June 2011 G485-Fields, Particles and Frontiers of Physics Watch

Lengalicious
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#361
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#361
(Original post by ChoYunEL)
A-scan in one direction only / range or distance or depth finding
B-scan uses a number of sensors or a sensor in different positions / angles (to build up a 2D/3D image)

A B-scan is not many A-scans
Lol dragged straight from the mark scheme, reading it right now =P
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Oh my Ms. Coffey
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#362
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#362
Do we have to remember how long a parsec is?
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anshul95
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#363
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#363
(Original post by ChoYunEL)
Question 3 ai) of the OCR 2824 Jan06 paper, it asks for the direction of flow of liquid.
In the Physics OCR A2 text book, page 122.
I used left hand rule for that particular question.

There is no induction and flow is already there.
that is correct. But because the generator is producing a current you have to use the right hand rule. Just because you are trying to find the direction of the force, doesn't mean you automatically use LHR as in this case you are trying to find the motion which PRODUCES the current i.e. this isn't the motor effect.
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Ralphus J
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#364
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#364
Hows this for the PET scan have i missed anything out?

Patient injected with radiopharmaceutical tagged with fluorine-18 (B+ emitter).

Patient surrounded by a ring of gamma detectors, which detect pairs of gamma rays coming from inside the patient and travelling in opposite directions.

The times at which they arrive at the detectors are compared and from this the positron at which they were emitted can be determined.

Gradually a 3D image of the distribution of radioactive tracer in the patient can be built up and from this any abnormal functioning can be deduced.

I havent said anything about the annihilation which was previously mentioned? HMMMMMMM...
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sulexk
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#365
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#365
(Original post by Ralphus J)
Hows this for the PET scan have i missed anything out?

Patient injected with radiopharmaceutical tagged with fluorine-18 (B+ emitter).

Patient surrounded by a ring of gamma detectors, which detect pairs of gamma rays coming from inside the patient and travelling in opposite directions.

The times at which they arrive at the detectors are compared and from this the positron at which they were emitted can be determined.

Gradually a 3D image of the distribution of radioactive tracer in the patient can be built up and from this any abnormal functioning can be deduced.

I havent said anything about the annihilation which was previously mentioned? HMMMMMMM...
Include some information with regards to the production of the gamma rays and that will be a fantastic explanation of PET.

Thank you!
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Lengalicious
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#366
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#366
(Original post by Ralphus J)
Hows this for the PET scan have i missed anything out?

Patient injected with radiopharmaceutical tagged with fluorine-18 (B+ emitter).

Patient surrounded by a ring of gamma detectors, which detect pairs of gamma rays coming from inside the patient and travelling in opposite directions.

The times at which they arrive at the detectors are compared and from this the positron at which they were emitted can be determined.

Gradually a 3D image of the distribution of radioactive tracer in the patient can be built up and from this any abnormal functioning can be deduced.

I havent said anything about the annihilation which was previously mentioned? HMMMMMMM...
Seems pretty spot on, I highly doubt we will be required to quote the specific radioactive isotope injected but yeh full marks
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Oh my Ms. Coffey
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#367
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#367
(Original post by Ralphus J)
Hows this for the PET scan have i missed anything out?

Patient injected with radiopharmaceutical tagged with fluorine-18 (B+ emitter).

Patient surrounded by a ring of gamma detectors, which detect pairs of gamma rays coming from inside the patient and travelling in opposite directions.

The times at which they arrive at the detectors are compared and from this the positron at which they were emitted can be determined.

Gradually a 3D image of the distribution of radioactive tracer in the patient can be built up and from this any abnormal functioning can be deduced.

I havent said anything about the annihilation which was previously mentioned? HMMMMMMM...
Mention the tracer is a Positron emitter which when it meets an electron annihilates giving off 2 gamma rays. The computer builds up a map of activity. Then write about metabolism and how that matches.
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Lengalicious
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#368
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#368
This sounds more like a biology thread than physics, LAMEEE
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Ralphus J
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#369
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#369
^ Thanks y'all, and Ms coffey that was the extra sentence i was looking for thanks.
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Ralphus J
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#370
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#370
Now im onto "outline the principles of magnetic resonance, with reference to precession of nuclei, Larmor frequency, resonance and relaxation times"....
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Bell
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#371
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#371
CAT scan help for people:

Patient surrounded by an array of detectors and an x-ray source

source rotated around patient, x-rays pass through same section of the body but from different angles

producing a 'slice' image of the patient

higher proton numbers are a cause of deeper blackening on film

attenuation is due to the photoelectric effect

contrast media can be used to enhance image

patient is moved slightly and process is repeated to produce another 'slice' and so on

computer builds 3D image of the 'slices'

and shows much greater detail than a simple x-ray

the image can be rotated on screen/viewed from different angles




hope it helps - likely to come up 5+ marker !
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Lengalicious
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#372
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#372
(Original post by Ralphus J)
Now im onto "outline the principles of magnetic resonance, with reference to precession of nuclei, Larmor frequency, resonance and relaxation times"....
- Protons posses magnetic property as they spin on their axis.
- Application of magnetic field causes proton to precess about its axis due to the magnetic interaction.
- The frequency of precession = the larmor frequency.
- If the frequency of AC in the radio coil = the precessional frequency the proton will resonate.
- Nuclear resonance causes the proton to gain energy boosting it up to a higher unstable energy level.
- The proton will lose this energy as radio waves after a certain time known as nuclear relaxation.
- Radio wave picked up by radio coils and interpreted by computer to form 3D image on screen.
- A series of smaller magnets are placed in the main magnet of the scanner to alter the otherwise uniform magnetic field so the relaxation times of the emitted Rwaves are different so the data can be interpreted.
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Lengalicious
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#373
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#373
(Original post by Bell)
CAT scan help for people:

Patient surrounded by an array of detectors and an x-ray source

source rotated around patient, x-rays pass through same section of the body but from different angles

producing a 'slice' image of the patient

higher proton numbers are a cause of deeper blackening on film

attenuation is due to the photoelectric effect

contrast media can be used to enhance image

patient is moved slightly and process is repeated to produce another 'slice' and so on

computer builds 3D image of the 'slices'

and shows much greater detail than a simple x-ray

the image can be rotated on screen/viewed from different angles




hope it helps - likely to come up 5+ marker !
Actually i beleive higher proton number = higher attenuation coefficient, therefore its not 'deeper blackening' but more 'better contrast' between the black and white as much of the x-ray is deflected or absorbed by the contrast medium.

'deeper blackening' implies that the contrast medium is causing a deeper blackening but this is not the case.
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Oh my Ms. Coffey
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#374
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#374
(Original post by Lengalicious)
Actually i beleive higher proton number = higher attenuation coefficient, therefore its not 'deeper blackening' but more 'better contrast between the black and white as much of the x-ray is deflected or absorbed by the contrast medium.
Attenuation coefficient is affected by density/Energy of photons. The amount of energy absorbed depends on atomic number according to revision guide.
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Lengalicious
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#375
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(Original post by Oh my Ms. Coffey)
Attenuation coefficient is affected by density/Energy of photons. The amount of energy absorbed depends on atomic number according to revision guide.
Lol.... attenuation is a property of a material. . . larger density of photons is irelevant. The ray of photons would become attenuated due to larger density of protons causing more interaction due to *wave particle duality* but larger density of photons would alter the attenuation of the ray by naught.

PS. im guessing thats a typo?
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Oh my Ms. Coffey
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#376
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#376
(Original post by Lengalicious)
Lol.... attenuation is a property of a material. . . larger density of photons is irelevant. The ray of photons would become attenuated due to larger density of protons causing more interaction due to *wave particle duality* but larger density of photons would alter the attenuation of the ray by naught.

PS. im guessing thats a typo?
Question 2aiii) Give 2 factors that determin the value of attenuation coefficient
Answer: Energy or frequency of photons/ Density or proton number which the beam passes through.
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Lengalicious
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#377
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(Original post by Oh my Ms. Coffey)
Question 2aiii) Give 2 factors that determin the value of attenuation coefficient
Answer: Energy or frequency of photons/ Density or proton number which the beam passes through.
Ok thats great but i don't get why you mentioned this in reply to my other reply? I was just saying that its wrong to say deeper blackening because the contrast medium absorbs the xrays photons before they would be able to pass to cause deeper blackening..
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sulexk
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#378
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#378
(Original post by Oh my Ms. Coffey)
Question 2aiii) Give 2 factors that determin the value of attenuation coefficient
Answer: Energy or frequency of photons/ Density or proton number which the beam passes through.
If you think about it the we are dealing with the absorption coefficient. When the barium meal is taken up or injected into the tissue of interest, this tissue is then a better absorber of X-rays and its edges show up more clearly. Now the absorption mechanism by which X-rays are absorbed is via the photoelectric effect- the contrast media are elements with high Z numbers-so they have many protons and electrons, many electrons with which the xrays can interact- note that in this case the absorption coefficient is directly proportional to the cube of the Z number, so higher Z number higher attenuation coefficient and hence a better contrast between soft tissues will show up on the film

I hope that helps!
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Nutterfly
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#379
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#379
(Original post by Oh my Ms. Coffey)
Do we have to remember how long a parsec is?
Yes, I'm fairly sure it said you do in the syllabus, (3.086x10_16) that and the light year - although you can always just calculate that one. =)
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Oh my Ms. Coffey
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#380
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#380
(Original post by Lengalicious)
Ok thats great but i don't get why you mentioned this in reply to my other reply? I was just saying that its wrong to say deeper blackening because the contrast medium absorbs the xrays photons before they would be able to pass to cause deeper blackening..

(Original post by sulexk)
If you think about it the we are dealing with the absorption coefficient. When the barium meal is taken up or injected into the tissue of interest, this tissue is then a better absorber of X-rays and its edges show up more clearly. Now the absorption mechanism by which X-rays are absorbed is via the photoelectric effect- the contrast media are elements with high Z numbers-so they have many protons and electrons, many electrons with which the xrays can interact- note that in this case the absorption coefficient is directly proportional to the cube of the Z number, so higher Z number higher attenuation coefficient and hence a better contrast between soft tissues will show up on the film

I hope that helps!

Yeah sorry I misread it.
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