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OCR Physics A Breadth of Physics (24th May 2016) Discussion

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1. (Original post by TheHobbit)
I thought it was 0.8/0.2 = 4 because it was the ratio between the smaller diameter/the larger one and a thinner wire will extend more so will have greater epe (so it must be greater than 1)
i got the same thing 4
2. https://docs.google.com/document/d/1...wjPsvdb0/edit#

unofficial markscheme link, pls add any answers you know you definitely got right
3. I said 16 for the wire one because the area would be squared to work out stress, and strain is a constant for the material so young mod is propotional to 1/a so 1/0.1^2 divided by 1/0.4^ = 16
4. (Original post by viktorh)
I used the weight of the cylinder/9.81=mass of the cyclinder

then I took the difference between the two newton meter readings = the upthrust on the cylider = Apgh

deduced h from that. Used H and A to find the volume

Mass/Volume = density.

Anyone else do it like this? answer I got was around 750
I did this

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5. (Original post by voltz)
I think I got the LED one completely wrong, I said the threshold voltage was too low and connect it in parallel? I was just guessing at that point
the LED was directed against conventional current flow.
6. (Original post by julesaquilina)
i think i put 6.5
Same, just not sure about the one with wave speed, I got 60+-0.3, I think the uncertainty is probably wrong.
7. (Original post by NthnBrwn)
the LED was directed against conventional current flow.
I wrote that but crossed it out:/
8. (Original post by suhaylpatel786)
I think that didnt work because you didn't take into fact the possible resistance of the extra branch and so the actual Vout would be different. But yh i got 6 too
That's it, thanks!

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9. (Original post by NthnBrwn)
the LED was directed against conventional current flow.
I got that too

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10. (Original post by Parhomus)
Same, just not sure about the one with wave speed, I got 60+-0.3, I think the uncertainty is probably wrong.
I got 60 +- 6 because I did 5% + 5% because you double the 4 to get one wavelength
11. (Original post by voltz)
For the pressure question with the cylinder was I write to say you can use Newton's third law and so find the cross sectional area and weight of the cyclider and sub that into the pressure equation (force/cross sectional area)?
I just wrote measure mass on scale. W=mg , A =pid^2/4 then P=W/A. To make it more precise I said repeat the measurement and average it.
12. Was the answer to the question for the LED (2nd last one) that it was the wrong way round?
13. (Original post by voltz)
I got 60 +- 6 because I did 5% + 5% because you double the 4 to get one wavelength
Yeah I got the 5% uncertainty so when I got the answer I did 60x5/100=0.3
14. (Original post by rafiuddin)
didn't the equation they give have thinner wire on top?
Yeah, and the thinner wire would extend more under the same force and so have greater EPE than the thicker one, so the ratio had to be greater than 1
15. Answers:

Denisty was 7500kgm^-3, not 750kgm^-3 as some peolle have suggested- how can a metal cylinder be less dense than water, considering it sank in the water?

Drift velocity was something like 1.7 x 10^-3. Lots of you probably forgot to find the number density (number per m^3) and just used the number of electrons they gave you (incorrect)

Momentum- 70 or 80ish for the velocity of the carbon atom.

Rest of the questions were pretty standard.
16. (Original post by Parhomus)
Yeah I got the 5% uncertainty so when I got the answer I did 60x5/100=0.3
But that actually =3 fk my calculator.
17. (Original post by Fish40)
Gradient i got was 400/25 = 16 as m^2s^-2 divided by m = ms^-2

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Yeah the gradient was 16 but that was equal to 2a as u^2/x = 2a, so a = 8 (I think)
18. (Original post by isaac_wood_)
Yeah the gradient was 16 but that was equal to 2a as u^2/x = 2a, so a = 8 (I think)
I subbed the values from the graph into the equation and got a to be 7.6...?
19. (Original post by Parhomus)
Same, just not sure about the one with wave speed, I got 60+-0.3, I think the uncertainty is probably wrong.
Nah that's right dw!

For the voltage, I didn't even spot the LED was the wrong way around...
20. there was a question about two object collide and they gave us a graph ask us to explain the force exert something how did you guys explain

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