HSC Physics Question Thread

[jamonwindeyer]

I don't know if anyone did Medical Physics but I'm having a hard time understanding the difference between T1 & T2 weighted for MRI.

Can anyone distinguish this for me?

Hey! It was my option (Jake's too), happy to lend a hand! This was probably the most difficult concept in the course for me, so I hope I can explain it in a way that is beneficial

Okay, so we know that when we expose nuclei to radio waves of an appropriate frequency, they resonate and precess, and this causes change to the net magnetisation vector. Relaxation is related to this vector.

T1 Relaxation: As nuclei are exposed to radio waves, they flip into anti-parallel alignments. This shrinks the component of the net magnetisation vector in the direction of the file. T1 Relaxation concerns this vector returning to its initial value (specifically, the T1 relaxation time is when the vector returns to 63% of the initial value), as nuclei dissipate their energy into the surrounding lattice. This diagram does a pretty good job showing what this looks like (ignore the equation, it's beyond this course):



T2 Relaxation: For reasons slightly beyond the scope of the courses, exposing precessing nuclei to radio frequencies causes the net magnetisation vector to shift into the transverse plane, perpendicular to the applied field. Remember that previously, there was no component of the vector in this direction, it was only parallel to the field. The result is that the precession of the nuclei as a whole loses coherency and phase. As the nuclei relax, those precessing about this transverse axis transfer their energy to those precessing parallel to the field, and the component of the vector perpendicular to the field shrinks. T2 Relaxation Time is when the vector shrinks back to 37% of its initial. Again, see diagram:



That is the complicated version: If you want simple, go for this instead. Applying a magnetic field rotates the net magnetisation vector 90 degrees, so it ends up perpendicular to the applied field. T1 Relaxation time measure the parallel component increasing to normal, T2 relaxation time concerns the perpendicular component shrinking away. This is kind of why the two numbers (63% and 37%) are related (adding to 100%), because they concern the same vector

I hope this makes sense! Understanding this demands a rock solid knowledge of everything else in this section; and then a lot of dedication and research to really wrap your head around it!

Oh! So for actually weighting to detect these relaxation times, a few things to note about MRI imaging radio waves first.

Repetition Time (TR): Elapsed time between pulses of radio waves
Echo Delay Time (TE): Time delay between the sending of radio waves and measurement of emitted signals

For T1 weighted images, we want to emphasise areas with short T1 relaxation time (meaning, they dissipate energy very quickly). This is achieved by minimising the repetition time, meaning that only nuclei with short T1 will have the time to dissipate their energy before the next pulse.

For T2 weighted images, we want to emphasise areas with a long T2 relaxation time (meaning, it takes a long (relative) time for them to return to their initial coherency, and thus, will emit MR signals for longer). We do this by maximising echo delay time, so that by the time we take the measurement, only nuclei with long T2 will still be emitting MR signals

[jamonwindeyer]

Hi!
I'm stumped. Is trajectory and projectile motion the same thing? 0.o

Hey! Essentially, like analysing the trajectory of a projectile is what we do in projectile motion

[conic curve]

Damm, no moderators did quanta to quarks did they?

[jamonwindeyer]

Damm, no moderators did quanta to quarks did they?

Spencerr did I believe, and Jake has a good knowledge of that Option from his own studies! Definitely ask any questions you have

[conic curve]

Spencerr did I believe, and Jake has a good knowledge of that Option from his own studies! Definitely ask any questions you have

Oh fantastic, no need to worry

[Klexos]

Hey! It was my option (Jake's too), happy to lend a hand! This was probably the most difficult concept in the course for me, so I hope I can explain it in a way that is beneficial

Okay, so we know that when we expose nuclei to radio waves of an appropriate frequency, they resonate and precess, and this causes change to the net magnetisation vector. Relaxation is related to this vector.

T1 Relaxation: As nuclei are exposed to radio waves, they flip into anti-parallel alignments. This shrinks the component of the net magnetisation vector in the direction of the file. T1 Relaxation concerns this vector returning to its initial value (specifically, the T1 relaxation time is when the vector returns to 63% of the initial value), as nuclei dissipate their energy into the surrounding lattice. This diagram does a pretty good job showing what this looks like (ignore the equation, it's beyond this course):

(Image removed from quote.)

T2 Relaxation: For reasons slightly beyond the scope of the courses, exposing precessing nuclei to radio frequencies causes the net magnetisation vector to shift into the transverse plane, perpendicular to the applied field. Remember that previously, there was no component of the vector in this direction, it was only parallel to the field. The result is that the precession of the nuclei as a whole loses coherency and phase. As the nuclei relax, those precessing about this transverse axis transfer their energy to those precessing parallel to the field, and the component of the vector perpendicular to the field shrinks. T2 Relaxation Time is when the vector shrinks back to 37% of its initial. Again, see diagram:

(Image removed from quote.)

That is the complicated version: If you want simple, go for this instead. Applying a magnetic field rotates the net magnetisation vector 90 degrees, so it ends up perpendicular to the applied field. T1 Relaxation time measure the parallel component increasing to normal, T2 relaxation time concerns the perpendicular component shrinking away. This is kind of why the two numbers (63% and 37%) are related (adding to 100%), because they concern the same vector

I hope this makes sense! Understanding this demands a rock solid knowledge of everything else in this section; and then a lot of dedication and research to really wrap your head around it!

Oh! So for actually weighting to detect these relaxation times, a few things to note about MRI imaging radio waves first.

Repetition Time (TR): Elapsed time between pulses of radio waves
Echo Delay Time (TE): Time delay between the sending of radio waves and measurement of emitted signals

For T1 weighted images, we want to emphasise areas with short T1 relaxation time (meaning, they dissipate energy very quickly). This is achieved by minimising the repetition time, meaning that only nuclei with short T1 will have the time to dissipate their energy before the next pulse.

For T2 weighted images, we want to emphasise areas with a long T2 relaxation time (meaning, it takes a long (relative) time for them to return to their initial coherency, and thus, will emit MR signals for longer). We do this by maximising echo delay time, so that by the time we take the measurement, only nuclei with long T2 will still be emitting MR signals

Wow .____, My teacher legit said a couple of sentences and pointed at the PIF, I'm going to have to try understand this and get back to you XD

Thank you, Jamon!

[jamonwindeyer]

Wow .____, My teacher legit said a couple of sentences and pointed at the PIF, I'm going to have to try understand this and get back to you XD

Thank you, Jamon!

Ahaha yep, my teacher was a legend and taught this extremely well, I got a lot of detail! You'll never be asked for this much in a HSC Exam, so take as much or little of it as you need to understand what is happening!

Feel free to quote specific bits you don't get and I can help clarify it for you, I did a bit of a content dump on you

[conic curve]

Ahaha yep, my teacher was a legend and taught this extremely well, I got a lot of detail! You'll never be asked for this much in a HSC Exam, so take as much or little of it as you need to understand what is happening!

Feel free to quote specific bits you don't get and I can help clarify it for you, I did a bit of a content dump on you

Loving your indepth explanations there Jamon 

[Neutron]

Hey guys! Having trouble with this 2011 HSC question (even though i really shouldn't):

A single turn coil is positioned in a region of uniform magnetic field with a strength of 0.2 T. The plane of the coil is at 45 degrees to the magnetic field. The coil is a square with 5cm sides and carries a current of 10.0A. Explain why the net force produced by the magnetic field on the coil is zero.

I didn't think the net force would be zero cause doesn't the coil rotate haha thank you!

[jakesilove]

Hey guys! Having trouble with this 2011 HSC question (even though i really shouldn't):

A single turn coil is positioned in a region of uniform magnetic field with a strength of 0.2 T. The plane of the coil is at 45 degrees to the magnetic field. The coil is a square with 5cm sides and carries a current of 10.0A. Explain why the net force produced by the magnetic field on the coil is zero.

I didn't think the net force would be zero cause doesn't the coil rotate haha thank you!

That's a totally understandable mistake to make! From memory, one side was labelled AB, and the other was labelled CD. Now, we know that the force on each size is going to be F=BILsin(theta). That will cause AB to 'rotate' in a particular direction, right? However, the force on CD will be exactly the same as the force on AB (same current, length etc.) but in the opposite direction! Therefore, the two forces EXACTLY cancel out. This means that the NET force on the coil is zero, but there IS a force on each side (which causes the rotor to rotate). I hope this makes sense; let me know if I can clarify further!

Jake

[brontem]

Hey can someone help me with part (ii) of this question?

[jakesilove]

Hey can someone help me with part (ii) of this question?

Hey!

I would structure an answer to include the following. First, we recall that moving particles have a wavelength (the de Broglie wavelength, with formula lambda=h/mv). Now, for orbiting electrons, there are only certain set 'distances' (ie. quantum numbers) that allow the electrons to orbit WITHOUT causing destructive interference with their own waves. This explains why there are set orbits around the nucleus. When a photon is incident on a Hydrogen atom, it is really going to hit an electron. If the photon has the exact right amount of energy to 'jump' the electron to another allowed energy level, it will be absorbed. However, most photons will have a different energy (calculated by E=hf), and so cannot be absorbed by the electron. Thus, they are reflected!

I hope that makes sense! Let me know if you need me to clarify anything.

Jake

[yaboiaderler]

Hiya, I have two content based questions:
1. Topic: Space
In special relativity, in regards to calculations of mass dilation, length contraction and time dilation, I have some difficulty in determining which values are which - how do you know/determine, for example in length contraction, which value is l_o and l_v?

2. Topic: Motors and Generators
In some multiple choice questions, I have come across questions that ask you to determine the direction of an eddy current when say, a metal sheet is  pulled in/out of a magnetic field. How do you actually determine the direction? (Lenz's Law is confusing)

Thanks!

[RuiAce]

Hiya, I have two content based questions:
1. Topic: Space
In special relativity, in regards to calculations of mass dilation, length contraction and time dilation, I have some difficulty in determining which values are which - how do you know/determine, for example in length contraction, which value is l_o and l_v?

2. Topic: Motors and Generators
In some multiple choice questions, I have come across questions that ask you to determine the direction of an eddy current when it its pulled in/out of a magnetic field. How do you actually determine the direction? (Lenz's Law is confusing)

Thanks!

The way I learnt how to distinguish between l_o and l_v is that instead of considering rest frame and moving frame, I started considering two things:
1) Which frame is your point of view. Your point of view is the l_o
2) Use logic. Length contracts, time and mass dilate. So if the wrong thing happens (say, your length got longer when it should've been shorter), just swap them around lol.

And for Lenz's law, I think about what's going on. Say a magnet with north pole is travelling downwards. The way I think about it is that it's introducing "too much north". So (not literally, but) by Lenz's law it wants to get rid of the north. How do we get rid of the north? Put a south back in there.

So I figure out the polarity first. THEN I choose to use the grip rule (or push rule where applicable).


The other mods can give more tips here. This was just me.

[jamonwindeyer]

Hiya, I have two content based questions:
1. Topic: Space
In special relativity, in regards to calculations of mass dilation, length contraction and time dilation, I have some difficulty in determining which values are which - how do you know/determine, for example in length contraction, which value is l_o and l_v?

2. Topic: Motors and Generators
In some multiple choice questions, I have come across questions that ask you to determine the direction of an eddy current when say, a metal sheet is  pulled in/out of a magnetic field. How do you actually determine the direction? (Lenz's Law is confusing)

Thanks!

Ditto with Rui above!! Common sense is the best way to check which is which in those equations; since it will always change depending on frame of reference as to which is which. Remember, as Rui said, that lengths get shorter as you watch something fly past, times/masses get larger as you watch something fly past

I'd love to give you an example of how to figure out the direction, but I'd prefer it to be something you are specifically having trouble with. Reckon you could snap a picture of an example?