OCR Physics A - G485: Fields, Particles & Frontiers of Physics - June 2012

Reply 380

Father Ted

5

Original post by AwesomeAkhilesh

Cheers for making the thread and posting links! :smile:

Yes Akhilesh, first one in there!

Hows the revision going buddy?

Reply 381

Jukeboxing

8

What mark is everyone aiming for?

Reply 382

Mathlete 4 the win

If you give your answer to 3 sig fig but the mark schemes answer is to 2 sig fig (no where in the question did it state the amount of sig figs) would I get the mark?

Reply 383

Picture~Perfect

19

Original post by 4 Mathlete the win

If you give your answer to 3 sig fig but the mark schemes answer is to 2 sig fig (no where in the question did it state the amount of sig figs) would I get the mark?

Yes, you would still get the mark.

Reply 384

danielpickwick

1

Original post by cepti

Hello

I'm been staring at this question from the red OCR book (pg 174) for about 30 minutes and still can't figure it out:

Q:The pi0 meson has its mass quoted as 135 Mev/c^2. It decays into two gamma rays

a) Calculate the mass of the meson in kg
b) Assuming that the meson is initially at rest, calculate the energy and hence the wavelength of the two gamma rays.

I got the right answer for a) (2.4 x10^-28kg) but for b) I keep getting 9.2 x10^-15, the book says it should be 1.98 x10^-14?
If anybody could help, I would be very grateful :biggrin:

Well you'd used E=mc^2 to find out the total energy that is produced when the pi meson decays. So:

E = 2.4x10^-28 * (3x10^8)^2
E = 2.16x10^-11J

Then if the pi meson produces two gamma photons, the total energy produced will be split equally between the photons so that each gamma-ray photon has half the total energy produced.

2.16x10^-11/2 = 1.08x10^-11J

Then use E=hc/λ and rearrange to get lambda:

λ=hc/E

λ = (6.63x10^-34*3x10^8)/1.08x10^-11
λ = 1.84x10^-14m

I know that's not quite the exact answer, but it's of the same order, I guess the book must have used more accurate values for the speed of light or something? Hope I helped :smile:

(edited 11 years ago)

Reply 385

Mathlete 4 the win

Original post by Picture~Perfect

Yes, you would still get the mark.

that's cracking, cheers

Reply 386

ReTurd

1

Could someone explain to me how ultrasound is used to measure the speed of blood flow in veins.. The theory and the maths.

Reply 387

stunnaman

Original post by DavidMRoper

if any body needs any help PM me! I tend to learn by answering peoples questions :smile:

can you tell me how to answer question 6 on the 'ALL PAST PAPER QUESTIONS FILE' ? the one where you have to draw voltage agaist time graph.. its confusing and the mark scheme is of no help!

Reply 388

ReTurd

1

Can anyone tell me why the book says that a proton has a spins of +0.5 but when you use the table to the right of the picture where is says that up and down quarks both have a spin of +0.5 so the total spin of a proton should add up to +1.5.
Anyone explain? Whats wrong here?

(edited 11 years ago)

Reply 389

stunnaman

Can some1 summerise Image Intensifier scrrens? What they are and what they do?

Reply 390

SamXi

7

Original post by stunnaman

Can some1 summerise Image Intensifier scrrens? What they are and what they do?

> X-rays are absorbed by a photographic film
> An intensifier can be used which produces and increased number of visible light photons from an X-ray photon
>These are then absorbed by the film
> Scans show the outline of soft tissue

Taken directly from the "essay type questions" attachment in the OP

Reply 391

ReTurd

1

Original post by SamXi

> X-rays are absorbed by a photographic film
> An intensifier can be used which produces and increased number of visible light photons from an X-ray photon
>These are then absorbed by the film
> Scans show the outline of soft tissue

Taken directly from the "essay type questions" attachment in the OP

Are image intenifiers and photomultipliers the same thing?

Reply 392

Mathlete 4 the win

Original post by ReTurd

Can anyone tell me why the book says that a proton has a spins of +0.5 but when you use the table to the right of the picture where is says that up and down quarks both have a spin of +0.5 so the total spin of a proton should add up to +1.5.
Anyone explain? Whats wrong here?

I'm pretty sure you don't need to know about spin. I could be wrong. There's a lot of stuff in that text book that's irrelevant in my opinion

Reply 393

username896004

Original post by ReTurd

Are image intenifiers and photomultipliers the same thing?

No there's a slight difference.

Image intensifier screens:
Photographic film is slotted inside two phosphur screens (scintillators) like sodium iodide. These screens convert a few X-ray photons into many many visible photons. This is useful as photographic film is basically **** at absorbing X-rays, so the patient exposure would need to be higher without these screens.

Image intensifiers can also include a device as follows:
Scintillator emits photons as described above, these liberate electrons from a photocathode, which are accelerated and focused by a series of anodes - then they strike a phosphur screen and emit visible light photons, which are absorbed by film. Advantage of this, is you can get rid of the film and use it to make a moving image.

Photomultipliers are very similar. They do not include the sintilator component. X-rays strike a scintillator, and then the visible light photons enter a photomutilplier . These liberate electrons by the photoelectric effect as before, and are accelerated as before. This produces an pulse of electrons - and electrical signal, which is fed into a computer. These specifically digitise the image.

Think, intensifier screens, bog standard X-ray, image intensifiers moving X-rays (and night vision goggles!) and photomultipliers used in digitisation of the image, ie in CAT scans and gamma cameras and PET scan!

David.

This was posted from The Student Room's iPhone/iPad App

Reply 394

username896004

Original post by ReTurd

Can anyone tell me why the book says that a proton has a spins of +0.5 but when you use the table to the right of the picture where is says that up and down quarks both have a spin of +0.5 so the total spin of a proton should add up to +1.5.
Anyone explain? Whats wrong here?

We don't need to know about spin states for the exam - that's just extra stuff

This was posted from The Student Room's iPhone/iPad App

Reply 395

username896004

Original post by ReTurd

Can anyone tell me why the book says that a proton has a spins of +0.5 but when you use the table to the right of the picture where is says that up and down quarks both have a spin of +0.5 so the total spin of a proton should add up to +1.5.
Anyone explain? Whats wrong here?

But for the sake of it :P you notice the table includes magnitude of spin, not direction? Up and down quarks have opposite spins - the sum of which produce +1/2 spin in the proton

This was posted from The Student Room's iPhone/iPad App

Reply 396

Picture~Perfect

19

Original post by ReTurd

Could someone explain to me how ultrasound is used to measure the speed of blood flow in veins.. The theory and the maths.

You don't need to know the mathematics involved but I will explain the theory.

An ultrasound transducer is pointed towards a major blood artery. The blood cells reflect the ultrasound. For blood travelling towards the transducer, the reflected ultrasound has a slightly higher frequency. The change in the frequency of the ultrasound is directly proportional to the speed of the blood.

Reply 397

Picture~Perfect

19

Original post by singh224

Do we need to know the hazards from the radioactivity as its not mentioned in the spec?

If it isn't in the specification then you don't need to know it.

Reply 398

B Jack

1

In the OCR Physics A book, it says that for an ultrasound transducer, the piezoelectric crystal is cut to a whole number of wavelengths so that it will resonate. Is this wavelength that of the transmitted ultrasound waves only? Because I'd imagine returning ultrasound waves would have varying wavelength, although I'm unsure about this :s-smilie:

Reply 399

username896004

Original post by B Jack

In the OCR Physics A book, it says that for an ultrasound transducer, the piezoelectric crystal is cut to a whole number of wavelengths so that it will resonate. Is this wavelength that of the transmitted ultrasound waves only? Because I'd imagine returning ultrasound waves would have varying wavelength, although I'm unsure about this :s-smilie:

You want the intensity of the pulsed ultrasound to be high don't you? Without resonance you'd need a large voltage to make the crystal contract a lot, to give sufficient amplitude. The transducer has a tuning device so you can change the resonant frequency of the crystal :smile:

the wavelength will be different yes, not so different as to differ massively from the resonant frequency

This was posted from The Student Room's iPhone/iPad App

OCR Physics A - G485: Fields, Particles & Frontiers of Physics - June 2012

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