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

1. (Original post by Seclusion)
You know e is proportional to v^2, so if you quarter energy, you must half the pd, so the original pd must be 4, as that's the only value that works in the situation. Just sub that into E=1/2CV^2.
ah, clever
2. Can someone help me with June 2015 section A question 8? I can't get 4R no matter what I try and since its one mark I must be missing something majorly.
3. (Original post by mangoli)
Can someone help me with June 2015 section A question 8? I can't get 4R no matter what I try and since its one mark I must be missing something majorly.
You know the original value is 2R when they are touching, as it's measured from center to center. The relationship between GF and r is an inverse square, so a decrease by 1/9, means the radius increased by 3, so you multiply 2R by 3, to get 6R. 6R - 2R gives you the separation.
4. Anyone know statistically which is most common in the multiple choice A,B,C or D, in case of having to guess?
5. (Original post by mangoli)
Can someone help me with June 2015 section A question 8? I can't get 4R no matter what I try and since its one mark I must be missing something majorly.
Using F = k/r2

F = k/(2R)2- as 2R is the distance between the centres of each sphere in contact
(1/9)F = k/(x.2R)2

as the force has been reduced by a factor of 9, the separation between the centres must have been increased by a factor of 3, so x = 3

the original separation distance is 2R, the new separation distance between the centres is 3 x 2R = 6R

so the separation distance between the surfaces is 6R - (R+R) = 4R
6. Im doing the January 2013 paper section B and the first question has a part about alpha particles and decay. Is this because it is a different spec or is the knowledge assumed from unit 1 and 2?
7. (Original post by ReeceFraser)
Im doing the January 2013 paper section B and the first question has a part about alpha particles and decay. Is this because it is a different spec or is the knowledge assumed from unit 1 and 2?
its the same spec, just assumed knowledge - not sure if its meant to be assumed from unit 1 or unit 5 - both have alpha decay
8. (Original post by Questioner1234)
Anyone know statistically which is most common in the multiple choice A,B,C or D, in case of having to guess?
Choice E.
9. (Original post by alevelstresss)
its the same spec, just assumed knowledge - not sure if its meant to be assumed from unit 1 or unit 5 - both have alpha decay
Cheers man, it just took me by surprise and i thought i may have printed a unit 5 paper by mistake
10. Anyone?
11. anyone got a quick method to work out questions involving radius and density? like the one above, always struggle on these
12. (Original post by physicsunit4lol)
Hey everyone!

I'm feeling averagely confident on unit 4 for monday (relying on a good multiple choice to boost my grade chances up) I just could really use some help on some minor things that I can't get my head around and if anyone could explain them it would be much appreciated

1) How does a transformer work? (I know this was a six marker/ and asked similarly with the ring going down the pole) but electromagnetic induction is my weakest area so an explanation about these two things such as the steps involved would be an amazing amount of help!

2) Can anyone explain the effects/how we know what polarity of the coil is north/south? for example when you put a magnet in north end first into a coil, how do we know whether the flux linkage increases/decreases? I struggle with determining the polarity of the magnet with the coil story.

Basically, if I had to rank my priority it would be how a transformer works, but aside from that anything anyone has to further help explain electromagnetic induction would be amazing as thats my weakest topic which I don't feel very good on. Aside from that

I wish everyone on here the best of luck for Monday morning and I'm sure we'll nail it (as long as AQA aren't going to mess up this paper)

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13. How do we know whether to use Fleming's left or right hand rule in a question?

Right hand rule is for generators isn't it?
14. (Original post by micycle)
Anyone?
g=(GM)/r^2

M= 4/3(pi*r^3*p) (p=density)

therefore g = (G*4/3*pi*r^3*p) all over r^2

cancelling r's gives g= G*4/3*pi*r*p

Planet P --> Q the radius doubles but the density halves. So g stays the same at 13.4

15. (Original post by micycle)
Anyone?

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16. is a damping force the same as a restoring force?
17. (Original post by WillRose)
anyone got a quick method to work out questions involving radius and density? like the one above, always struggle on these
No, I think they just take ages. Cri

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18. (Original post by micycle)
Anyone?
M = (4/3)πr3ρ

the density of Q is half the density of P

so using Using g = GM/r2

the ratio will be (r3ρ)/r2

13.4 x [23 x(1/2)] / (2)2 = 13.4

so its 13.4 N kg-1
19. (Original post by Music With Rocks)
How do we know whether to use Fleming's left or right hand rule in a question?

Right hand rule is for generators isn't it?
left hand rule is for fields and stuff, right hand rule is just for generators
20. (Original post by WillRose)
Q7 no you just divide the 1.42 time by root 0.37

Q18 work out the resistance using v=ir
then use the v=v(0)e^-t/rc
with 4.5 as v and 5 as v(0) because o the 10% thing, then rearrange for t

thanks will!, but for q18, i got 1053 as the answer and not 1000 hmm :\

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