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

1. (Original post by yellowfan1555)
Hi,

For the question asking for the time constant: It said it took 45 seconds for 80% of the voltage to decay. So, I did 45/80, for the time for 1 % to decay. I then said the time constant was the time for it to decay to ~ 37% ((1/e)*100) and so I did (1-1/e)*100 which roughly gave 63% as expected. I did this 63% timesd by the value of 45/80 to give 63% lost- and I got like 35.6... which is wrong. Why is this wrong?
It doesn't decay at a constant rate. You can't just do 45/80.
2. (Original post by tophatmenis)
What did people but as units for epsilon naught x Gravitational constant?
C?
3. (Original post by Mango Milkshake)
It doesn't decay at a constant rate. You can't just do 45/80.
Damn. I was initially doing it correctly too, but I decided to switch methods. How many marks was that? 3?
4. (Original post by yellowfan1555)
Damn. I was initially doing it correctly too, but I decided to switch methods. How many marks was that? 3?
yes it was 3 marks, don't worry about we've all messed up somewhere on this paper

for that question for future reference so you know, we had no values of what v inital was and no graph. we know that the voltage fell by 80% in 45s though. so as its exponential, we can simply say that the inital voltage was 1v and it fell by 80% to 0.2v

then its just a bit of rearranging into: 45s/ln(0.2/1) = around 20-25s something in that area is what i got
5. I don't think anyone does, but when someone gets a hold of the multiple choice, could they post it so we can get an unofficial markscheme of it? as i'd really like to know what i averaglely got on it to reassure me for section B, I was aiming for 18ish but some got me stuck between 2
6. (Original post by Yo12345)
What did people get for the gravitational potential at the centre of the 2 masses?

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-4GM/d?

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7. (Original post by Yo12345)
-4GM/d?

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same, g.f.s would be 0 but potential is what you said, thank god i got that
8. (Original post by Nathan2580)
same, g.f.s would be 0 but potential is what you said, thank god i got that
I hope it is. Is it definitely correct?

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9. Vote if you haven't yet voted- http://www.thestudentroom.co.uk/show....php?t=4176850
10. (Original post by Yo12345)
I hope it is. Is it definitely correct?

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its the same question as asked in our paper but a few years back, different answers to choose but the core question was to find the potential at the mid point between 2 masses, and the answer is -4pi bla bla bla, we got it right
11. I think it will be about 51 or 52 for an A. It's much harder to tell for A* and full ums though.
12. Answers to the MC here: http://www.thestudentroom.co.uk/show...4#post65961084
13. (Original post by C0balt)
NatSci, you? I applied to imp too
Maths
14. (Original post by Nathan2580)
yes it was 3 marks, don't worry about we've all messed up somewhere on this paper

for that question for future reference so you know, we had no values of what v inital was and no graph. we know that the voltage fell by 80% in 45s though. so as its exponential, we can simply say that the inital voltage was 1v and it fell by 80% to 0.2v

then its just a bit of rearranging into: 45s/ln(0.2/1) = around 20-25s something in that area is what i got
Thanks man Lol yeah i'll need to know that next year when i'm resitting haha
15. (Original post by notanooblol)
Maths
Cool, I could never do a maths degree haha
16. Would 59-62 be an A*?

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17. (Original post by Yo12345)
Would 59-62 be an A*?

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A* boundary should be 57/58 so yes
18. Do you have to get a certain number of questions right on the multiple choice to get a good grade overall?
19. I can't help but think that this is gonna have 2014 level high gb lol
20. (Original post by ella97)
Do you have to get a certain number of questions right on the multiple choice to get a good grade overall?
Nope, its total mark that counts

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