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# AQA Physics PHYA4 - 20th June 2016 [Exam Discussion Thread] watch

1. (Original post by johnapplebottom)
ah **** yeah your right i see, i put the negative in brackets will i lose the marks?
Most likely because the minus shouldn't be there
2. (Original post by DesignPredator)
Here are my answers to Written, the question parts might not be correct.

1ai) 8.26 (will be a range)
ii) 2.6x10^-4

bi)28
ii)5.9x10^4
iii) bottom box ticked

2ai) Derivation
ii) 6.06x10^6 (3SF only)

b) Downward Curve staring at 9.81
Minimum at g=0 at a point slightly offset from halfway closer to Venus
Rises up to 8.87

3a) V=mv/N

b) Derivation

ci) 216
ii) 1.15x10^-19 kgms-1 OR Ns
iii) Anti electron neutrino released also so Beta minus particles have range of Kinetic Energies upon release.

4a) Small amplitude oscillation as only valid for small angular amplitudes.

b) -Set up light string with Bob on the end,
attach top of string to clamp stand boss.
-Measure length from top to centre of Bob.
-Release the Bob so it oscillates with small amplitude.
-Use stop clock to measure time taken for 20 oscillations.
-Divide this by 20 for mean T
-Plot T^2 against L
-Repeat procedure

c) -Student value 4x true value
-Time period half so all values for T^2 will be 1/4 the true
-State equation or show that g is inversely proportional to gradient.

5a) Lenz's Law: Direction of induced EMF/current is always in a direction that opposes the change that caused it.
Faraday's Law: Induced EMF is proportional to rate of change of flux linkage.

b) -Change in flux linkage when current flows.
-EMF induced in wheel (or whatever it was called)
-Current induced in the wheel as good conductor.
-Current carrying conductor (wheel) opposes the field due to coil, therefore there is a force against the direction of motion.

c) Wheel not in contact with electromagnet so no wear, but in brake pads there is wear due to friction.

More energy used or less effective or heating.
Do you remember the marks for each?
3. (Original post by Jonsmith98)
I think both answers for increased power drain and electrical failure will be valid. Although my view is that it would be unreasonable to use a powered braking system, because if you park your car on a hill your battery will eventually go flat and then your car will have f***Ed off down the hill by the time you come back with your shopping.
Not directly related to the question on the paper but doesn't the magnetic braking system only work when the discs are spinning and so wouldn't be used if parking on a hill. I think there would still have to be a mechanical parking brake.
4. (Original post by duncant)
what did people say about the B- emission any why the equations wouldnt work? I said that B emission is an electron which isnt a nucleon, which is what the equations represent?
Because beta negative emission is a three body process, as an (anti)neutrino is produced. The (anti)neutrino will take some momentum away.

I was a bit stupid and put just neutrino, will I lose the mark?
5. (Original post by duncant)
what did people say about the B- emission any why the equations wouldnt work? I said that B emission is an electron which isnt a nucleon, which is what the equations represent?
I put mass number doesnt change as the electron has negligible mass. I also put that not all energy is KE but that was a side note not sure about it
6. (Original post by johnapplebottom)
no wat, 220 was the nuclean number before it decayed by alpha particle thing
so after it was 220-4 = 216
Oh yeah, f***k

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7. What will 45 raw marks get you?
8. How many marks was the derivation for Venus and then working it's R out
I said (T/(2*pi))^2 on x axis, l on the y axis - which is right, as gradient is g.

You would possibly lose 1 mark for not saying 'evaluate (T/(2*pi))^2.
I have no idea why I did it but I said plot 'lengths against time' and the gradient would be g. Would I get any marks for the graphical method bit since I mentioned the gradient would be g?
10. (Original post by james2mid)
Not directly related to the question on the paper but doesn't the magnetic braking system only work when the discs are spinning and so wouldn't be used if parking on a hill. I think there would still have to be a mechanical parking brake.
Yeah it only works when the disks are spinning, otherwise there is no rate of change of flux linkage through the disks.
11. (Original post by particlestudent)
How many marks was the derivation for Venus and then working it's R out
4 marks
12. (Original post by kingaaran)
Was the direction from X to Y or Y to X
I said 4F and X to Y
13. i think an A* would be 56 marks or 57
14. (Original post by johnapplebottom)
for RC i got 202 what did everyone else get
and for R i got some big number 10^5 i think

someone confirm if this is right pls i used v=vo e^-t/rc
where v = 0.8vo

45/(-ln(0.8) = RC

I did the exact same thing unfortunately - noticed my mistake coming out.

You get that time constant if you took Q/Q0 to be 0.8 rather than 0.2...

Sorry - hope we didn't lose many marks for it and get ECF for the resistance
15. Can I write that the electron mass is very small for the beta decay question
16. (Original post by Jordenwilder1998)
because g iversly proportional to t and t is 4 ti,mes smaler
No, T^2 is 4 times smaller
17. (Original post by Euclidean)

Can anyone confirm with me?

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Lmao my first few I saved I got, I feel like there is some I missed out

(I added in X to try and line them up but wasn't very effective.

hmm they don't match up very well haha
After the 6 Marker there was a 3 Mark Question on about Time Period being miscalculated, 3 Marks on Electromagnetic Laws (maybe 4 pretty sure it was 3 though). A 3 marker on Lenz's Law and then finally a 2 Marker on Disadvantages of Electromagnetic Brakes so you have missed:

The electromargnetic laws part was definitely a 3-marker. I lost a least a mark for saying .... rate of change of magnetic flux density.
As soon as I came out of the exam, I was like bruh.
19. (Original post by particlestudent)
Do you remember the marks for each?
Q1 2+3+3+2+1
Q2 2+3+3
Q3 2+4+1+4+1
Q4 1+6+3
Q5 3+3+2

Missing 1 mark can't remember where it was.
20. (Original post by particlestudent)
How many marks was the derivation for Venus and then working it's R out
2 and 3 I think

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