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AQA Physics PHYA4 - Thursday 11th June 2015 [Exam Discussion Thread] Watch

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    (Original post by boombozx)
    Attachment 407693

    guys I really need your help. I don't know why C is correct and the other answers are invalid Attachment 407693
    C is the only answer which would let the coil cut the field lines, which would produce a force allowing the coil to spin.
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    (Original post by Fred Cantoni)
    Sort of 20-23, was just wondering if that was good enough, or if everyone was getting 25!


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    Ah I don't think I know anyone getting 25s! I'd say anything 20 or above is very good, definitely good enough to not let the multiple choice bring your grade down for that paper
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    (Original post by Fred Cantoni)
    What are you guys generally scoring on the multiple choice section?!
    ~22 at the moment. You?

    (Original post by PotterPhysics)
    Leaning on a generalization of my first solution. We let the three charges be q_1,q_2,q_3 and let the midpoints between (q_1,q_2),(q_2,q_3),(q_3,q_1) be x,y,z respectively. Then letting (wlog) the sides of the triangle be 2, and as before letting the constant for V\propto \frac{Q}{r} be k, we have k(\frac{q_1}{1}+\frac{q_2}{1})=x  \Leftrightarrow k(q_1+q_2)=x, and similarly k(q_2+q_3)=y and k(q_3+q_1)=z. Hence adding up we have 2k(q_1+q_2+q_3)=x+y+z, giving q_1+q_2+q_3=\frac{x+y+z}{2k}. (Call this equation 1)

    Now, since the figure is an equilateral triangle, its area is \frac{s^2\sqrt{3}}{4} where s is its side length (I leave this as an exercise to the reader), therefore since our triangle has s=2 then plainly its area is \sqrt{3}. Then from the formula \frac{s^3}{4R}=\frac{s^2\sqrt{3}  }{4} (where R is the circumradius, and both sides give the area of the triangle) we have R=\frac{2}{\sqrt{3}}. Therefore, at the center of the triangle the potential is k\frac{q_1}{R}+k\frac{q_2}{R}+k\  frac{q_3}{R}=\frac{k\sqrt{3}}{2}  (q_1+q_2+q_3). Then substituting equation (1) into this last expression yields the answer \frac{k\sqrt{3}}{2}(\frac{x+y+z}  {2k})=\frac{\sqrt{3}}{4}(x+y+z).
    Thanks for the explanations

    (Original post by Leonacatherine)
    I was doing a past paper, last quesiton on june 2011 and it said that in a transformer a high voltage introduces insulation problems and raises safety issues.
    I dont understand how a high voltage can affect insulation can anyone explain?
    I believe the question (albeit ambiguously) is referencing the need for a step down transformer before voltage is supplied to the home. For domestic applications, a much lower voltage is needed to be considered safe and non-hazardous to humans. For many applications, if a much higher voltage surges through them then they will break, making them unsafe.


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    @CD: Np

    This is also why a fuse is used in a transformer.
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    (Original post by PotterPhysics)
    @CD: Np

    This is also why a fuse is used in a transformer.
    I see. Is it a fuse in the primary coil I take it? Or both?


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    (Original post by CD223)
    I see. Is it a fuse in the primary coil I take it? Or both?


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    Yes, primary coil. There was a question about this in a past paper -- from 2013 papers IIRC.
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    (Original post by CD223)
    ~22 at the moment. You?



    Thanks for the explanations



    I believe the question (albeit ambiguously) is referencing the need for a step down transformer before voltage is supplied to the home. For domestic applications, a much lower voltage is needed to be considered safe and non-hazardous to humans. For many applications, if a much higher voltage surges through them then they will break, making them unsafe.


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    Thanks
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    (Original post by PotterPhysics)
    Yes, primary coil. There was a question about this in a past paper -- from 2013 papers IIRC.
    So a fuse is used in the primary coil if the current gets too high in the secondary?
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    (Original post by Leonacatherine)
    So a fuse is used in the primary coil if the current gets too high in the secondary?
    Yes, it prevents the transformer from overheating/supplying excessive current.
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    (Original post by PotterPhysics)
    Yes, primary coil. There was a question about this in a past paper -- from 2013 papers IIRC.
    Yeah I thought so, just wanted to make sure I remembered correctly haha.


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    (Original post by Lau14)
    Keep an eye on how much time you're spending on each half - you shouldn't be exceeding the recommended 45 minutes on the multiple choice I'd say.

    In the multiple choice half, look for proportionality in questions. Also look for contradicting statements (particularly on true or false type questions) - they put these in to speed it up, but they're often missed.

    Don't forget to leave time to fill in the multiple choice answer sheet! You might forget about this because in mocks you probably just circle the answer on the paper, but there is in fact a separate sheet to fill in.

    Obviously never leave multiple choice questions blank - you've got a one in four chance of getting a question right even if you've got absolutely no clue (if you've got the time though, aim to make educated guesses where possible!).

    Decide whether doing multiple choice or written part is best for you, and whether starting or finishing with six markers works better. Remember that you don't have to do the paper in order! (May seem obvious, but a lot of people try and do everything in order and lose a lot of time when they get stuck).

    If you get stuck, move on and come back to it. Get all your guaranteed "I can definitely do this right now" marks before you try and figure out anything that's confusing you.
    Thank you!! Did not know there was an answer sheet for mc but makes sense.


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    (Original post by CD223)
    Yeah I thought so, just wanted to make sure I remembered correctly haha.


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    Yeah, it's also a good idea to do past papers twice (or more if you have time) as questions are repeated each year. (Note: Just checked: it was JAN12, not 2013.)
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    (Original post by PotterPhysics)
    Yeah, it's also a good idea to do past papers twice (or more if you have time) as questions are repeated each year. (Note: Just checked: it was JAN12, not 2013.)
    Good idea! How many papers have you done? All of them? I feel unprepared tbh


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    if you're finding the unit 4 multiple choice section difficult, this guy will get you 25/25
    https://www.youtube.com/user/BurdittPhysics
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    (Original post by zaybun)
    Thank you!! Did not know there was an answer sheet for mc but makes sense.


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    Yeah, it's got 1-50 on it for some reason with A, B, C, D circles next to each one and you colour in the right little circle for each question I think.
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    (Original post by Lau14)
    Yeah, it's got 1-50 on it for some reason with A, B, C, D circles next to each one and you colour in the right little circle for each question I think.
    For those people who insist on doing the paper twice


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    Really, really hope Milikans oil drop experiment comes up this year as it's my strongest "difficult" topic throughout the whole course :lol:

    That being said, I bet you guys have no idea what I'm talking about considering I think I'm the only one to be doing option 2D

    Also I should have posted this on the PH5 thread but Im not watching that thread
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    (Original post by CD223)
    For those people who insist on doing the paper twice


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    Confuse the examiners by answering 26 questions :P
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    (Original post by AR_95)
    Really, really hope Milikans oil drop experiment comes up this year as it's my strongest "difficult" topic throughout the whole course :lol:

    That being said, I bet you guys have no idea what I'm talking about considering I think I'm the only one to be doing option 2D

    Also I should have posted this on the PH5 thread but Im not watching that thread
    We both share the same misfortune lol. I'm doing that option too. Have done any question on that unit? I've heard the questions on special relativity are a bit vague


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    (Original post by AR_95)
    Really, really hope Milikans oil drop experiment comes up this year as it's my strongest "difficult" topic throughout the whole course :lol:

    That being said, I bet you guys have no idea what I'm talking about considering I think I'm the only one to be doing option 2D

    Also I should have posted this on the PH5 thread but Im not watching that thread
    I got genuinely freaked out until you said it's an option topic!!

    Here's the PHYA5 thread http://www.thestudentroom.co.uk/show....php?t=3047351


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