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February 25, 2020, 02:25:17 pm

Author Topic: Harder Physics Questions  (Read 9594 times)  Share 

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Edmund

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Re: Harder Physics Questions
« Reply #30 on: February 01, 2010, 09:59:21 pm »
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I'm allowed to ask questions right? I need help with this:

Magnetic resonance imaging (MRI) is a noninvasive technique for visualising the living human body. It depends on a phenomenon called nuclear magnetic resonance (NMR) which occurs when certain atoms are place in very strong magnetic fields. All nuclei that have odd mass numbers exhibit NMR as do all nuclei that have even mass numbers but odd atomic numbers. Nuclei having both even mass numbers and even atomic numbers do not exhibit NMR.


The number of neutrons in the nucleus of an atom in which NMR occurs is always

A  odd
B  even
C  odd if the number of protons is even
D  even if the number of protons is odd

Also explain why
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appianway

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Re: Harder Physics Questions
« Reply #31 on: February 01, 2010, 10:04:34 pm »
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n(protons) +n(neutrons) = mass number
n(protons) = atomic number

Therefore if n(protons) + n(neutrons) is odd, NMR is exhibited.
This staement means that if n(protons) is odd, n(neutrons) is even and vice versa.

Taking the second statement
n(protons) + n(neutrons) = even for NMR to occur
n(protons) = odd
Thus n(neutrons) = odd.

Therefore A is always true... I think.

Edmund

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Re: Harder Physics Questions
« Reply #32 on: February 01, 2010, 10:28:46 pm »
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Unfortunately that's wrong :( (Took me a while to figure out though)

A  The number of neutrons in the nucleus of an atom in which NMR occurs is always odd

Not always true. Look up lithium on the periodic table. Mass number 19 and atomic number (proton) 9, so even number of neutrons (10). NMR occurs in this nucleus, and has even neutrons ;)

According to the information provided, odd mass number and even atomic number exhibit NMR. An example is Beryllium (Mass 9, atomic 4). W can deduce that the number of neutrons is 5 and this is odd. We can then conclude that the number of neutrons in the nucleus of an atom in which NMR occurs is always odd if the number of protons is even.

D can't be the answer because fluorine can exhibit NMR (mass of 19 and atomic of 9 - both odd)

Not much of Physics, but perhaps logic? :P Any better way of coming to this conclusion?
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QuantumJG

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Re: Harder Physics Questions
« Reply #33 on: February 01, 2010, 10:49:09 pm »
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Thermodynamics, physical optics, special relativity, electromagnetism. :)

Oh, and how can I forget... quantum. :)

I probably need practise in geometrical optics too, seeing as it's my least favourite area...

lol same here.

I wouldn't mind some special relativity or some tricky electromagnetism questions.

Also sorry for attacking you (cthulhu), I just got annoyed at the question. Also what are you studying at uni?
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kamil9876

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Re: Harder Physics Questions
« Reply #34 on: February 01, 2010, 11:04:24 pm »
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Find necessary and sufficient conditions for a region in to be the unit ball for some norm.
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polky

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Re: Harder Physics Questions
« Reply #35 on: February 01, 2010, 11:35:40 pm »
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mark_alec here:

Do you accept university level questions in those areas? That is, questions that require the use of more advanced techniques than are available in the VCE curriculum.
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QuantumJG

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Re: Harder Physics Questions
« Reply #36 on: February 02, 2010, 12:06:18 am »
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I'm allowed to ask questions right? I need help with this:

Magnetic resonance imaging (MRI) is a noninvasive technique for visualising the living human body. It depends on a phenomenon called nuclear magnetic resonance (NMR) which occurs when certain atoms are place in very strong magnetic fields. All nuclei that have odd mass numbers exhibit NMR as do all nuclei that have even mass numbers but odd atomic numbers. Nuclei having both even mass numbers and even atomic numbers do not exhibit NMR.


The number of neutrons in the nucleus of an atom in which NMR occurs is always

A  odd
B  even
C  odd if the number of protons is even
D  even if the number of protons is odd

Also explain why

Awesome question (we looked at this in nuclear physics).

So a nucleon such as a proton or a neutron has a spin of +/- 1/2. Let's first look at a proton which can take two quantum states (+/- 1/2), now let's say we have two hydrogen atoms together where one proton is -1/2 and the other is +1/2. Now if I put in a magnetic field the -1/2 proton gains energy by being anti-aligned with the magnetic field, whilst the other drops to a lower energy level and this energy difference can be used to find the resonant frequency.

Sorry for the digression to physics, but it is a physics forum.

Now basically as long as you have either an odd number of neutrons or protons or both, you get NMR.

e.g. C-11(p-spin,n-spin) could be C-11(-,-1/2) & C-11(-,+1/2).

So the answer is C, why?

C says that the number of neutrons must be odd if Z is even.

NMR occurs for,

odd n & even p, odd n & odd p and even n & odd p, but it doesn't happen if even n & even p.  C basically states this.

A is wrong since NMR can occur if n is even (iff p is odd)
B is wrong since NMR can't occur if n is even and p is even
D is wrong since NMR can occur if n is odd and p is odd.  
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QuantumJG

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Re: Harder Physics Questions
« Reply #37 on: February 02, 2010, 12:08:46 am »
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Find necessary and sufficient conditions for a region in to be the unit ball for some norm.

what???
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Cthulhu

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Re: Harder Physics Questions
« Reply #38 on: February 02, 2010, 12:12:48 am »
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QuantumJG: I study Physics

mark_alec here:

Do you accept university level questions in those areas? That is, questions that require the use of more advanced techniques than are available in the VCE curriculum.
I'm wondering the same thing as mark. The thread is "harder" physics questions... but how hard do you want them....

QuantumJG

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Re: Harder Physics Questions
« Reply #39 on: February 02, 2010, 01:31:01 am »
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2008: Finished VCE

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appianway

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Re: Harder Physics Questions
« Reply #40 on: February 02, 2010, 05:02:10 pm »
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Whoops, my logic was obviously very flawed for the NMR question. I just treated it as a logic question, but just realised that I misread it as NMR always occurs when... rather than the number of neutrons is always.

Oh, and to marc_alec: Perhaps things that use 1st year knowledge but don't use complicated maths (so that eager VCE students can attempt them).
« Last Edit: February 02, 2010, 05:09:24 pm by appianway »

Cthulhu

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Re: Harder Physics Questions
« Reply #41 on: February 02, 2010, 06:55:59 pm »
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Heres a quantum-type question:

Through what angle must a 200keV photon be scattered by a free electron so that the photon loses 10% of its energy?

appianway

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Re: Harder Physics Questions
« Reply #42 on: February 02, 2010, 06:57:31 pm »
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Presuming an elastic collision?

Edmund

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Re: Harder Physics Questions
« Reply #43 on: February 02, 2010, 10:04:00 pm »
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Since the photon loses energy, isn't it an inelastic collision (Compton Scattering)?
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appianway

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Re: Harder Physics Questions
« Reply #44 on: February 02, 2010, 10:08:16 pm »
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It's compton scattering, and I presume that energy is conserved, as energy is transferred to the electron... however, it's more of a relativistic transfer of energy (that doesn't make much sense; I just meant that you use the relativistic equations for energy), I think.

So under non-relativistic guidelines, the energy isn't conserved (ie the energy of the photon + energy of the electron is less after the collision), but if you consider relativity, the energies should be the same.

I guess that's not really elastic, but that's what I was getting at.