You are Here: Home >< Physics

# G485 Physics A 13/6/2013 My unoffical mark Scheme watch

1. OCR Physics A G485 FP and FoP 13 June 2013

Usual disclaimers. These are just my answers. They are not in any sense official and I havent seen the mark scheme.
They may contain errors and typos.
Other methods are also possible.

I DONT KNOW THE GRADE BOUNDARIES

First impression? I've covered all that lot in class - but there are some obscure bits of the syllabus.

Q1 Routine capacitor question to get you started.

a) 1/C = 1/C1 + 1/C2 =1/100 + 1/300 so S = 300/4 = 75uF
C = C1 + C2 = 75+500 = 575 uF (2)
b) i) Various ways to do this. Work out 6/e - read off t and use t=CR.
V=vo exp (-t/RC) ln(2) = 42/RC so R = 1.2E5 ohms (3)
ii) When t = 30s V=3.6V
Energy = 1/2 CV^2 so energy loss = (1/2 500E-6 x 6^2) - (1/2 500E-6 x 3.6^2)
(no you cant use 6-3.6)^2
= 5.76E-3J (3)
Total 8

Q2 a) Q= Ne so n = 2.8E-9 / 1.6E-19 = 1.75E10 (1)
b) F = Q1Q2 / 4 pi eo r^2 = 2.8E-9 x 2.8E-9 / 4 pi 8.85E-12 c (2.0E-2)^2 = 1.76E-4 N (2)
c) i) Weight of sphere / tension in string (1)
ii) Draw a vector triangle. (AS stuff)
Tan(theta) = F / weight = 1.76E-4/6.5E-5 x 9,81 = 0.276 so theta = 15.4 degrees (3)
Total 7
Nothing new here. We've seen this problem before.

Q3 a) Electric field is to LEFT. (Electron is negative charge so force is to right) (1)
b) i) 1500eV (Doh) (1)
ii) 1/2 mv^2 = qV so v=2.3E7 ms-1 (3)
c) Yay - a velocity selector! Been teaching this for years and its never come up - until now.
i) If undeflected FE (up) = FB (down)
Eq = Bqv so v = E/B (2)
ii) If make B bigger, Bqv increases so Fb is bigger than Fe and particle will deflect down. (2)
Total 9.
Should be Ok but part c will throw some - and if you missed electrons are negative, you'll lose a shed-load of marks.

Q4 a) Magnetic Flux = product of the magnetic flux density and the area perpendicular to the flux density. (1)
b) i) R = rho L/ A = 1.7E-8 x 130 / pi x (4.6E-4)^2 = 3.32 ohms
so I = V/R = 24/3/32 = 7.22A (3) (Bit of AS stuff there)
ii) emf = (-) d (N phi )/ dt
150 = (1100 x 0.090 x 1.3E-3) / t
so t = 8.58E-4s (3)
Total 7
Still on familiar ground - and there are a lot harder induced emf questions than this.

Q5 a) i) Need to know atoms are 1E-10 to 1E-9 so answers is 1E4 to 1E5 (1)
ii) 1) p = h /lambda = 6.63E-34 / 1E-14 = 6.63E-20 kgms-1 (2)
2) (Relativistic electron - as v approaches c) mass increases with speed (1)
b) i) Different number of neutrons / nucleons (1)
ii) uud
iii) p+ -> no + e+ + v
uud -> udd so U->d (2)
iv) charge / lepton no / baryon no conserved (NOT mass or energy but mass-energy shoudl be OK) (1)
v) B+ proton rich ie Z>N B- neutron rich ie N>Z (1)
Total 10
Seems Ok to me.

Q6 a) random = cant predict when will decay / cant affect when will decay (1)
b) 1.1/100 = exp ( - lambda t)
lambda = ln2 /7.1E8 = 9,76E-10 y-1
so t = ln(0.01) / 9,76E-10 = 4.6E9y (3)
c) i) estimate A for highest point -> A = 60 ish (Fe - 56) (1)
ii) Only produce energy when BE increases.
Fusion = joining nuclei together to make a larger nucleus
this would decrease BE for 100Mo so cant produce energy this way (1)
iii) I agree with the complaints- what is the second page?? Stupid instruction.
Mbe = 1.329E-26
Mass of separate particles = 4 x mp + 4 x mn
= 1.3392E-26 kg
so mass defect = 1.02E-28 kg
so BE = mc^2 = 9.18E-12J
so BE/nuc = 9.18E-12/8 = 1.15E-12J (4)
I know many will use 1u = 1.661E-27. This is a wrong method but should score some marks.
(You should know how to calculate mass defects)
Total 10
Bit trickier this one. It's starting to get harder.

Q7 a) Electomagnetic radiation / travel at speed of light / wavelengths 1E-9 to 1E-12 (2)
b) hf = mc^2(electron ) + mc^2(positron) f = c/ lambda
so lambda = hc/2mc\^2 = 1.2E-12 (3)
1/3 if you just remembered the value of the energy as 1.1MeV
c) Barium
X rays not absorbed very well by soft tissue (low Z) so doesnt show up on X ray image
Barium has high Z so good absorber and shows outine of stomach or bowel on pic after patient ingests barium 'meal' (3)
Total 8
Routine bit of physics. Been asked before.

Q8 a) Activity = no of particles emitted per unit time / no of nuclei decaying per unit time (1)
b) Energy rate = activity x energy of 1 photn
= 500E6 x 2.2E-14 = 1.1E-5 js-1 (2)
c) Gamma camera - bit obscure this for long question!
lead collimator - block of lead with thousands of holes which only allow gamma photons to enter in one direction
so improving picture
NaI crystal (scintillator) - emits a flash of light when gamma phton hits it
photomultiplier tubes - convert flash of light to electrical signal and amplify
Some cameras have more than 1 crystal andc an get 3D images. (5)
Total 8
I can see quite a few just skipping this question entirely.

Q9 a)Sound (longitudinal) waves that are above audible frequency (>25kHz) (typically a few Mhz used ) (2)
b) Emit: alternating PD is placed across crystal which makes it vibrate at this frequency and emit usound.
Receive: usound makes crystal vibrate at this freq which creates an alternating PD at this freq across crystal. (2)
c) Impedance matching. - gel has same acoustic impedance as skin ( z = rho x c)
Otherwise almost 100% of usound would reflect at air / skin interface
Ir = (z1-z2)^2 / (z1+z2)^2 so if z1 = z2 no reflection
(Air has a much lower acoustic impedance than skin) (2)
d) When a wave reflects off a moving object , its frequency changes ( Doppler effect)
if object is moving toward then f increase
if object is moving away then f decreases
dF is prop to velocity
so if measure change of f , can calcuate velocity of blood flow. (4)
Total 10
Been expecting usound - but all describe and no calculations

Q10 a) d = vel x time = 3E8 x 3.16E7 = 9.5E15 m (1)
b) (on diagram) 1AU = dist from sun to earth
1pc = distance from sun/earth to star
1 second = angle between earth to star to sun (1)
c) i) d = 2.1E7 ly = 2.1E7 x 9.5E15 m = 2.1E7 x 9.5E15/3.1E16 = 6.44E6 pc (2)
ii) E = P t so t = 1E44/4E26 = 2.5E17s = 7.9E9 y (2)
d) Black hole - has mass / prob has singularity(?) at centre / prob(?) has infinite(?) density at centre /
has escape velocity > speed of light so nothing can escape.
- can have charge / has a Hawking temperature ... (2)
Total 8
Messy

Q11 a) i) usually we do this the other way round which is easier
t = 1/Ho = 2.31E-18s-1
= 2.31E18 x 1E6 X 3.09E16 / 1E3 Mpc/kms-1 = 71.4 (3)
ii) d(lambda) / lambda = v/c v = Hoxd

so d(lambda) = 656 x 71.4 x 50 x 1E3 / 3E8 = 7.8nm
Distant galaxies are red-shifted so lambda = 656 + 7.8 = 663.9 nm (4)
b) Big bang ; universe starts from a singularity ie point of infinite density and temperature.
Universe expands and cools - quark soup / nuclei / fusion / atoms
When atoms form universe becomes transparent and gamma photons travel freely through whole universe
CMB is this radiation stretched as universe expands and cools
it is ISOTROPIC (same in all directions) and HOMOGENEOUS ( same throughout universe)
Has a black body spectrum of temp 2.7K ; max intensity at 1.1cm (5)
c) as universe expands and cools , it will stretch to longer wavelength / lower temp
d) open universe means keeps on expanding forever so line must continue upwards
accelerating means gradient should increase. (1)
Total 15
Not nice last question.

Overall/ My impression is there weren't too many easy marks on offer for simple definitions and sub into simple equation.
Second half was a lot harder than first half.
Gonna have to work for these marks.

Ignore Jan boundaries - they aren't typical.

I'd expect a touch lower than last June.

Good Luck.
2. Really appreciate this, thanks! Would I be penalised for every significant figure error I made? I read somewhere that they only take away a single mark throughout the paper?
3. Hi, just asking but will i get any marks for a 3 mark question if i put the wrong answer, but i actually have the correct answer and equation and things written down on the paper?
It was just i thought 12,000 Ohms was too large for the answer to 1b, so i put the wrong answer, but i did actually work it out correctly...
4. Thank you so much!

Got between 82-75 (depending on how harsh I mark it), I need around 115 UMS for an A so hopefully the A grade is about 76>.
5. (Original post by teachercol)
x
Thank you!

Few queries though,

Why wouldn't they accept 1.02Mev for pair production, it arrives at the same answer.

Random is the fact that we can't predict when an individual nucleus will decay.

For the improvement, I said you could have thinner, longer tubes so it absorbs more gamma rays and they different ones won't appear over eachother.
Reference that one gamma photon produces many light photons

Didn't it ask for 2 properties of Ultrasound waves?

Are you sure the Quark soup/fusion etc will gain marks? It didn't really ask for that

Also, for the last one, I didn't know whether to draw an open universe like we have in the book or one where the expansion is accelerating. I went for open.
6. (Original post by KD35)
Thank you!

Also, for the last one, I didn't know whether to draw an open universe like we have in the book or one where the expansion is accelerating. I went for open.
I drew an open line as well. But looking at wikipedia and google it seems we were wrong
990350b.jpg
7. (Original post by Ryan Gregg)
I drew an open line as well. But looking at wikipedia and google it seems we were wrong

[IMG]990350b.jpg[/IMAGE]
Looks like the book trolled us...It wasn't clear in their wording whether they actually wanted a universe where the expansion was accelerating..
8. Counted 50 or so marks, but i can't really remember any of my answers, so it's more of a guess XD
Just hope i've got the UMS for a C overall and then i can say goodbye to physics forever...if not, looks like i'm resitting :/
9. (Original post by KD35)
Looks like the book trolled us...It wasn't clear in their wording whether they actually wanted a universe where the expansion was accelerating..
Yeh It never even crossed my mind, it was never mentioned in the book (I think) so I just assumed it was their OCR physics jargon wording for "draw the graph of an open universe". Oh well, it was only 1 mark
10. (Original post by KD35)
Thank you!

Few queries though,

Why wouldn't they accept 1.02Mev for pair production, it arrives at the same answer.

Random is the fact that we can't predict when an individual nucleus will decay.

For the improvement, I said you could have thinner, longer tubes so it absorbs more gamma rays and they different ones won't appear over eachother.
Reference that one gamma photon produces many light photons

Didn't it ask for 2 properties of Ultrasound waves?

Are you sure the Quark soup/fusion etc will gain marks? It didn't really ask for that

Also, for the last one, I didn't know whether to draw an open universe like we have in the book or one where the expansion is accelerating. I went for open.
You'll have to use the masses to get 1.02mev rather than just quote it. They never allow you to 'remember' an answer.

Random = cant predict is fine

Gamma camera just asked ' how good quality image' achieved.
More detail on collimator is fine.

Yes - it asked for two properties of ultrasound - not sure what they expect here. There isn't much more to say.

Quark soup stuff was just there to explain where CMB originates. Just saying early gammas would be enough. I'm not sure if they expect any detail about transparency but it doesn't hurt to include it.

Nasty last question.. It just says 'sketch a suitable graph to illustrate an open universe. As long as your graph keeps going up it should be Ok however it curves. It was something that I discussed as a 4th possibility with my classes. - that the graph might curve the other wave due to 'dark energy' but it isn't in any of the books.
11. (Original post by antonio23)
Really appreciate this, thanks! Would I be penalised for every significant figure error I made? I read somewhere that they only take away a single mark throughout the paper?
That's true.
And only for using 1sf.

They will penalize you if your answers aren't correct though to the number of figures you gave.
12. (Original post by turnip storm)
Hi, just asking but will i get any marks for a 3 mark question if i put the wrong answer, but i actually have the correct answer and equation and things written down on the paper?
It was just i thought 12,000 Ohms was too large for the answer to 1b, so i put the wrong answer, but i did actually work it out correctly...
Probably get 2/3. If you write 2 different answers - its a contradiction and loses a mark.
13. (Original post by teachercol)
You'll have to use the masses to get 1.02mev rather than just quote it. They never allow you to 'remember' an answer.

Random = cant predict is fine

Gamma camera just asked ' how good quality image' achieved.
More detail on collimator is fine.

Yes - it asked for two properties of ultrasound - not sure what they expect here. There isn't much more to say.

Quark soup stuff was just there to explain where CMB originates. Just saying early gammas would be enough. I'm not sure if they expect any detail about transparency but it doesn't hurt to include it.

Nasty last question.. It just says 'sketch a suitable graph to illustrate an open universe. As long as your graph keeps going up it should be Ok however it curves. It was something that I discussed as a 4th possibility with my classes. - that the graph might curve the other wave due to 'dark energy' but it isn't in any of the books.

For Ultrasound, I included that it's a transverse wave that can only travel through a medium.

I'm going to keep my hopes up about the 1.02Mev one, because there was a fair amount of working after it too that seems like it could equate to 3. It seems harsh to penalise for it especially as they've included the value in most of the books, and they didn't state or imply any correct way to go about it.
14. nice one. 95 hopefully.
15. (Original post by KD35)
For Ultrasound, I included that it's a transverse wave that can only travel through a medium.

I'm going to keep my hopes up about the 1.02Mev one, because there was a fair amount of working after it too that seems like it could equate to 3. It seems harsh to penalise for it especially as they've included the value in most of the books, and they didn't state or imply any correct way to go about it.
er - its a longitudinal wave ....
16. (Original post by fkh_95)
nice one. 95 hopefully.
Well done! Think I got about the same
What do you think the 150/150 UMS boundary will be?
17. (Original post by teachercol)
er - its a longitudinal wave ....
Sorry, thats what I meant, I promise! :P come to think of it I probably could have made that mistake in the exam
18. teachercol what if i write for the 2 forces on the ions attached to spring gravitational force and strong force? and for the random part that the nuclei in the sample have each got the same probability of decaying per unit time?
19. (Original post by JG1027)
teachercol what if i write for the 2 forces on the ions attached to spring gravitational force and strong force? and for the random part that the nuclei in the sample have each got the same probability of decaying per unit time?
Strong force only acts at short distances within atomic nuclei so I doubt that will be accepted. And in the book for random it mentions:

Unable to predict when an individual nuclei will decay,
Each nucleus has the same chance of decaying per unit time.

So you'd definitely get that mark.
20. (Original post by Ryan Gregg)
Strong force only acts at short distances within atomic nuclei so I doubt that will be accepted. And in the book for random it mentions:

Unable to predict when an individual nuclei will decay,
Each nucleus has the same chance of decaying per unit time.

So you'd definitely get that mark.
thanks but would i then get the mark for gravitaional force because thats the same as the weight of the ball

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: February 27, 2014
Today on TSR

### Cambridge interviews

Find out which colleges are sending invitations

### University open days

• University of East Anglia
Fri, 23 Nov '18
• Norwich University of the Arts
Fri, 23 Nov '18
• Edge Hill University
Sat, 24 Nov '18
Poll
Useful resources

Can you help? Study help unanswered threadsStudy Help rules and posting guidelinesLaTex guide for writing equations on TSR

## Groups associated with this forum:

View associated groups

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE