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

Original post by randlemcmurphy

To be honest I am up to the point where all I have to do is remember specific points from markschemes. Compared to G484, G485 just requires a lot more memorizing, compared to application, for example in G484 there are (although only 1/2 marks) some tough suggest style questions. Whereas for G485 as long as you can remember things you should do fairly well.

What do you mean by "suggest style questions"?

Reply 362

randlemcmurphy

11

Original post by sagar448

What do you mean by "suggest style questions"?

For example there was a G484 question where you had to "Suggest why the value of the mass calculated in (ii) will be different from the actual massof the racket." And it was because the person holding the racket also had a change in momentum. I isn't that difficult when you think about it for a while, but under exam conditions I would probably have to skip it, in fear of wasting too much time.

Reply 363

Original post by randlemcmurphy

For example there was a G484 question where you had to "Suggest why the value of the mass calculated in (ii) will be different from the actual massof the racket." And it was because the person holding the racket also had a change in momentum. I isn't that difficult when you think about it for a while, but under exam conditions I would probably have to skip it, in fear of wasting too much time.

THAT is exactly and the only reason I lost marks on my mock and have been losing marks on PP. I still manage an A* or an A but questions such as that just give me too much stress during examination and I just end up skipping them.. :c

Reply 364

I've decided this is my least favourite exam, because there's so much information you need to know for all the written answer questions. But understanding it isn't enough, you have to get the awkward mark scheme points too.

Reply 365

randlemcmurphy

11

Original post by Elcor

I've decided this is my least favourite exam, because there's so much information you need to know for all the written answer questions. But understanding it isn't enough, you have to get the awkward mark scheme points too.

It would be alright, if they didn't have the annoying QWC marking point, for some of the medical physics questions. Or odd one mark questions like the one where the student incorrectly suggested it was due to the photoelectric effect, which I still don't see how it is incorrect....

Reply 366

Original post by randlemcmurphy

It would be alright, if they didn't have the annoying QWC marking point, for some of the medical physics questions. Or odd one mark questions like the one where the student incorrectly suggested it was due to the photoelectric effect, which I still don't see how it is incorrect....

Photoelectric effect has an electron emission there was no electron being emitted (I think we are talking about the same question) e.e

Reply 367

Need help with two things from the specification:
What would you say for:

Describe the use nuclear fission as an energy source

AND

Compare and contrast decay of radioactive nuclei and decay of charge on a capacitor a C--R circuit

Thanks :smile:

Reply 368

L'Evil Fish

OP

18

Original post by sagar448

Need help with two things from the specification:
What would you say for:

Describe the use nuclear fission as an energy source

AND

Compare and contrast decay of radioactive nuclei and decay of charge on a capacitor a C--R circuit

Thanks :smile:

Fission as an energy source, isn't that just nuclear reactors? Uranium bombarded with slow moving neutrons, mention control rods, fuel rods and a moderator

Both decays are exponential
Both have a constant associated with it
Differences, well obviously they're different things.
Discharge depends on two things, not just lambda

Reply 369

Original post by sagar448

Need help with two things from the specification:
What would you say for:

Describe the use nuclear fission as an energy source

AND

Compare and contrast decay of radioactive nuclei and decay of charge on a capacitor a C--R circuit

Thanks :smile:

Describe the use nuclear fission as an energy source

Neutrons are emitted into a fission reactor and are slowed by the moderator to become 'thermal neutrons'
Thermal neutrons are adsorbed by the atoms of a heavy element like Uranium
This causes the nucleus to fission, forming smaller nuclei and releasing more neutrons which go on to cause more fissions
Hence a chain reaction occurs
Control rods adsorb neutrons and stop the reaction from going out of control
Binding energy per nucleon for products is greater than that of Uranium, so energy is released as heat
Heat turns a turbine which generate electricity

Compare and contrast decay of radioactive nuclei and decay of charge on a capacitor a C--R circuit

Both decays are exponential
Rate of radioactive nuclei decay depends on the decay constant
Rate of charge decay on capacitor depends on the capacitance of the capacitor and the resistance of the circuit

Reply 370

Original post by Elcor

X

Original post by L'Evil Fish

X

Thanks guys, I didn't realise the first question was just describing the nuclear reactor. :tongue:

Thank you for the second one though. :biggrin:

Reply 371

Guys I'm pretty confused about the 'image intensifiers' or scintillators (also not sure if they're the same thing). I believe that in the revision guide they mention some sort of image intensifying device in X-ray imaging, fluoroscopy and gamma cameras. Could someone explain how that thing works in each case, and say what it's actually called? I lost a couple of marks in this question about image intensifiers http://gyazo.com/021a4a8ebe6d84b0152063a4f94069b1

I think I'm mixing them up or something

Reply 372

Original post by Elcor

Guys I'm pretty confused about the 'image intensifiers' or scintillators (also not sure if they're the same thing). I believe that in the revision guide they mention some sort of image intensifying device in X-ray imaging, fluoroscopy and gamma cameras. Could someone explain how that thing works in each case, and say what it's actually called? I lost a couple of marks in this question about image intensifiers http://gyazo.com/021a4a8ebe6d84b0152063a4f94069b1

I think I'm mixing them up or something

They are basically the exact same thing. But the term scintillator is used in gamma camera while the intensifier is used in questions with the above link. In the gamma camera I don't think there is a device but the visible light photons go through a collimator to get to the scintillator. While in the X-ray thing there is a film (intensifier) which intensifies a visible photon, it's placed under the patient so a moving image can be produced..

If you're still not clear, ask. :biggrin:

EDIT:
Oh yes they are used in something called Fluoroscope but it's not needed for the exam. In the device the intensifier converts low intensity x-ray beams to bright visible photons (same as scintillator)

(edited 8 years ago)

Reply 373

Original post by sagar448

They are basically the exact same thing. But the term scintillator is used in gamma camera while the intensifier is used in questions with the above link. In the gamma camera I don't think there is a device but the visible light photons go through a collimator to get to the scintillator. While in the X-ray thing there is a film (intensifier) which intensifies a visible photon, it's placed under the patient so a moving image can be produced..

If you're still not clear, ask. :biggrin:

EDIT:
Oh yes they are used in something called Fluoroscope but it's not needed for the exam. In the device the intensifier converts low intensity x-ray beams to bright visible photons (same as scintillator)

So scintillator converts a gamma photon to thousands of visible photons and an image intensifier converts an X-ray into thousands of visible photons, but what's all the stuff about electrons in that mark scheme?

Reply 374

Hilton184

9

Original post by Elcor

Guys I'm pretty confused about the 'image intensifiers' or scintillators (also not sure if they're the same thing). I believe that in the revision guide they mention some sort of image intensifying device in X-ray imaging, fluoroscopy and gamma cameras. Could someone explain how that thing works in each case, and say what it's actually called? I lost a couple of marks in this question about image intensifiers http://gyazo.com/021a4a8ebe6d84b0152063a4f94069b1

I think I'm mixing them up or something

See page 17

That question is silly, because a very similar question came up in a later paper with complete different marking points. In a separate paper they said an image intensifier doesn't use photoelectric effect, even though it does because of the photocathode which releases electrons when visible light photons are incident upon it, as this mark scheme suggests!

So basically what I would do is say image intensifier consists of a phosphor screen and a photocathode. And then role off the more recent mark scheme which corresponds to this question.

Posted from TSR Mobile

Reply 375

Original post by Elcor

So scintillator converts a gamma photon to thousands of visible photons and an image intensifier converts an X-ray into thousands of visible photons, but what's all the stuff about electrons in that mark scheme?

So basically you know how theres this vacuum thing that converts accelerated electrons to X-rays? Well there is this thing called an X-ray image intensifier which is kind of like a machine so it works like this:

X-ray photons penetrate the input window of the vacuum case. The input phosphor absorbs the x-ray photons and converts them into optical photons (a phenomenon called luminescence). These optical photons are converted to photoelectrons at the photocathode. The photoelectrons are accelerated by the electric field produced by the strong electric potential difference of the image intensifier and are collected at the output phosphor. Each accelerated electron produces many optical photons at the output phosphor.

That link up there is a really old paper, there is another paper which asks like for image intensifier and stuff and it's kind of new so i'd rely on that one because it makes much more sense. If you go up through the posts on this forum you'll find it, a guy posted something about it.

:biggrin:

EDIT: yeah do what Hilton said, but read this my post ,I explain the image intensifier thingy :3

(edited 8 years ago)

Reply 376