said.

[Bubbly_bluey]

Hey thanks for responding - It's attached to a galvanometer which measures current so a large current would induce a large deflection not as a direct result of voltage. However voltage apparently causes a large current which is what I don't understand.

oml lol sorry yeah galvanometers measure current. But the maths would still work in this case you would use Np/Ns=Is/Ip. rearrange to make Is subject. Is=(Np x Ip) / Ns. So yes it would be B. Sorry that was a really bad mistake XD. Hope this clears it up