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# Physics A G485. My answers and mark scheme Tweet

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1. Physics A G485. My answers and mark scheme
I know there's another thread. But here's my version as promised.

Usual discaimer. These are just my answers and are in no sense official or anything to do with OCR
They may contain typos and erros.

My first reaction as a teacher looking at paper was one of relief. I'd covered everything in a lot of detail and told my lot to revise fission reactors / MRI / evolution of universe in detail ...I dont think it was
especially hard - at least question by question , it seemed straightforward. Several of my students were
pressed for time - which is quite good - there was a lot of writing on that paper.

Ok here's the answers ...

1 a) One Farad = 1 Coulomb / 1 Volt (or in words) (1)
b) i) Gradient = v/Q = 1/Capacitance. (1)
ii) Area under graph = Energy stored on capacitor (1)
c) Min C is when in series.
1/C = 1/100 + 1/200 +1/500 = 17/1000 so C = 1000/17 = 58.8 uF (3)
d) i) Q=It = 4.0E-3 x 60 = 2.4C (2)
ii) E = 1/2 Q^2/C = 28.8J (2)
Total 10.
Nice easy opener. A lot harder Cap questions than that around.

2 a) i) Arrow towards centre of circle (1)
ii) Force is perp to motion so no work done so no change in energy (1)
b) F = mv^2/r = 9.1E-31 x (6.0E7)^2/24E-2 = 1.37E-15N (2)
c) F = Bqv so B = 1.37E-15/(1.6E-19 x 6.0E7) = 1.42E-3T (2)
d) 2mc^2 = 2hc/lambda so lambda = h/mc = 6.6E-34/(9,1E-31 x 3.0E8) = 2.4E-12m (3)
Total 10
Not too bad for a mag field queston. I was hoping for a hard velocity selector / crossed E/B fields question.

3 a) i) T = 10E-3s so f = 1/T = 100Hz (1)
ii) Flux linkage = NBA so B = 2E-2/(400x x1.6E-3) = 3.125E-2 T (3)
iii) Emf = rate of change of flux linkage
so use gradient at point where crosses axis (or differentiate P=Pocos(2 pi f t)
I got about 13v. ( Must show working ) (3)
b) P=V^2/R = 13^2/150 = 1.13W (2)
Total 9
Some easy marks there. Only grad of graph should cause any problems.

4 a) Conservation of momentum so inital momentum of alpha = final momentum of gold nucleus
or argue in terms of repulsive forces and newton3
Force that slows down alpha also acc gold nucleus to left.
I dont think ansers in terms of energy will score anything. (2)
b) Lines that (come from centre of each charge) outwards, curving away from other charge (2)
c) F = Q1Q2/(4 pi eo r^2) = (79 x 1.6E-19 x 2 x 1.6E-19) / (4 pi eo x 6.0E-14^2)
= 10.1N (3)
d) 1/R^2 graph. Easiest is to double r each time and / force by 4
so F=10N at 6 (x10-14m) then F=2.5N at 12 and F = 0.6 at 24. (2)
Total 9.
Not sure about this one. Calc is easy but I'd expect a lot of people to not put charge in correctly.
First one to cause real problems .

5 a) Some of mine missed the word neutron in question! (Bah)
No of neutrons = (234-94) - (4-2) = 142 or just use 234-92 = 142 (1)
b) i) KE= 5.6MeV = 5.6 x 1E6 x 1.6E-19 = 8,96E-13J (1)
ii) KE = 1/2 mv^2 so v = sqrt(2 x 8.96E-13 / 6.65E-27) = 1.64E7 ms-1 (2)
c i) Power = Activity x energy of 1 event
so A = 62/8.96E-13 = 6.92E13 Bq (1)
ii) lambda= ln2 / t1/2 = 0.693/(88 x 3.16E7) = 2.49E-10 s-1 (2)
iii) A = lambda N so N = 6.92E13/2.49E-10 = 2.78E23 (2)
n=N/NA = 2.78E23/6.02E23 = 0.462 mol
m = n x Mm = 0.462 x 0.24 = 0.11 kg (1)
Total 10.
Seems straightforward but an unusual order of calc. Usually we go m -> N -> A -> power
This time we went in the opposite direction and that seems to have thrown people.

6 a) Emitted neutrons can go on to initiate more fission events wheich releases more neutrons.
These neutrons in turn cause more fissions and rate can increase exponentially
(or just draw a picture) (1)
b) Fuel rods: contain enriched U-235, where fission happens.
Control rods: B or Cd. Absorb neutrons. lowered to slow down reaction .
Moderator: water in PWR / Graphite in AGR. Slow dwon neutrons by colliosn with moderator nuclei.
doesnt absorb neutrons. Ideally mass od nucleu = mass of neutrons
used to extract energy from neutrons
slow neutrons more efficient /greater chance of initiating fission of U-235
c) i P of reactor x 22% = Output power so P = 3.0E9 x 100/22 = 1.36E10 W (1)
ii) E = Pt = 1.36E10 x 8.64E4 = 1.18E15J (1)
iii) No of fissions needed = 3.68E25/3.2E-11 = 3.68E25
so mass od U needed = 3.68E25 x 3.9E-25 = 14.4kg (2)
d) Waste is very radioactive ie high activity mainly beta - emittors.
Mixture of short and long half lives. Beta- causes damage to tissue so health risk if exposed.
Generates a lot of heat so very hot. (2)
Total 12.
Routine bit of book work + calculation that is very similar to last question.
If you got that wrong you probably got this one wrong too.

7 a) Em radiation / photons / oscillating E and B fields / wavelength 1E-9 -> 1E-12. etc (2)
b) X ray photon is scattered by electron and loses energy / increases wavelength.(2)
c) i) Io = inital intensity of X rays before enter material (1)
ln2 = u x 1/2 so x1/2 = ln2 /3.3 = 0.21cm (3)
YOu can work in metres but dont need to. NB 3.3 cm-1 = 3.3E2 m-1 not 3.3E-2 m-1
d) High Z material put into body so absorbs X rays and allows imaging of soft tissue
eg Barium into stomach / bowel so look for blockages etc (2)
Total 10
Seems easy enough.

8 a) protons precess in magentic field at Larmor frequnecy
freq depends on B
gradient coils create unique value of B (and f) at each point
RF transmitter fires radio pulse at one freq. Proton precessing at that freq resonates
ie absorbs energy and moves to excited state
After relaxation time, it deexcites and drops back to lower level emitting pulse of RF at same freq.
Detected by RF detector. Measure relaxtion time
Sweep through range of frequencies so do this for each point in filed.
Relaxtion time depedns on proton environment ie tissue type.
Computer builds up 3D image from relaxation times - can make false colour or look at particular range of times.
Can look at 3D image of organs eg brain, muscle tears, bleeds ....
(7)
b) PET uses B+ emitters to create gammas that are detected. MRI doesnt use ionising radiaton
so MRI much safer.
PET looks at cellular activity - cancerw MRI looks at tissue defects.
PET can be used on patients with implants/pacemakers. MRI cant.
PET quick minutes, MRI can take up to an hour.
etc (2)
Total 9
Routine bookwork - everyone predicted this would come up this year

9 a) Fusion: 2 small nuclei join together to make a larger one
High energies/temp - to overcome Coulomb repulsion between positive nuclei
High densities to give chance for two protons to collide
High confinement times to allow many fusion events
Grav compression responsible for all these. (4)
b) short life - lives fast and dies young
becomes red giant when H runs out in core
burns He and expands and contracts as burns through different elelments (up to Fe)
may become red supergiant
supernova
becomes neutron star or black hole (if big enough) (3)
Total 7
More routine bookwork. premium on bullet pointing this stuff. Easy to waffle and miss marks.

10 a) F = Gm1m2/r^2 = 6.67E-11 x (1E11 x 1E30)^2 / (4E22^2) = 4.17E26n (3)
b) Universe is expanding.
(newtons explanation in terms of equilibrium only works in infinite universe which is ruled out by opening statement)
c) Unified forces - expends cools and forces split
(Inflation?) : period of rapid expansion
Quarks/antiquarks+leptons+photons formed
Matter/antimatter symmetry broken
Continues to expand and cool
Hadrons form / annihilation of matter+antimatter leaving only matter
Fusion H->He (+Li)
Atoms form. UNiverse becomes tranparent
Small density fluctuations -> grav collapse -> stars and galaxies
(6)
d) i) Absorption spectrum consist of dark lines on a continuous spectrum. Formed by absorption of certain freqnecies when continuous spectrum passes through cool gas.
Freq absorbed are same as one emitted by hot gas
hf = E2-e1 etc (2)

ii) dlambda/lambda = v/c so v = (480 - 393.4)/393.4 x 3E8 x 1E-3 = 66000 kms-1
v = Ho x d so d = 66000/50 = 1320 Mpc. (3)
Total 15

so there we have it.

I'm not sure what to make of this one. I thought it was pretty straightforward but the verdict here is it was hard.
Some of mine thought it was Ok - but they made too many mistakes.

I dont know what the grade boundaries will be
I dont know how many UMS makrs you'll get

Col
Last edited by teachercol; 12-06-2012 at 15:29. Reason: Omission of one Q
2. Re: Physics A G485. My answers and mark scheme
Thanks, really appreaciated

+REP

Edit

I used 12 volts here b) P=V^2/R = 13^2/150 = 1.13W (2)

So got 0.96W

(Original post by teachercol)

I think I lost about 16 marks, maybe more.
Last edited by Jukeboxing; 12-06-2012 at 13:47.
3. Re: Physics A G485. My answers and mark scheme
Thanks, I think i got 90/100
4. Re: Physics A G485. My answers and mark scheme
(Original post by jamesvernon)
Thanks, I think i got 90/100
Nice, looks like 150/150 UMS for you then
5. Re: Physics A G485. My answers and mark scheme
I think you missed out the absorption spectrum question? Found it far harder than January 2012 and other past papers so I'm expecting the boundaries to drop, similar consensus at my school. Thanks for the mark scheme.
6. Re: Physics A G485. My answers and mark scheme
(Original post by highway-61)
I think you missed out the absorption spectrum question? Found it far harder than January 2012 and other past papers so I'm expecting the boundaries to drop, similar consensus at my school. Thanks for the mark scheme.
I just took another look at the Jan 12 paper, it was much easier.

Easier calculations and less written questions, so boundaries should go back to usual.

Edit:

The average mark on the Jan 12 was about 10 marks higher than the other papers, which is probably the reason why the boundaries were much higher.

It was 60 as opposed to 48-50.
Last edited by Jukeboxing; 12-06-2012 at 14:49.
7. Re: Physics A G485. My answers and mark scheme
what do you reckon 61 would be?
so annoyed if i sat this paper last year the mock I got was 69/100 which was an A and this paper...evil
Last edited by pshewitt1; 12-06-2012 at 14:43.
8. Re: Physics A G485. My answers and mark scheme
Is it an average UMS of 90%+ for an A*, or 90% raw paper marks over the 2 a2 papers?
9. Re: Physics A G485. My answers and mark scheme
(Original post by jamesvernon)
Is it an average UMS of 90%+ for an A*, or 90% raw paper marks over the 2 a2 papers?
Yh you need 90% overall at A2 and 80% overall for your whole A level.
10. Re: Physics A G485. My answers and mark scheme
So is the 90% Raw marks or 135 UMS?

I.E. Do i need to get 90/100 or 135/150 UMS to get an A* given that i got 90% in the first A2 module.

EDIT:

So i got 75/90 UMS in Newtonian World. = 83%

So i need 96% im UMS for This paper right? So 145 UMS? :s
Last edited by jamesvernon; 12-06-2012 at 15:05.
11. Re: Physics A G485. My answers and mark scheme
(Original post by jamesvernon)
So is the 90% Raw marks or 135 UMS?

I.E. Do i need to get 90/100 or 135/150 UMS to get an A* given that i got 90% in the first A2 module.

EDIT:

So i got 75/90 UMS in Newtonian World. = 83%

So i need 96% im UMS for This paper right? So 145 UMS? :s
At A2 you need atleast 270/300 UMS

For the whole A level you need 480/600 UMS, then you'll be awarded A*.

So 75 + 145 = 220 UMS, that means you'll need at least 50/60 from practical mark to total up 270 UMS.
Last edited by Jukeboxing; 12-06-2012 at 15:25.
12. Re: Physics A G485. My answers and mark scheme
(Original post by highway-61)
I think you missed out the absorption spectrum question?
Oops so I did - will add it.
13. Re: Physics A G485. My answers and mark scheme
(Original post by Jukeboxing)
At A2 you need atleast 270/300 UMS

For the whole A level you need 480/600 UMS, then you'll be awarded A*.

So 75 + 145 = 220 UMS, that means you'll need at least 50/60 from practical mark to total up 270 UMS.
Don't remember my practicals.
Let's hope for the best :3
14. Re: Physics A G485. My answers and mark scheme
(Original post by jamesvernon)
Don't remember my practicals.
Let's hope for the best :3
If you do get 90/100, that will surely equate to 150/150 UMS.

So you would need a 45/60 in practical.

Why cant you remember, when did you do the practical?

I got 72/90 for unit 4 in Jan but i'm resitting it to try get an A*.
15. Re: Physics A G485. My answers and mark scheme
I used 12^2 /150=0.96W for maximum voltage used but i think your answer is correct because u can use your answer carried forward.

http://imgur.com/7OZCV The field lines should cross the vertical asymptote between the particles at the midpoint.

for 7.a it asks two properties of X-rays you could mention reflection, refraction, diffraction, travel at speed of light as it e.m wave, transverse wave, can be polarized,

5.c.iii. it says the molar mass is 0.24kg and asked for mass. surely the mass would the molar mass multiplied by the relative atomic mass to give 56.4kg

6.d. exposure due to skin to highly ionizing

4.a just realized it is to do with momentum as the particle suddenly stops so energy wouldn't gained marks, if the alpha particle starts to decelerate and gold atom begins to accelerate then energy would be involved...- nasty question

9.a I mentioned that hydrogen nuclei had to overcome the electrostatic repulsive force so it is able to experience the strong nuclear force to cause a collision would that gain marks?
16. Re: Physics A G485. My answers and mark scheme
on the first question, they mentioned 'steady current'. Can you still use the equation for alternating current?
17. Re: Physics A G485. My answers and mark scheme
On the Show that something is 12V question, I did:

Nphi = 0.02cos(200pi x t)

So the gradient would be:

4pi x sin (200pi x t), which is about 12.6V, would I get the marks?
18. Re: Physics A G485. My answers and mark scheme
I think I got about 76/100 by marking harshly, could be up to +7/8 marks possibly, depends on the examiner I suppose
19. Re: Physics A G485. My answers and mark scheme
(Original post by Father Ted)
I think I got about 76/100 by marking harshly, could be up to +7/8 marks possibly, depends on the examiner I suppose
wow,nice one jov! think i got around the same!
let's hope grade boundaries are nice!
time to start revising for my next exam now haha!
20. Re: Physics A G485. My answers and mark scheme
(Original post by joefoxon)
On the Show that something is 12V question, I did:

Nphi = 0.02cos(200pi x t)

So the gradient would be:

4pi x sin (200pi x t), which is about 12.6V, would I get the marks?
I hope they would, but you have to remember this is physics A, and in this syllabus we aren't meant to differentiate anything, this syllabus expects you to make a gradient triangle (which is a bit ridiculous). The physics B syllabus involves differentation etc.
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