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

Honestly, it's weird, the right answer for image intensifier stuff has changed a couple of times, but it generally agreed that the stuff on the 2010 paper I mention is wrong.

Anyway, does anyone know a good way to word the movement of the stuff that's rotating about the patient in a CAT scan? The Jan 2013 mark scheme has this but I've got no idea how to actually write it:
"X-ray beam passes through the patient at different angles / X-ray tube rotates around the patient
A thin fan-shaped beam is used (AW)
Images of ‘slices’ through the patient (in one plane are produced with the help of computer software)
X-ray tube / detectors are moved along (the patient for the next slice through the patient)"
To me it almost seems like it's repeating itself. The best I would have would be "a think fan-shaped beam of X-rays is moved up and down a rotating axial that moves around the patient, these X-rays are then detected by detectors that are also being moved to correspond with the fan-shaped beam of X-rays".

Can anyone word it better?

an X ray tube rotates around the patient, producing a thin fan shaped beam of x rays. as the x ray photons pass through the patient they are attenuated, and the amount of attenuation depends on the type of tissues they have passed through.

The detector on the other side of the patient measures the attenuation of the photons and identifies the tissue type, this is performed all around the patient to build up a 2-D image slice. Repeating this process along the body of the patient produces many 2D slices that when stacked atop each other creates a 3D image.

should include everything in there i think. any better?

Reply 741

boristeve

2

Original post by ninjasinpjs

Lenz's law :

Is the opposing force due to the magnetic field created by the current generated?
In other words, the magnetic field initially there and the magnetic field due to the current act against each other which 'causes the oppose to change', is this what lenz's law saying?

does this image help? the emf is opposed to the change that caused it. so when the changing flux is maximal negative, the emf is at a maximal positive, and visa versa. equally when the gradient of the top graph is 0, i.e. the rate of change of flux is 0, the emf is 0 too.
hope this helps

(edited 8 years ago)

Reply 742

nothepreacher

18

Original post by boristeve

does this image help? the emf is opposed to the change that caused it. so when the changing flux is maximal negative, the emf is at a maximal positive, and visa versa. equally when the gradient of the top graph is 0, i.e. the rate of change of flux is 0, the emf is 0 too.
hope this helps

What are the experiments related to magnetism or electromagnetism, and capacitors that we need to be able to describe? Are there any others from other topics too?

Reply 743

boristeve

2

Original post by nothepreacher

What are the experiments related to magnetism or electromagnetism, and capacitors that we need to be able to describe? Are there any others from other topics too?

only experiment i can recall off the top of my head is rutherfords scattering experiment, to prove the structure of the atom.

Most of the alpha particles weren’t scattered ⇒ The gold atoms are mostly empty space
Some of the alpha particles were scattered through large radius ⇒ Small dense positivenucleus.

those two conclusions are all we need to know I think

Reply 744

ninjasinpjs

2

Original post by boristeve

does this image help? the emf is opposed to the change that caused it. so when the changing flux is maximal negative, the emf is at a maximal positive, and visa versa. equally when the gradient of the top graph is 0, i.e. the rate of change of flux is 0, the emf is 0 too.
hope this helps

so when it says oppose, its not referring to forces?

also, if i push a magnet on the north side towards a coil how do i know what direction the induced current is traveling in the coil?

Reply 745

Oraeng

9

What determines if an incoming x-ray photon undergoes the photoelectric effect or the Compton effect? It it to do with the photon's energy?

Reply 746

L'Evil Fish

OP

18

Original post by Oraeng

What determines if an incoming x-ray photon undergoes the photoelectric effect or the Compton effect? It it to do with the photon's energy?

Yeah

Higher energy will be Compton effect

Reply 747

boristeve

2

Original post by ninjasinpjs

so when it says oppose, its not referring to forces?

also, if i push a magnet on the north side towards a coil how do i know what direction the induced current is traveling in the coil?

use flemings left hand rule to work out the current direction i think

and no, i don't think its referring to forces. lenz law is; The direction of the induced e.m.f. is such that it opposes the change causing it.
so no mention of forces, just emf and changing magnetic flux.

Reply 748

ttttepp

Does anyone have a link for the june 2014 paper? Or is it not online anywhere?

Reply 749

ninjasinpjs

2

Lenz's Law [ SOLVED]

So when a wire moves through a magnetic field, there is an induced current, we can use Flemming RHR here to check the directions of force, induce current and direction of the field.

At the same time, there will be an opposition to the force that is creating the induced current, we call this the motor effect, this force can be found using the LHR where first finger is the direction of the field, middle finger is the direction of induced current and thumb the motor effect.

The reason why there is this opposing force ( motor effect) is because of the conservation of energy. If there was not motor effect ( opposing force) we would just be adding energy to the system and no energy is being lost. Which breaks the law of conservation of energy.

So if what i'm saying is true, if you were to cut a magnetic field with a wire you should feel some resistance due to the motor effect.

Hope this makes sense, and helps anyone else struggling to understand Lenz's law too.

Reply 750

nothepreacher

18

Original post by ttttepp

Does anyone have a link for the june 2014 paper? Or is it not online anywhere?

Hey, Sagar posted it on here.

http://www.thestudentroom.co.uk/showthread.php?t=2947411&page=28&p=56711289#post56711289

Reply 751

BecauseFP

2

Original post by boristeve

an X ray tube rotates around the patient, producing a thin fan shaped beam of x rays. as the x ray photons pass through the patient they are attenuated, and the amount of attenuation depends on the type of tissues they have passed through.

The detector on the other side of the patient measures the attenuation of the photons and identifies the tissue type, this is performed all around the patient to build up a 2-D image slice. Repeating this process along the body of the patient produces many 2D slices that when stacked atop each other creates a 3D image.

should include everything in there i think. any better?

I'm not too sure, I just wanna make sure that I tick all of the boxes for how the detector and fan thing move.

Reply 752

Oraeng

9

Thanks

Original post by L'Evil Fish

Yeah

Higher energy will be Compton effect

Do we need to know exactly how image intensifiers work? Is just that they are 'a phosphor screen which converts x-ray photons into many visible light photons which are absorbed by the film' all we need to know?

Reply 753

L'Evil Fish

OP

18

Original post by Oraeng

Thanks

Do we need to know exactly how image intensifiers work? Is just that they are 'a phosphor screen which converts x-ray photons into many visible light photons which are absorbed by the film' all we need to know?