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

https://docs.google.com/document/d/18mLVuo38imOSCLm3O-Wl6nR8m201CIBH0e7wjPsvdb0/edit#

unofficial markscheme link, pls add any answers you know you definitely got right

Reply 341

sanchit117

2

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

Reply 342

Gilo98

14

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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Reply 343

NthnBrwn

1

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.

Reply 344

Parhomus

8

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.

Reply 345

voltz

6

Original post by NthnBrwn

the LED was directed against conventional current flow.

I wrote that but crossed it out:/

Reply 346

Oooskar

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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Reply 347

xsrajabx

1

Original post by NthnBrwn

the LED was directed against conventional current flow.

I got that too

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Reply 348

voltz

6

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

Reply 349

Parhomus

8

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.

Reply 350

spaceman11

12

Was the answer to the question for the LED (2nd last one) that it was the wrong way round?

Reply 351

Parhomus

8

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

Reply 352

TheHobbit

1

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

Reply 353

richpanda

16

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.

Reply 354

Parhomus

8

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.

Reply 355

isaac_wood_

5

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)

Reply 356

spaceman11

12

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...?

Reply 357

sanchit117

2

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...

Reply 358

chenhao96

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

Reply 359

Parhomus

8

Original post by richpanda

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.

I remember getting that I couldn't remember if it was 10^-4 or 3 :P for the drift velocity. Considering electrons travel slowly it made sense.

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

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