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# OCR Physics A G485 FPFoP Unofficial mark scheme

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Why bother with a post grad? Are they even worth it? Have your say! 26-10-2016
1. (Original post by teachercol)
I think you should use the diameter to find the angle subtended.
But do you not use the distance between as center to center? So you are creating a right-angled triangle when none exists.
2. (Original post by teachercol)
OCR Physics A G485 FP and FoP 28/6/16

Usual disclaimers. These are just my quick answers . They may and probably will contain errors and typos.

My students were pretty happy when they came out of the exam room yesterday.
First impression is that this was pretty straight down the middle.
Not much in the way of long extended answers

Here we go

Q1 a) 1T = 1N / 1A x 1m = 1 kg ms-2 / 1 Cs-1 x 1m = kg s-1 C-1 (2)
b) i) Bqv = mv^2 / r so B = mv / qr = (9.11E-31 x 7.0E6)/(1.6E-19 x 2.5E-2) = 1.59E-3 (3)
ii) T = 2 pi r / v = 2 pi x 2.5E-2 / 7.0E6 =2.2E-8s (1)
iii) If v doubles and m, B, and q are same then r is prop to v, so r doubles
T is prop to r/v so stays constant. (2)
Total: 8

Q2 a) Direction should be + to -
Field lines must meet plates at 90 (2)
b) i) Grad = dy/dx = 1.0E21/8.0E27 = 1.25E-7
E = Q / 4 pi eo r^2 so Grad = Q / 4 pi eo so Q = 4 pi eo x grad
= 1.19E-17 C (2)
ii) Charge decreases by 2e (so now Q = 1.358E-17C)
Grad is prop to Q so will decrease (2)
c) i) mg = Eq = Vq/d
4/3 pi r^3 rho = Vq/d so q = 1.12E-18C (3)
ii) N=Q/e = 7 (1)
Total: 10

Q3 a) i) Mag flux linkage = product of no of turns on coil and magnetic flux through 1 turn of coil (1)
ii) 1) L = N x 2 pi r so N = ? / 2 pi r (1)
2) N phi = NBA = L / 2 pi r x B x pi r^2 = BrL/2 (1)
b) i) Faraday's law says induced emf - rate of change of magnetic flux linkage (1)
ii) emf = - gradient -> -cos graph (2)
c) Ac supply creates alternating current in primary coil which creates alternating flux in primary
Core 'conducts' flux through secondary coil.
Alternating flux in secondary induces an alternating emf in secondary (2)
Total: 8

Q4 a) In series so same charge
v=Q/C so V is prop to 1/C
so if B has twice the capacitance it has half the PD. (2)
b) 1/C = 1/200 + 1/200 so C = 100uF
R = 18 + 9 = 27k so tau = 27E3 x 100E-6 = 2.7s (3)
c) i) Initial I = 1.5E-4A from graph
so Vo = IR = 6.0v
so Qo = CV = 7.2E-3C (2)
ii) 2 x 40k in parallel means resistance halves so time constant halves
so graph starts at twice the current (3.0) and has quarter value at each 20s
Total: 9

Q5 a) i) 2 x 1n0 (1)
ii) 0e-1 + antineutrino (2)
b) i) No of decay/s = 2000/9.0E-3 = 2.22E15
so Activity = 2.22E15
lambda = ln2/half life = 2.293E-10 s-1 (convert y to s!)
N = A / lambda = 8.906E24
n = N/NA = 14.79 moles so m = 0.238x 14.79 = 3.52 kg (4)
ii) W = Pt = 120/1000 x 24 = 2.88 kW.h (2)
Total: 9

Q6 a) Hadrons experience strong force (1)
b) u +2/3 ; d -1/3 (1)
c) proton = uud (1)
d) uud + udd = uud + uud + pi-
so pi- = d +ubar(3)
e) i) E = mc^2 where E = energy m = change in mass c = speed of light in a vacuum (1)
ii) Min KE = 1.4E8eV = 2.24E-11 J
so m = E/c^2 = 2.29E-28 kg (2)
Total: 9

Q7 a) 1) fission occurs with heavy nuclei; fusion with light
2) fission: 1 nucleus splits into 2 smaller ones; fusion two small nuclei join to form 1 larger one (2)
b) Nuclei need to get close enough for strong force to pull them together
so need enough energy to overcome coulomb repulsion
KE = 3/2 kT so need high temperatures
Need a reasonable chance that collision will occur so need lots of nuclei ie high pressures (4)
c) i) Bombard nucleus with thermal neutrons. Neutron is absorbed by nucleus.
Unstable nucleus splits ( releasing more neutrons) (2)
ii) 3/2 kT = 1/2 mv^2
T = 300+273 = 573K so v = 3736 ms-1 (3)
Total: 11

Q8 a) Photoelectric effect
Photon absorbed; electron released
Compton scattering
Photon scattered with lower energy at an angle and electron emitted
Pair production
Photon absorbed and electron/positron pair created.
1st and 3rd involve photon being absorbed so transmitted intensity decreases. (4)
b) i) E = hf = hc/ lambda = .141E14J (2)
ii) Gradient = -mu so mu = - gradient = 0.20 (2)
Total: 8

Q9 a) Frequency of radio waves = angular frequency / 2 pi = 6.37E6 Hz
so lambda = 3.0E8/6.37E6 = 4.7m (3)
b) relaxation time = time between proton being excited to higher energy state and dropping back to ground state (1)
Total:4

Q10 a) piezoelectric effect: when apply PD to a crystal, it expands or contracts. (1)
b) i) z = product of density and speed of sound in material (1)
ii) In patient, ultrasound will reflect off boundaries between different tissue types (media with different z) (1)
c) z muscle = 1.38E6; z fat = 1.69E6
so Ir/I = (z2-z1)^2 / (z2+z1)^2 = 0.0961/9.42 = 1.0%
so 99% is transmitted (3)
Total: 6

Q11 a) tan(theta) = 1.3E20/2.4E22 so theta = 0.31 degrees (1)
b) dlambda / lanbda = v/c so d lambda = 656.3 x 2.5E5 /3.0E8 = 0.547 nm (2)
c) GMm/R^2 = mv^/r so M - v^r/G = 6.09E40 kg
so N = M/m = 3.03E10 stars (4)
Total: 7

Q12 a) Initially all 4 forces unified
Grav force splits first; primordial quark soup
Strong force separates; leptons form from photons; more matter than antimatter
weak and EM force separate; protons and neutrons formed ; p:n = 4:1; annihilation results in matter only
Fusion forms He and Li; matter is plasma
Atoms form; recombination; universe is transparent; first light CMB formed (5)
b) Hubble: for distant galaxies, velocity of recession is prop to distance (1)
c) i) rho = 8 x 1.673E-27
= 8 H0^2 / 8 pi G so Ho = 2.73E-18 s-1 (3)
ii) t = 1 / Ho = 3.6E17 s = 1.14E10 y (2)
Total: 11

Hopefully that adds up to 100.

I wouldn't expect grade boundaries to be below what they were last year.
Good exam paper. Well done examiners.
Hi teachercol , thank you as usual for your report. Going to be a long one I'm afraid.2)a) (i) any credit for saying field lines are of equal spacing/parallel when they shouldn't be? 3)a) (i) I simply wrote no. of turns of coil x flux density x area of wire perp to field, does this gain credit?b) (i) I stupidly drew a positive cos graph with a slightly larger amplitude, both marks lost?4)c) (i) did this weirdly but think I got the same answer as you. Found the time constant using cap x res, then calculated the charge lost 'Q1' during that time using loss in current x time constant. Then used Q = (Q+Q1) e^-1 to find Q after time constant, then added that to Q1 for original charge... 5) b) (i) found the decay constant however didn't get the correct number 'N' value. However I did my N value divided by Avogadro x molar mass, how many marks?6)a) hadrons are made up of quarks?e) (i) I defined E as the BE (for a particular nucleus) and M as the mass defect (of the same nucleus)..?11) a) I did it with the diameter as well. Is that a typo or the correct answer? 12) a) any credit forniverse cooled as it expanded, to 3K. CMBR redshifted from gamma radiation due to loss in energy (as energy prop to temp)? Realise that this is extremely tedious for you but if you have the time, I'd appreciate your feedback!
3. (Original post by King Ahmed)
For q2c i) I was able to find the weight and find an expression for charge but got the wrong answer wil I lose any marks since this is was a ' show that 'question.
Generally there are marks for each step.
4. (Original post by TheFarmerLad)
Hi teachercol , thank you as usual for your report. Going to be a long one I'm afraid.2)a) (i) any credit for saying field lines are of equal spacing/parallel when they shouldn't be? 3)a) (i) I simply wrote no. of turns of coil x flux density x area of wire perp to field, does this gain credit?b) (i) I stupidly drew a positive cos graph with a slightly larger amplitude, both marks lost?4)c) (i) did this weirdly but think I got the same answer as you. Found the time constant using cap x res, then calculated the charge lost 'Q1' during that time using loss in current x time constant. Then used Q = (Q+Q1) e^-1 to find Q after time constant, then added that to Q1 for original charge... 5) b) (i) found the decay constant however didn't get the correct number 'N' value. However I did my N value divided by Avogadro x molar mass, how many marks?6)a) hadrons are made up of quarks?e) (i) I defined E as the BE (for a particular nucleus) and M as the mass defect (of the same nucleus)..?11) a) I did it with the diameter as well. Is that a typo or the correct answer? 12) a) any credit forniverse cooled as it expanded, to 3K. CMBR redshifted from gamma radiation due to loss in energy (as energy prop to temp)? Realise that this is extremely tedious for you but if you have the time, I'd appreciate your feedback!
2ai yes - the field isn't uniform
3ai you've defined magnetic flux along the way - should be OK
b) -1 for not getting it inverted
4ci you cant use charge = current x time if current is not constant. so 0/2
5bi its a long question so probably ecf in there.
6a) quarks will be OK
ei) I wouldn't give it.
11a) dia will be Ok. You can use radius then double angle but you'll get same answer with an angle that small.
12a)cool and expand should be worth a mark/ mention of CMB should be worth a mark too.
5. (Original post by Aph)
But do you not use the distance between as center to center? So you are creating a right-angled triangle when none exists.
You can use the radius then double the angle but for small angles youll get the same answer.
6. teachercol , on the first question how many marks would i lose for using 5x10^-2 as the radius through the question?

Also would i get a mark on drawing the emf graph for doing a sinusoidal curve with correct period and amplitude? Thankyou
7. @teachercol, what do you reckon the A* and A boundaries will be like for this paper?
8. (Original post by CoIIins)
teachercol , on the first question how many marks would i lose for using 5x10^-2 as the radius through the question?

Also would i get a mark on drawing the emf graph for doing a sinusoidal curve with correct period and amplitude? Thankyou
a) probably cost you a mark - did the same thing myself first time
b)a sin graph probably wont score any marks - amplitude isn't relevant. Id expect 1 mark for a cos graph and 2 for -cos.
9. (Original post by Asurat)
@teachercol, what do you reckon the A* and A boundaries will be like for this paper?
Probably much the same as last year - but until I've seen a few papers I don't know
10. (Original post by teachercol)
Probably much the same as last year - but until I've seen a few papers I don't know
PRSOM, thanks for your thoughts
11. For 8bii) Gradient = -mu so mu = - gradient = 0.20 (2)
Would I get marks for calculating the maximum intensity and one further along the line (using e^the y value) and putting it into I=I0e^-ux ? I think I got 0.20
12. hey ! for differences between fusion and fission i said that fission can be induced by thermal neutrons, do you think that may get the mark ?
13. (Original post by Laurasaur)
For 8bii) Gradient = -mu so mu = - gradient = 0.20 (2)
Would I get marks for calculating the maximum intensity and one further along the line (using e^the y value) and putting it into I=I0e^-ux ? I think I got 0.20
I expect so.
14. (Original post by DAWG123)
hey ! for differences between fusion and fission i said that fission can be induced by thermal neutrons, do you think that may get the mark ?
I doubt it.
15. (Original post by teachercol)
..

How detailed do you have to be for 2a) ?

I simply put that the direction and the shape was wrong. Would this give me 0/2??

I think I may have lost quite a few marks for 12a)

I forgot to mention that all forces were unified however I stated
-Fg seperated first in primordial quark soup stage.
-Fs seperated next, leptons from photons.
-Fw and Fe seperated, quarks were only found in atoms.

How many marks do you think i'd lose?

Other than that I've seem to have gotten everything so overall a good paper !

Thanks
16. (Original post by calcumm)
How detailed do you have to be for 2a) ?

I simply put that the direction and the shape was wrong. Would this give me 0/2??

I think I may have lost quite a few marks for 12a)

I forgot to mention that all forces were unified however I stated
-Fg seperated first in primordial quark soup stage.
-Fs seperated next, leptons from photons.
-Fw and Fe seperated, quarks were only found in atoms.

How many marks do you think i'd lose?

Other than that I've seem to have gotten everything so overall a good paper !

Thanks
Direction should score a mark. Shape on its own wont score.

Hard to say for 12a. I don't know exactly what will and what wont score marks.
17. (Original post by teachercol)
Direction should score a mark. Shape on its own wont score.

Hard to say for 12a. I don't know exactly what will and what wont score marks.
Do you think i will score a mark for saying field lines should be curved and the distance between field lines should be different because the field is not uniform? I also mentioned the direction.
18. (Original post by Ramil Ahmadov)
Do you think i will score a mark for saying field lines should be curved and the distance between field lines should be different because the field is not uniform? I also mentioned the direction.
Seems reasonable to me
19. (Original post by teachercol)
Seems reasonable to me
Thank you very much for your reply. Can you remind me what this question was about? I can remember all the questions and answers except this one which i absolutely forgot.

Q8 ii) Gradient = -mu so mu = - gradient = 0.20 (2)

20. It was a graph of Ln(I) against x for absorption of X rays.

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