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

sulexk
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#521
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#521
(Original post by Oh my Ms. Coffey)
Can anyone tell me about EMF produced by a AC. Generator.

Is this right?

EMF is Max when coil is parallel to the magnetic fields, EMF is 0 when it is perpendicular.
That is 100% correct!
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PhilliChilli
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(Original post by scotzbhoy)
X-rays came up in June '10

Heres some notes i made earlier on i hope that if you can actually open the attachment that they prove useful :awesome:

Computer Axial Tomography :

How it works :
• The patient lies in a vertical right of X-Ray detectors
• The X-Ray tube rotates around the ring, exposing the patient to a fan shaped beam of Xrays from all directions
• Detectors opposite the tube send electronic records to a computer
• The computer software builds up a 3-D image of the patient
• The radiographer can view images of “slices” through the patient on the computer screen.
The technique is called computerised axial tomography because it relies on a computer to control the scanning motion and to gather and manipulate the data to produce images. Because the X-Ray tube rotates around the axis and because it produces images of “slices” through the patient (tomo – slice).
Advantages of CAT Scans:
• They can produce images that show 3D relationships between different tissues
• They can distinguish tissues with quite similar densities (attenuation coefficients)
• CAT Scans use X-Rays and any exposure to ionising radiation carries a risk for the patient. A single scan can expose the patient to a radiation dose equal to several years worth of background radiation. THIS IS NOT THE CASE WITH MRI SCANS!!

Positron Emission Tomography :

PET scanning is another technique which uses the fact that gamma rays can emerge from a source inside the body. As with CAT scans this too involves taking slices of the body. The radiopharmaceuticals used here contain radioisotopes that emit positrons, they are beta + emitters.
However, it is not the gamma photon we care about. Were interested in the electron-positron interaction, once emitted they collide and are annihilated! Their mass is released as energy, two gamma photons at 180 degrees to each-other. In PET scanning, it is these two gamma photons that are detected.

A PET scanner looks like a CAT scanner but it is of course detecting gamma rays not X-Rays.
The patient is injected with a radiopharmaceutical, in this case a form of glucose tagged with fluorine-18. This tends to accumulate in tissues with a high rate of respiration. In this case doctors will be looking at brain function so the tracer will be taken up by active cells in the brain. The patient is surrounded by a ring of gamma detectors, these detect a pair of gamma rays coming from inside the patient and travelling in opposite directions. The times at which they arrive are compared and from this the position of which they were emitted can be determined. A 3D image of the distribution of the radioactive tracer in the patient is built up and from this any abnormal functioning can be deduced.

• Collimator – columns of lead plates, any gamma photons are absorbed.
• Scintillator – detects only photon energy of gamma rays and converts them into visible light.
• Photomultiplier tube – produces electrical pulses for each photon on light. Arranged in hexagonal array as a single p.m.tube. Incident gamma photon strikes Scintillator crystal, a single light photon releases a single electron from the photocathode (photoelectric effect)

sorry if this looks like too much info
Attached files
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Oh my Ms. Coffey
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(Original post by M_I)
Photoelectric effect - a photon with about 30keV of energy hits and electron in the tungsten atom, the electron is ejected, another electron fills the gap in the lower energy level shell, releasing a photon

Compton Scattering - a photon with about 0.5 - 5 MeV of energy hits an electron and knock it out, photon loses energy and is scattered.

Would I need to add any more info?

--------------------------------------------------------

How do radiographers try to produce a sharp image while minimising the radiation dose?
By moving the detection plate closer to the patient and the X-ray tube further
Keeping the patient still
A lead grid to reduce 'fogging'
A florescent screen which scintillates when hit which also helps the image produce quicker
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MarieLyon
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(Original post by Oh my Ms. Coffey)
Can anyone tell me about EMF produced by a AC. Generator.

Is this right?

EMF is Max when coil is parallel to the magnetic fields, EMF is 0 when it is perpendicular.
you need to remember that the gradient of the magnetic flux linkage against time graph is the induced emf.
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Oh my Ms. Coffey
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(Original post by sulexk)
That is 100% correct!
So an EMF/T graph looks like a cos graph?
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Stasiaxx
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What grades did you get in the feb 2011 paper?
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MarieLyon
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(Original post by Oh my Ms. Coffey)
So an EMF/T graph looks like a cos graph?
the flux linkage one is the cos graph and the induced emf is the sin graph.
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JONNYCMW
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#528
(Original post by apo1324)
I get 0.05Ohm. :L Damn.
I think its because average voltage is half that of original V, so half V? Since when it discharges, the voltage decreases over time
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muffingg
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(Original post by PhilliChilli)
Heres some notes i made earlier on i hope that if you can actually open the attachment that they prove useful :awesome:

Computer Axial Tomography :

How it works :
• The patient lies in a vertical right of X-Ray detectors
• The X-Ray tube rotates around the ring, exposing the patient to a fan shaped beam of Xrays from all directions
• Detectors opposite the tube send electronic records to a computer
• The computer software builds up a 3-D image of the patient
• The radiographer can view images of “slices” through the patient on the computer screen.
The technique is called computerised axial tomography because it relies on a computer to control the scanning motion and to gather and manipulate the data to produce images. Because the X-Ray tube rotates around the axis and because it produces images of “slices” through the patient (tomo – slice).
Advantages of CAT Scans:
• They can produce images that show 3D relationships between different tissues
• They can distinguish tissues with quite similar densities (attenuation coefficients)
• CAT Scans use X-Rays and any exposure to ionising radiation carries a risk for the patient. A single scan can expose the patient to a radiation dose equal to several years worth of background radiation. THIS IS NOT THE CASE WITH MRI SCANS!!

Positron Emission Tomography :

PET scanning is another technique which uses the fact that gamma rays can emerge from a source inside the body. As with CAT scans this too involves taking slices of the body. The radiopharmaceuticals used here contain radioisotopes that emit positrons, they are beta + emitters.
However, it is not the gamma photon we care about. Were interested in the electron-positron interaction, once emitted they collide and are annihilated! Their mass is released as energy, two gamma photons at 180 degrees to each-other. In PET scanning, it is these two gamma photons that are detected.

A PET scanner looks like a CAT scanner but it is of course detecting gamma rays not X-Rays.
The patient is injected with a radiopharmaceutical, in this case a form of glucose tagged with fluorine-18. This tends to accumulate in tissues with a high rate of respiration. In this case doctors will be looking at brain function so the tracer will be taken up by active cells in the brain. The patient is surrounded by a ring of gamma detectors, these detect a pair of gamma rays coming from inside the patient and travelling in opposite directions. The times at which they arrive are compared and from this the position of which they were emitted can be determined. A 3D image of the distribution of the radioactive tracer in the patient is built up and from this any abnormal functioning can be deduced.

• Collimator – columns of lead plates, any gamma photons are absorbed.
• Scintillator – detects only photon energy of gamma rays and converts them into visible light.
• Photomultiplier tube – produces electrical pulses for each photon on light. Arranged in hexagonal array as a single p.m.tube. Incident gamma photon strikes Scintillator crystal, a single light photon releases a single electron from the photocathode (photoelectric effect)

sorry if this looks like too much info
Hey, thanks for sharing. Do you have any more notes like these?
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scotzbhoy
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(Original post by MarieLyon)
onlt the contrast media and intensifiers nothing to do with compton scattering... photoelctric effect and that positron one.
There was also a section on intensity & attenuation, and one on how x-rays interact with matter, which would include Compton scattering, photoelectric effect and pair production.
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MarieLyon
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(Original post by PhilliChilli)
Heres some notes i made earlier on i hope that if you can actually open the attachment that they prove useful :awesome:

Computer Axial Tomography :

How it works :
• The patient lies in a vertical right of X-Ray detectors
• The X-Ray tube rotates around the ring, exposing the patient to a fan shaped beam of Xrays from all directions
• Detectors opposite the tube send electronic records to a computer
• The computer software builds up a 3-D image of the patient
• The radiographer can view images of “slices” through the patient on the computer screen.
The technique is called computerised axial tomography because it relies on a computer to control the scanning motion and to gather and manipulate the data to produce images. Because the X-Ray tube rotates around the axis and because it produces images of “slices” through the patient (tomo – slice).
Advantages of CAT Scans:
• They can produce images that show 3D relationships between different tissues
• They can distinguish tissues with quite similar densities (attenuation coefficients)
• CAT Scans use X-Rays and any exposure to ionising radiation carries a risk for the patient. A single scan can expose the patient to a radiation dose equal to several years worth of background radiation. THIS IS NOT THE CASE WITH MRI SCANS!!

Positron Emission Tomography :

PET scanning is another technique which uses the fact that gamma rays can emerge from a source inside the body. As with CAT scans this too involves taking slices of the body. The radiopharmaceuticals used here contain radioisotopes that emit positrons, they are beta + emitters.
However, it is not the gamma photon we care about. Were interested in the electron-positron interaction, once emitted they collide and are annihilated! Their mass is released as energy, two gamma photons at 180 degrees to each-other. In PET scanning, it is these two gamma photons that are detected.

A PET scanner looks like a CAT scanner but it is of course detecting gamma rays not X-Rays.
The patient is injected with a radiopharmaceutical, in this case a form of glucose tagged with fluorine-18. This tends to accumulate in tissues with a high rate of respiration. In this case doctors will be looking at brain function so the tracer will be taken up by active cells in the brain. The patient is surrounded by a ring of gamma detectors, these detect a pair of gamma rays coming from inside the patient and travelling in opposite directions. The times at which they arrive are compared and from this the position of which they were emitted can be determined. A 3D image of the distribution of the radioactive tracer in the patient is built up and from this any abnormal functioning can be deduced.

• Collimator – columns of lead plates, any gamma photons are absorbed.
• Scintillator – detects only photon energy of gamma rays and converts them into visible light.
• Photomultiplier tube – produces electrical pulses for each photon on light. Arranged in hexagonal array as a single p.m.tube. Incident gamma photon strikes Scintillator crystal, a single light photon releases a single electron from the photocathode (photoelectric effect)

sorry if this looks like too much info
thank you so much for this!!!!!!!!!!
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MarieLyon
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(Original post by scotzbhoy)
There was also a section on intensity & attenuation, and one on how x-rays interact with matter, which would include Compton scattering, photoelectric effect and pair production.
sorry about all the typos in that then. Okay thanks - I just can't stand x-rays!
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sulexk
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#533
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(Original post by Oh my Ms. Coffey)
So an EMF/T graph looks like a cos graph?
if the coil is starting in the horizontal position. Then it will look like a cos graph
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yokabasha
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(Original post by Oh my Ms. Coffey)
Can anyone tell me about EMF produced by a AC. Generator.

Is this right?

EMF is Max when coil is parallel to the magnetic fields, EMF is 0 when it is perpendicular.
Actually I don't think that is, that's what I originally thought due to the use of cos x
. But to increase emf, you need to increase the change in magentic flux linkage, to increase linkage, you need to increase the flux, for max flux, it needs to be perpendicular the field.
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Oh my Ms. Coffey
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(Original post by MarieLyon)
the flux linkage one is the cos graph and the induced emf is the sin graph.
As rate of change Flux Linkage / Rate of change t = EMF

A graph of Flux linkage would be a max when the wire is perpendicular as BANcosx would be max. The gradient would be the EMF from the above formula.
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MarieLyon
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(Original post by sulexk)
if the coil is starting in the horizontal position. Then it will look like a cos graph
yeah what they said!
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MarieLyon
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(Original post by Oh my Ms. Coffey)
As rate of change Flux Linkage / Rate of change t = EMF

A graph of Flux linkage would be a max when the wire is perpendicular as BANcosx would be max. The gradient would be the EMF from the above formula.
depending on where the coil is cutting the field, yeah.
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sulexk
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(Original post by yokabasha)
Actually I don't think that is, that's what I originally thought due to the use of cos x
. But to increase emf, you need to increase the change in magentic flux linkage, to increase linkage, you need to increase the flux, for max flux, it needs to be perpendicular the field.
But the induced emf = rate of change of magnetic flux linkage. When magnetic flux linkage is maximum- the rate of change of magnetic flux linkage is zero.
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yokabasha
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(Original post by sulexk)
But the induced emf = rate of change of magnetic flux linkage. When magnetic flux linkage is maximum- the rate of change of magnetic flux linkage is zero.
Yh I agree with that, but the coil needs to be perpendicular to the field for maximum magnetic flux/ linkage.
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anshul95
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(Original post by M_I)
Photoelectric effect - a photon with about 30keV of energy hits and electron in the tungsten atom, the electron is ejected, another electron fills the gap in the lower energy level shell, releasing a photon

Compton Scattering - a photon with about 0.5 - 5 MeV of energy hits an electron and knock it out, photon loses energy and is scattered.

Would I need to add any more info?

--------------------------------------------------------

How do radiographers try to produce a sharp image while minimising the radiation dose?
well actually that is an incorrect definition of the photoelectric effect as applied to X-rays. Don't worry I know the CGP book says this, but the extra energy of the photon (which of course has to above the work function) becomes the energy of the electron. http://www.upscale.utoronto.ca/PVB/H...yInteract.html
You are explaining the characteristic wavelengths which occur when the electrons hit the tungsten target (this is not the photoelectric effect as it does not release an electron but an x-ray photon).
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