A2 Physics A FP and FoP June 2014

OCR A2 Physics A G485 FP and FoP June 2014
Usual disclaimer. These are just my answers and opinions. They are in no sense official.
They may contain errors, omissions and typos.
Talking to my students as they came out, first impression is that it was a straight down the middle paper. Not too sure about that. There were a few topics that I went into in class in a lot of detail (Q5,Q7) which might prove difficult. Not many 'memory' questions and a lot of working out. We'll see. I thought G484 was OK but the general consensus around here was that it was hard.
Anyway here we go.

Q1 a) Seen this lots of times. Make sure your arrows go + to - and your lines meet the surfaces at 90 [2]
b) i) B is the bigger charge.
Resultant is zero nearer A. Fields must be equal. E is prop to Q/ r^2 so if E is same and r is bigger Q must be bigger (WTTE) [1]
ii) + on left (Force is to R) . Curves like 1/r^2. passes through 0 at X. Negative from X to B . (force is to left) curving downwards [3]
Tricky. Expecting not too many to score 3/3 on that.
c) E = k / r^2 g = k' / r^2 so ratio = k / k' and distance factor cancels out.
(Both inverse square laws so get weaker at same rate) [1]
TOTAL 7
Some easy bits; some tricky bits.

Q2 a) i) Standard: Force due to mag field is always perp to velocity. [1]
ii) because F is perp to v, no work is done / no acceleration in direction of travel so speed is constant (NOT velocity!) so KE is constant. [1]
b) Bqv = mv^2/r so mv = Bqr = 0.20E-2 x 3.2E-19 x 15E-2 = 9.6E-24 [3]
c) v = 9.6E-24/6.6E-27 = 1454 ms-1 so KE = 1/2 x 6.6E-27 x 145362 = 6.98E-21J [2]
d) Tricky.Does circular motion with centre of circle accelerating to right uniformly and circles get bigger as v increases.
( [2]
TOTAL 9
Same applies here. Some easy bits with a sting at the end.

Q3 a)i) Same proton number / same charge / same no of electrons / same nuclear density [1]
ii) All nuclei have a mass defect which is the mass of the separate particles - mass of the nucleus. It is due to the binding energy of the nucleus which is the mass/energy needed to separate the nucleus against the strong force into its separate particles. [3]
b) proton and anti - neutrino. [2]
Half lfe = time for half the neutrons in a sample to decay .
c) i) F = Q1Q2/ 4 pi eo r^2 = 1.6E19^2 / 4 x pi x 8.85E-12 x 1E-14^2 = 2.3N [2]
ii) 70 keV = 70E3 x 1.6E-19 J = 3/2 kT
so T = 2/3 x 70E3 x1.6E19/1.38E-23 = 5.4E8 K [3]
iii) average KE = 70 keV but some have much more than this
Quantum mechanical tunneling [1]
iv) Energy released = 18MeV = 18E6 x 1.6E-19J = mc^2
so m = 3.2E-29 kg [2]
v) D-He is harder to fuse than D-T because higher charge on he means more repulsion so need higher temps. [2]
TOTAL Pretty reasonable. Seen all this before.

Q4 a) Time constant = time for charge to drop to 1/e of initial value [1]
b) Any two values that multiply to give 5 eg 1uF and 5Mohm [1]
c)i) Area of wire = pi x (0.12E-3/2)^2 = 1.131E-8 m62
R = rho L/ A = 4.9E-7 x 5.0 / 1.131E-8 = 216.6ohm
tau = CR = 0.010 x 216.6 = 2.17s [3]
II) electrons move from X to + plate on cell due to potential difference
at same time electrons move from - plate of cell to Y
Effectively electrons move from X to Y
so X gets + charge and Y gets - charge
Charge is conserved so Qx = Qy. [3]
iii) Energy stored on capacitor = 1/2 Cv^2 = 1/2 x 0.010 x 12^2
= mcdT so dT = 3.41 C [4]
iv) if double PD then 4x energy stored so temp rise is 4x bigger [2]
TOTAL 14. Seems Ok to me.

Q5 a) Induced emf is prop/equal to the rate of change of magnetic flux linkage [1]
b) Energy for emf comes from work down in pushing magnet
so magnet must be repelled (lenz's law)
so must be N pole of face [1]
c) i ) No change in flux linkage so rate of change = 0 so no emf. [1]
ii) I do variations on this question every year in class.
from t=3 to t = 5 , flux is increasing at increasing rate so emf is negative and increasingly negative.
from t = 5 to t=8, flux is constant so emf = 0
from t = 8 to t=10, flux is decreasing at decreasing rate so, emf is + dropping to zero.[3]
Total 6.
Last bit is hard if you've not seen it before

Q6 a) i) proton is uud so antiproton is C [1]
ii) Zero. (its a pi meson) [1]
b) i) Proton (add up the top and bottom numbers) [1]
ii) lambda = ln2/ t = 0.693/5700x3.16E7 = 3.86E-12 s^-1
A = lamda N so N = 1.1E19/3.86E-12 = 2.85E30
no of moles = N/NA = 2.85E30/6.0E23 = 4.75E6 mol
mass = molar mass x n = 14E-3 x 4.75E6 = 6.66E5kg [3]
c) Nucleus absorbs a neutron; becomes unstable and splits into two smaller nuclei emitting 2 or3 more neutrons [2]

d) Moderator : slows down neutrons from high energies (MeV) to thermal energies (1/10eV) by repeated collisions between moderator nuclei and neutrons
slow neutrons have a better chance of being absorbed and causing fission.
Control rods : absorb neutrons and control rate of reaction [4]
TOTAL 12 Seems OK to me

Q7 a) i) Energy level = allowed value of energy that an isolated atom can have. [1]
ii) f = c/lambda so f =3.0E8/7.2E-11 = 4.167E18Hz
E=hf = 6.6E-34 x 4.167E18 = 2.76E-15J
This is difference between levels so energy of level = -3.2+2.76 = -0.44E-15J [3]
Last bit might catch a few out.
iii) If double V, then double energy so f doubles so lambda halves [2]
b) i) I=Ioexp(-ux) so I/Io = exp(-0.96 x 2.3) = 0.11 [3]
ii) Bone:fat = 2.8:0.9 Muscle:fat=0.96:0.90
Bigger ratio so bone:fat will have better contrast [1]
TOTAL 10: Again seems OK apart from last bit of aii

Q8 a) Array of microwave transmitters and receivers
send out pulses of ultrasound
which reflects off boundary between media
of different acoustic impedance ( = density x velocity)
Brightness tells us amplitude of return pulse
Position determined by time delay
etc [3]
b) F-18 attached to glucose molecule
Injected / ingested into patient
Concentrates at sites of high metabolic activity
F-18 emits beta+ which annihilate with electrons
emitting 2 gamma photons at 180
Circular array of gamma cameras detect gammas and pick up line
Position along line determined by difference in time of arrival
(sketch) [4]
TOTAL 7 Routine memory work

Q9 a) r = 10km/2 = 6.0km.
density = M/V = 2.0E20 / (4/3 pi x 6.0E3 ^3) = 2.2E18 kg m-3 [3]
b) g = GM / R^2 so ratio = (1/r^2)sirius / (1/r^2) sun
= 1.4E10 [2]
c) d = 8.6ly = 2.6 pc so angle = 1/d = 0.38" [2]
d) dlambda / lambda = v/c = 7600 / 3.oE8 x 100% = 0.0025% [2]
e) No - hubble's law only applies to distant GALAXIES moving away from us. [1]
TOTAL 10 Easy

Q10 a) Universe is ISOTROPIC and HOMOGENEOUS [2]
b) Microwave background is isotropic - same intensity in all directions
has a temp of 2.7K
has a black body continuous spectrum - hot dense material
was formed by gammas formed shortly after big bang being stretched to microwaves. [3]
c) rhoc = 8Ho^2/8 pi G so Ho = 2.33E-18s
so Age = 1/Ho = 4.295E17s = 13.6E9 y [3]
TOTAL 8 . Seems Ok to me.

Overall. Not bad at all. Similar to previous papers in many respects.
Good paper I think. Nice balance of easy / hard. Calculate / memory
and one or two new bits to stretch.

Grade boundaries. Pretty much same as last year.

Good Luck everyone.

I think for the X-ray question, when you said the fraction is 0.11 it should actually be 0.89? Because its the fraction absorbed, not reflected.

Crazily, I think I did not that well on G484, I think I got like 40 to 43, depending on the mark scheme. I thought all hope was lost for my B in physics.

But now this has come up, I think I've got about 85 in this paper, that might have just got me my b, or at least very close to it.

We'll. Have. To. Wait. And. See.

BRING ON RESULTS DAY.

Haha I'm quite the opposite, I like G484 and got around 52, this unit always messes me up because of the amount of writing.

But yep,
August 14th...here we come

Posted from TSR Mobile

Yea isnt the absorbed fraction 0.89?

Do you know what question 6 b i was, where his answer was "a Proton"?

decay of a neutron i believe, so what was the missing particle

No, that was 3b. There's a different one that a Proton was the answer to as well!

Was it not what i was trying to do in the edit above?
if not then i cant remember

OCR A2 Physics A G485 FP and FoP June 2014
Usual disclaimer. These are just my answers and opinions. They are in no sense official.
They may contain errors, omissions and typos.
Talking to my students as they came out, first impression is that it was a straight down the middle paper. Not too sure about that. There were a few topics that I went into in class in a lot of detail (Q5,Q7) which might prove difficult. Not many 'memory' questions and a lot of working out. We'll see. I thought G484 was OK but the general consensus around here was that it was hard.
Anyway here we go.

Q1 a) Seen this lots of times. Make sure your arrows go + to - and your lines meet the surfaces at 90 [2]
b) i) B is the bigger charge.
Resultant is zero nearer A. Fields must be equal. E is prop to Q/ r^2 so if E is same and r is bigger Q must be bigger (WTTE) [1]
ii) + on left (Force is to R) . Curves like 1/r^2. passes through 0 at X. Negative from X to B . (force is to left) curving downwards [3]
Tricky. Expecting not too many to score 3/3 on that.
c) E = k / r^2 g = k' / r^2 so ratio = k / k' and distance factor cancels out.
(Both inverse square laws so get weaker at same rate) [1]
TOTAL 7
Some easy bits; some tricky bits.

Q2 a) i) Standard: Force due to mag field is always perp to velocity. [1]
ii) because F is perp to v, no work is done / no acceleration in direction of travel so speed is constant (NOT velocity!) so KE is constant. [1]
b) Bqv = mv^2/r so mv = Bqr = 0.20E-2 x 3.2E-19 x 15E-2 = 9.6E-24 [3]
c) v = 9.6E-24/6.6E-27 = 1454 ms-1 so KE = 1/2 x 6.6E-27 x 145362 = 6.98E-21J [2]
d) Tricky.Does circular motion with centre of circle accelerating to right uniformly and circles get bigger as v increases.
( [2]
TOTAL 9
Same applies here. Some easy bits with a sting at the end.

Q3 a)i) Same proton number / same charge / same no of electrons / same nuclear density [1]
ii) All nuclei have a mass defect which is the mass of the separate particles - mass of the nucleus. It is due to the binding energy of the nucleus which is the mass/energy needed to separate the nucleus against the strong force into its separate particles. [3]
b) proton and anti - neutrino. [2]
Half lfe = time for half the neutrons in a sample to decay .
c) i) F = Q1Q2/ 4 pi eo r^2 = 1.6E19^2 / 4 x pi x 8.85E-12 x 1E-14^2 = 2.3N [2]
ii) 70 keV = 70E3 x 1.6E-19 J = 3/2 kT
so T = 2/3 x 70E3 x1.6E19/1.38E-23 = 5.4E8 K [3]
iii) average KE = 70 keV but some have much more than this
Quantum mechanical tunneling [1]
iv) Energy released = 18MeV = 18E6 x 1.6E-19J = mc^2
so m = 3.2E-29 kg [2]
v) D-He is harder to fuse than D-T because higher charge on he means more repulsion so need higher temps. [2]
TOTAL Pretty reasonable. Seen all this before.

Q4 a) Time constant = time for charge to drop to 1/e of initial value [1]
b) Any two values that multiply to give 5 eg 1uF and 5Mohm [1]
c)i) Area of wire = pi x (0.12E-3/2)^2 = 1.131E-8 m62
R = rho L/ A = 4.9E-7 x 5.0 / 1.131E-8 = 216.6ohm
tau = CR = 0.010 x 216.6 = 2.17s [3]
II) electrons move from X to + plate on cell due to potential difference
at same time electrons move from - plate of cell to Y
Effectively electrons move from X to Y
so X gets + charge and Y gets - charge
Charge is conserved so Qx = Qy. [3]
iii) Energy stored on capacitor = 1/2 Cv^2 = 1/2 x 0.010 x 12^2
= mcdT so dT = 3.41 C [4]
iv) if double PD then 4x energy stored so temp rise is 4x bigger [2]
TOTAL 14. Seems Ok to me.

Q5 a) Induced emf is prop/equal to the rate of change of magnetic flux linkage [1]
b) Energy for emf comes from work down in pushing magnet
so magnet must be repelled (lenz's law)
so must be N pole of face [1]
c) i ) No change in flux linkage so rate of change = 0 so no emf. [1]
ii) I do variations on this question every year in class.
from t=3 to t = 5 , flux is increasing at increasing rate so emf is negative and increasingly negative.
from t = 5 to t=8, flux is constant so emf = 0
from t = 8 to t=10, flux is decreasing at decreasing rate so, emf is + dropping to zero.[3]
Total 6.
Last bit is hard if you've not seen it before

Q6 a) i) proton is uud so antiproton is C [1]
ii) Zero. (its a pi meson) [1]
b) i) Proton (add up the top and bottom numbers) [1]
ii) lambda = ln2/ t = 0.693/5700x3.16E7 = 3.86E-12 s^-1
A = lamda N so N = 1.1E19/3.86E-12 = 2.85E30
no of moles = N/NA = 2.85E30/6.0E23 = 4.75E6 mol
mass = molar mass x n = 14E-3 x 4.75E6 = 6.66E5kg [3]
c) Nucleus absorbs a neutron; becomes unstable and splits into two smaller nuclei emitting 2 or3 more neutrons [2]

d) Moderator : slows down neutrons from high energies (MeV) to thermal energies (1/10eV) by repeated collisions between moderator nuclei and neutrons
slow neutrons have a better chance of being absorbed and causing fission.
Control rods : absorb neutrons and control rate of reaction [4]
TOTAL 12 Seems OK to me

Q7 a) i) Energy level = allowed value of energy that an isolated atom can have. [1]
ii) f = c/lambda so f =3.0E8/7.2E-11 = 4.167E18Hz
E=hf = 6.6E-34 x 4.167E18 = 2.76E-15J
This is difference between levels so energy of level = -3.2+2.76 = -0.44E-15J [3]
Last bit might catch a few out.
iii) If double V, then double energy so f doubles so lambda halves [2]
b) i) I=Ioexp(-ux) so I/Io = exp(-0.96 x 2.3) = 0.11 [3]
ii) Bone:fat = 2.8:0.9 Muscle:fat=0.96:0.90
Bigger ratio so bone:fat will have better contrast [1]
TOTAL 10: Again seems OK apart from last bit of aii

Q8 a) Array of microwave transmitters and receivers
send out pulses of ultrasound
which reflects off boundary between media
of different acoustic impedance ( = density x velocity)
Brightness tells us amplitude of return pulse
Position determined by time delay
etc [3]
b) F-18 attached to glucose molecule
Injected / ingested into patient
Concentrates at sites of high metabolic activity
F-18 emits beta+ which annihilate with electrons
emitting 2 gamma photons at 180
Circular array of gamma cameras detect gammas and pick up line
Position along line determined by difference in time of arrival
(sketch) [4]
TOTAL 7 Routine memory work

Q9 a) r = 10km/2 = 6.0km.
density = M/V = 2.0E20 / (4/3 pi x 6.0E3 ^3) = 2.2E18 kg m-3 [3]
b) g = GM / R^2 so ratio = (1/r^2)sirius / (1/r^2) sun
= 1.4E10 [2]
c) d = 8.6ly = 2.6 pc so angle = 1/d = 0.38" [2]
d) dlambda / lambda = v/c = 7600 / 3.oE8 x 100% = 0.0025% [2]
e) No - hubble's law only applies to distant GALAXIES moving away from us. [1]
TOTAL 10 Easy

Q10 a) Universe is ISOTROPIC and HOMOGENEOUS [2]
b) Microwave background is isotropic - same intensity in all directions
has a temp of 2.7K
has a black body continuous spectrum - hot dense material
was formed by gammas formed shortly after big bang being stretched to microwaves. [3]
c) rhoc = 8Ho^2/8 pi G so Ho = 2.33E-18s
so Age = 1/Ho = 4.295E17s = 13.6E9 y [3]
TOTAL 8 . Seems Ok to me.

Overall. Not bad at all. Similar to previous papers in many respects.
Good paper I think. Nice balance of easy / hard. Calculate / memory
and one or two new bits to stretch.

Grade boundaries. Pretty much same as last year.

Good Luck everyone.

Crazily, I think I did not that well on G484, I think I got like 40 to 43, depending on the mark scheme. I thought all hope was lost for my B in physics.

But now this has come up, I think I've got about 85 in this paper, that might have just got me my b, or at least very close to it.

We'll. Have. To. Wait. And. See.

BRING ON RESULTS DAY.

I'm in exactly the same position haha. Think I got a max of about 45 for g485, but for this I just went through and got around 88

I have a question, I put sort of what you said for the emf graph, but after the exam I realised isn't there e.m.f. induced even though magnetic flux density is constant? Because the area is changing as it moves across?

Also, for the Intensity x ray one, didn't it ask for the amount absorbed? which would be 1-0.11 = 0.89?

Yes, you're right about the 0.89 thing, and don't worry about the first part, the area isn't changing as it moves across because it is moving horizontally, not vertically.

By the way, do you know what question 6b was, where Teachercol said "a Proton" as the answer??