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OCR PHYSICS B G494~ 11th June 2015 AM ~ A2 Physics Watch

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    (Original post by Mars256)
    I said that another cable could be attached at the bottom to it swinging. What did everyone put for it being unpractical to damp the lift?
    I said that the lift needs to be able to 'oscillate' as that's it's function - to move up and down. Damping it would make energy transfers really inefficient so a lot of energy would be required to move it up and down.

    Could be utterly wrong though.
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    (Original post by james-aitken)
    For the number of collisions per second question, did you have to estimate a value for the collision time to work out the force per particle?
    Pressure is Force/area so Force = pressure x are. Force is rate of change of momentum (mv/t). Rearrange to work out delta T is you know mass and velocity.

    Then frequency is 1/T
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    I wrote adding a damper would require a huge mass that would make the building too top-heavy.
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    (Original post by Rhetorical Hips)
    Can anyone remember the value they got for epsilon and the two capcitance questions? Epsilon was a tiny number, 10^-20 I think, while the first capacitance value was something like 10-2 Farads and the second was like 20 to 40 microfarads?
    Got like 3.96x10^-20 for epsilon. Anyone else get 0.94uA for current leakage?
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    For the rate of collision one, I rearranged to find F, then found F exerted per particle by using change in momentum over time equals force, assuming that t=1 second. Then divided F by F exerted per particle. Bit of a messy method I know.
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    (Original post by Minecraft27)
    I wrote adding a damper would require a huge mass that would make the building too top-heavy.
    I put that it would require the cable to be buffered or something along those lines and then I couldn't coil around motor?
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    Anyone doing an unofficial markscheme?
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    (Original post by Lewis7253)
    Got like 3.96x10^-20 for epsilon. Anyone else get 0.94uA for current leakage?
    How did you work out current leakage? I rearranged C=Q/V to get inital current and final current, then divide change in current over time (60 seconds). I think it came out in the order of 10^-5 Farads and thus 10^1 microfarads.
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    Why couldn't we have attached the lift to something at the bottom?


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    (Original post by Lewis7253)
    Did anyone else do the actuvation energy one. Was tricky. Required natural logs and complex rearring. Think i got something like 3.96x10^-20J
    I got that! Joke that it was a 2 marker - I saw an almost identical question in a past paper that was a 4 marker!
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    (Original post by Rhetorical Hips)
    How did you work out current leakage? I rearranged C=Q/V to get inital current and final current, then divide change in current over time (60 seconds). I think it came out in the order of 10^-5 Farads and thus 10^1 microfarads.
    Badsically the same way. Did V/t to get volts per second then used Q=CV to get charge per second which is current was like 9.4x10^-7A of 0.94uA
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    (Original post by Rhetorical Hips)
    How did you work out current leakage? I rearranged C=Q/V to get inital current and final current, then divide change in current over time (60 seconds). I think it came out in the order of 10^-5 Farads and thus 10^1 microfarads.
    I got 0.94


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    (Original post by Lewis7253)
    Got like 3.96x10^-20 for epsilon. Anyone else get 0.94uA for current leakage?
    I got both of those!

    Dropped 4 or 5 marks by running out of time expecting 41-43 to be an A
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    (Original post by urz13)
    I got that! Joke that it was a 2 marker - I saw an almost identical question in a past paper that was a 4 marker!
    Yeah I was worried I was on the wrong track when I notice it was only 2 marks for all that work!
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    (Original post by Rhetorical Hips)
    How did you work out current leakage? I rearranged C=Q/V to get inital current and final current, then divide change in current over time (60 seconds). I think it came out in the order of 10^-5 Farads and thus 10^1 microfarads.
    It was using dQ/dt. Find Q when V=0.12 and then dt=60s giving an answer of 0.94uA, I believe
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    (Original post by Lewis7253)
    Badsically the same way. Did V/t to get volts per second then used Q=CV to get charge per second which is current was like 9.4x10^-7A of 0.94uA
    Darn I must've got that one wrong then
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    what the actual **** was that section B, I got all the higher mark questions fine, but ironically left like four 2 markers.

    What was that work out the mass of the Earth from R and density question all about?
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    Thoughts on A* boundary? I'd estimate about 47-49. Depends how many finished the paper as I found it very time consuming, many people in my school missed the entire last question due to running out of time.
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    (Original post by ZestyX0r)
    what the actual **** was that section B, I got all the higher mark questions fine, but ironically left like four 2 markers.

    What was that work out the mass of the Earth from R and density question all about?
    Density=Mass/Volume therefore Mass=Density x Volume, you were given a rough estimate of density and volume=4/3 x pi x radius cubed

    EDIT on the topic of that question, what reason did you guys give for G being to large?
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    (Original post by urz13)
    Thoughts on A* boundary? I'd estimate about 47-49. Depends how many finished the paper as I found it very time consuming, many people in my school missed the entire last question due to running out of time.
    Really?
    I didn't find that


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