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OCR Physics A G485 - Frontiers of Physics - 18th June 2015

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Reply 220
Avatar for Elcor
Elcor
14
Original post by BecauseFP
So, does anyone know for sure what we need to know about Mass Spectrometers?


Might be wrong but I think it's just:

Vaporised, ionised, accelerated through an electric field, deflected through a magnetic field and detected.
Original post by Elcor
Might be wrong but I think it's just:

Vaporised, ionised, accelerated through an electric field, deflected through a magnetic field and detected.


I thought it was

Velocity selector where Bqv=Eq

So v = E/B

Only particles with right Velocity can pass through, then after that it enters a circular chamber where only magnetic field is present so they undergo circular motion

And then using the charge to mass ratio, and relative presence, you can determine their isotopes and that

Not sure what's actually needed I have to say, I just remember him teaching thay
Reply 222
Avatar for Hazza_
Hazza_
1
Does anyone understand the Doppler scan formula derivation on page 204? It says that for a reflection, the new frequency is c/(c-2v)*f. Everything I can find online says that it is actually (c+v)/(c-v)*f.

edit: Nevermind, it turns out that it is an approximation that only works for large values of c and relatively small values of v. The book doesn't tell you this of course.

http://www.wolframalpha.com/input/?i=plot+%28c%2Bv%29%2F%28c-v%29+and+c%2F%28c-2v%29+where+c%3D3*10^8
(edited 8 years ago)
Original post by Hazza_
Does anyone understand the Doppler scan formula derivation on page 204? It says that for a reflection, the new frequency is c/(c-2v)*f. Every resource I can find online says that the new frequency is actually (c+v)/(c-v)*f. These aren't the same thing.


upload a picture
Reply 224
Avatar for Hazza_
Hazza_
1
Original post by chem@uni
upload a picture


Don't worry I found the answer, if you compare the graphs they are close enough at small speeds.
Didn't go into school, did June 2010

It seems a good 80+ will guarantee an A*
Original post by MO2898
If they have increased the time allowed from 1 hour to 1 hour 15 for unit G484 (Newtonian world) does that mean there will be an extra question in the paper? And how can one find out about the mark needed for an A* as they don't show it on the OCR online grade boundaries? (Did Edexcel AS physics so this is all new to me) Thanks!


If you google for example june 2011 90% conversion rates, then it shows you the grade boundaries needed for A*'s in all A2 units
Reply 227
Avatar for Elcor
Elcor
14
Original post by L'Evil Fish
Didn't go into school, did June 2010

It seems a good 80+ will guarantee an A*


74 was an A* for that paper, but it's generally about mid 80s for the 4 most recent ones. I think that's what we should expect.
Reply 228
Avatar for Elcor
Elcor
14
A mark scheme says the binding energy definition is "The energy required to separate the nucleons in a nucleus". Would I not get the marks for saying "The energy released when a nucleus is formed from its nucleons"? This is is the same thing, but it's not in the mark scheme.
Original post by Elcor
A mark scheme says the binding energy definition is "The energy required to separate the nucleons in a nucleus". Would I not get the marks for saying "The energy released when a nucleus is formed from its nucleons"? This is is the same thing, but it's not in the mark scheme.


Possibly but may as well update it to match the one in the mark scheme
Original post by Elcor
A mark scheme says the binding energy definition is "The energy required to separate the nucleons in a nucleus". Would I not get the marks for saying "The energy released when a nucleus is formed from its nucleons"? This is is the same thing, but it's not in the mark scheme.


That's very good way of thinking about things but a nucleus cannot be formed from its constituents in that way because for that to happen the strong nuclear force must have a very long range to 'grab' the constituents together forming the nucleus. We still have difficulty fusing two protons(hydrogen atoms) because the strong nuclear force has a very short range and only attracts the nucleons at a very small distance. The electrostatic repulsion doesn't allow that to happen.
So we need an enormous amount of energy to form a nucleus from its constituents that the binding energy released after formation would seem negligible.


Posted from TSR Mobile
Original post by Elcor
74 was an A* for that paper, but it's generally about mid 80s for the 4 most recent ones. I think that's what we should expect.


Ah it should be fine then even if it is Mid 80s!

This is penultimate paper so I feel it'll be a shocker like last year's AS
Reply 232
Avatar for Elcor
Elcor
14
Original post by Mehrdad jafari
That's very good way of thinking about things but a nucleus cannot be formed from its constituents in that way because for that to happen the strong nuclear force must have a very long range to 'grab' the constituents together forming the nucleus. We still have difficulty fusing two protons(hydrogen atoms) because the strong nuclear force has a very short range and only attracts the nucleons at a very small distance. The electrostatic repulsion doesn't allow that to happen.
So we need an enormous amount of energy to form a nucleus from its constituents that the binding energy released after formation would seem negligible.


Posted from TSR Mobile


That's a good explanation, thank you. Doesn't it depend on which way is energetically favourable though? As long as the binding energy per nucleon increases, it will release energy. What you're saying is true for heavy elements that prefer fission but not lighter elements. Is this wrong?

Posted from TSR Mobile
(edited 8 years ago)
Original post by Elcor
That's a good explanation, thank you. Doesn't it depend on which way is energetically favourable though? As long as the binding energy per nucleon increases, it will release energy. What you're saying is true for heavy elements that prefer fission but not lighter elements. Is this wrong?

Posted from TSR Mobile


No worries.
Yeah thats right but a nucleus doesn't form naturally from its constituents. It would be easier for lighter elements to to form a nucleus but even that requires a lot amount of energy let alone heavy elements.


Posted from TSR Mobile
Just finished the course today at school, so far I think I'm good on all topics except Nuclear physics, and kind of lost on the mass spectrometer thing, Will need 83 for an A, how are you guys doing?
Original post by whyalwaysme??
Just finished the course today at school, so far I think I'm good on all topics except Nuclear physics, and kind of lost on the mass spectrometer thing, Will need 83 for an A, how are you guys doing?


We finished last week!
Can anyone explain the need for gradient coils in MRI a bit? I know they're used to vary the magnetic field strength slightly throughout the body to vary the Larmor frequency of the protons, but can anyone explain why this needs to be done?
(edited 8 years ago)
Original post by BrokenS0ulz
Can anyone explain the need for gradient coils in MRI a bit? I know they're used to vary the magnetic field strength slightly throughout the body to vary the Larmor frequency of the protons, but can anyone explain why this needs to be done?


You are right they are used to vary magnetic field strength slightly throughout the body to vary larmor frequency. This needs to be done because in order for the proton to gain large amounts of energy to go into the precession state the larmor frequency needs to be equal to the frequency of precession. Only when it precesses the proton will release energy in the form of radio frequency so it can be detected through the coils. This will then be used to pinpoint location with the time delay etc..
Original post by sagar448
You are right they are used to vary magnetic field strength slightly throughout the body to vary larmor frequency. This needs to be done because in order for the proton to gain large amounts of energy to go into the precession state the larmor frequency needs to be equal to the frequency of precession. Only when it precesses the proton will release energy in the form of radio frequency so it can be detected through the coils. This will then be used to pinpoint location with the time delay etc..


So why do we need to change it as we go down the body?

I write the answers and get the marks, but I don't understand why we want different larmor frequencies

Wouldn't that just flip different protons idk
Original post by L'Evil Fish
So why do we need to change it as we go down the body?

I write the answers and get the marks, but I don't understand why we want different larmor frequencies

Wouldn't that just flip different protons idk


As far as I know I think the frequency at which a proton or group of protons in a space in 3D resonates depends on the strength of the local magnetic field around the proton or group of protons. So then by applying additional magnetic fields that vary, specific slices to be imaged can be selected, and an image is obtained. In laymen terms each group of protons in different parts of the body have like different density and strength of magnetic field so frequency needs to be varied over different parts. Doesn't matter if different protons do flip over because they're just looking for time delays. So tumours will have a higher time delay than normal tissue.I am quite sure this will not be asked in the exam. But it's good to know :biggrin:
(edited 8 years ago)

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