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AQA A Physics Unit 4 24th Jan 2012

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    (Original post by maths134)
    If you guys want to spend a day working it out, you welcome to...
    Oh come on, its about helping each other out. I've helped people on this thread with questions for example and on the ocr chemistry F324 thread I spent ages doing the same (collating questions) for everyone else to use as well-check if you don't believe me. But you spent the time doing it, so it's your choice. Good luck either way
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    Hi all, I hope someone can help me out... The question is regarding capacitance.

    A capacitor is discharges through a fixed resistor, how long does it take the pd across the capacitor to fall from V_0 from V? (Assuming all values are known, aside the time)

    I thought you would use the equation below, but I can't rearrange for t :'( (Never did maths)

    V=V_0 e^(-t⁄RC)

    Thank you in advance
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    (Original post by schenker)
    Hi all, I hope someone can help me out... The question is regarding capacitance.

    A capacitor is discharges through a fixed resistor, how long does it take the pd across the capacitor to fall from V_0 from V? (Assuming all values are known, aside the time)

    I thought you would use the equation below, but I can't rearrange for t :'( (Never did maths)

    V=V_0 e^(-t⁄RC)

    Thank you in advance
    Rearranging V=V_0 e^(-t⁄RC) for t
    is simple if you do A-level maths, but since you don't here it is

    t=RC In(V_0/V), In is the inverse of the exponential function.
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    (Original post by albus)
    Rearranging V=V_0 e^(-t⁄RC) for t
    is simple if you do A-level maths, but since you don't here it is

    t=RC In(V_0/V), In is the inverse of the exponential function.
    Gah, damn maths I was leaving the e in :P

    Thank you!
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    jus done the june 2011 paper. i think i notices a couple of errors. anyone whos done it notice any ?
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    (Original post by kimmey)
    jus done the june 2011 paper. i think i notices a couple of errors. anyone whos done it notice any ?
    have u got the markscheme for it?
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    (Original post by kimmey)
    jus done the june 2011 paper. i think i notices a couple of errors. anyone whos done it notice any ?
    no not really... we did it as a mock and have been through it and i don't remember any errors...
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    (Original post by schizopear)
    no not really... we did it as a mock and have been through it and i don't remember any errors...
    i noticed 1 error. q14 it says work out the electic potential energy

    now to to this u would usually do v=q/4piepsilon0r then sub v into w=qv to find the energy.

    so this question should actually be work out the electric potential. the word energy should not be there. also another less relevant point is that energy is a scalar so there is no need for the +/- signs.
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    (Original post by handsome7654)
    have u got the markscheme for it?
    no my teacher marked it
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    (Original post by kimmey)
    no my teacher marked it
    Ohhh!
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    Does anybody know the grade boundaries from previous papers? I hear they are particularly low.
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    (Original post by LastHype)
    Does anybody know the grade boundaries from previous papers? I hear they are particularly low.
    http://web.aqa.org.uk/UMS/
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    (Original post by kimmey)
    i noticed 1 error. q14 it says work out the electic potential energy

    now to to this u would usually do v=q/4piepsilon0r then sub v into w=qv to find the energy.

    so this question should actually be work out the electric potential. the word energy should not be there. also another less relevant point is that energy is a scalar so there is no need for the +/- signs.
    i didn't take into consideration the multiple choice paper so there may be mistakes in the paper then...
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    http://www.physicsteachers.com/pdf/A...W-MS-Jun11.pdf

    that is the mark scheme for jun 2011 unit 4
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    Mistake on qu 23 of circular motions posted on previous page I think
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    Just done Jan 2011 Mult. choice.

    Can anyone help me out on question 2? I've tried everything! It'll be something simple probably.

    Also for question 21, I used BANcosθ and got 3.8x10^-3 Wb turns, so I assumed the flux linkage would increase by this amount. The answer is in fact B, can anyone explain why?

    http://store.aqa.org.uk/qual/gce/pdf...W-QP-JAN11.PDF
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    (Original post by lee_vassallo)
    Can anyone help me out on question 2? I've tried everything! It'll be something simple probably.
    Did you divide the diameter by 2 to get the radius?
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    (Original post by lee_vassallo)
    Also for question 21, I used BANcosθ and got 3.8x10^-3 Wb turns, so I assumed the flux linkage would increase by this amount. The answer is in fact B, can anyone explain why?

    http://store.aqa.org.uk/qual/gce/pdf...W-QP-JAN11.PDF
    BANcos50 = 3.78x10^-3
    BANcos0 = 5.88x10^-3

    5.88x10^-3 - 3.78x10^-3 = 2.1x10^-3

    Flux linkage is maximum when θ=0 because cos0=1, so it increases by 2.1x10^-3 Wb turns.
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    Oh thanks I get it.

    For question 2 I tried to find angular speed using w=θ/t and then for the avg. speed v=rw (and divided by pi) but to no avail
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    (Original post by lee_vassallo)
    Oh thanks I get it.

    For question 2 I tried to find angular speed using w=θ/t and then for the avg. speed v=rw (and divided by pi) but to no avail
    For Q2, note that the disc turns through an angle of 30 degrees in 20ms. Hence, it will turn through 360 degrees in 240ms.

    Then, you apply the formula for velocity (not angular): v = 2pir / t.

    => v = 2pi(60x10-3) / 240x10-3

    => v = 0.25(2pi) = 0.5pi = A

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