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

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    (Original post by Lau14)
    Confuse the examiners by answering 26 questions :P
    Confuse the examiner by answering 42 questions, because you always wonder about the meaning of life


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    (Original post by CD223)
    Confuse the examiner by answering 42 questions, because you always wonder about the meaning of life


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    What are you guys talking about? Are there more questions on the exam papers I didn't know??


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    (Original post by Mehrdad jafari)
    What are you guys talking about? Are there more questions on the exam papers I didn't know??


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    http://www.thestudentroom.co.uk/show...3#post56207383

    Aha


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    (Original post by Mehrdad jafari)
    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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    I haven't done any questions other than the ones on the discovery of the electron. I've done a few practices on special relativity before and a tiny bit today. Hopefully I should have all of turning points practised with all 5 papers tomorrow. I think because they missed out Milikans experiment that it should be on it this year.

    Tbh I find turning points quite interesting. Looking back, it seems like the easiest option out of the 4 but I haven't really looked at the other 3. Does seem like the medical physics one can be a pain
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    (Original post by AR_95)
    I haven't done any questions other than the ones on the discovery of the electron. I've done a few practices on special relativity before and a tiny bit today. Hopefully I should have all of turning points practised with all 5 papers tomorrow. I think because they missed out Milikans experiment that it should be on it this year.

    Tbh I find turning points quite interesting. Looking back, it seems like the easiest option out of the 4 but I haven't really looked at the other 3. Does seem like the medical physics one can be a pain
    That's true. For me studying it makes unit 4 acceptable.
    I've heard the astro option is the hardest alongside medical one. Though i found the waves chapter a bit abstract and hard to tolerate


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    (Original post by CD223)
    Confuse the examiner by answering 42 questions, because you always wonder about the meaning of life


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    I wonder if any of them would notice!
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    (Original post by Lau14)
    I wonder if any of them would notice!
    Probably if they had 700 other papers to mark which answered 25


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    (Original post by Mehrdad jafari)
    That's true. For me studying it makes unit 4 acceptable.
    I've heard the astro option is the hardest alongside medical one. Though i found the waves chapter a bit abstract and hard to tolerate


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    waves as in wave particle duality? Definitely finding it the most boring. Only thing I can get my head around easily is the photoelectric effect.
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    (Original post by Lau14)
    I wonder if any of them would notice!
    Don't think they even look at them, they just shove it under an OMR machine I think


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    (Original post by AR_95)
    waves as in wave particle duality? Definitely finding it the most boring. Only thing I can get my head around easily is the photoelectric effect.
    Yeah, photoelectric is cool. I cannot make sense of the duality nor did I at AS. And the electron microscopes at the end of the chapter are just horrible. I guess we are far from the truth in waves business lol


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    (Original post by CD223)
    Probably if they had 700 other papers to mark which answered 25


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    True, it would stand out then
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    (Original post by CD223)
    Good idea! How many papers have you done? All of them? I feel unprepared tbh


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    Yes, once each I think, though I'm sure I've left out a few since I tend to skip around and not work through them in linear order (june12 --> jan13, etc). I only started fairly recently because I tend to forget everything if I start too early, but I'm planning to work through them at least once more (twice would be ideal) before the exam. If you feel unprepared I suggest locating your weaker topics and working through the topic-specific papers here: http://www.physicsandmathstutor.com/...on/aqa-unit-4/ This will really give you a boost for a given topic.
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    (Original post by PotterPhysics)
    Yes, once each I think, though I'm sure I've left out a few since I tend to skip around and not work through them in linear order (june12 --> jan13, etc). I only started fairly recently because I tend to forget everything if I start too early, but I'm planning to work through them at least once more (twice would be ideal) before the exam. If you feel unprepared I suggest locating your weaker topics and working through the topic-specific papers here: http://www.physicsandmathstutor.com/...on/aqa-unit-4/ This will really give you a boost for a given topic.
    Same here! Thanks for that.

    I hope I can get through them all again once. Not sure I will


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    Can someone help me on this question. Nor lamely really good at capacitance but this one has thrown me, I know it has something to do with E is proportional with V^2 but cant seem to figure it out for some reason. Do you have to set up two simultaneous equations one with V^2 and one with (V-2)^2. Have no clue. Thanks

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    (Original post by Jimmy20002012)
    Can someone help me on this question. Nor lamely really good at capacitance but this one has thrown me, I know it has something to do with E is proportional with V^2 but cant seem to figure it out for some reason. Do you have to set up two simultaneous equations one with V^2 and one with (V-2)^2. Have no clue. Thanks

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    That's a difficult one burdittphysics on YouTube does great examples
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    (Original post by Jimmy20002012)
    Can someone help me on this question. Nor lamely really good at capacitance but this one has thrown me, I know it has something to do with E is proportional with V^2 but cant seem to figure it out for some reason. Do you have to set up two simultaneous equations one with V^2 and one with (V-2)^2. Have no clue. Thanks

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    It's the hardest Capacitance question I've seen. June 2014, right?

    Essentially, as E \propto V^2, this means that, if we set E = 1600\mu J, then when V has decreased by 2.0V, E has become \frac{E}{4} = 400\mu J.

    As E has decreased by a factor of 4 and E \propto V^2, V must have decreased by a factor of 2 \left(\sqrt{\frac{V^2}{4}}\right  ).

    As V has halved by decreasing by 2.0V, the initial pd must have been 4.0V.

    This follows that, as E = \frac{1}{2}CV^2,

    C = \frac{2 \times 1600 \times 10^{-6}}{4^2}

    

\therefore C = 200 \times 10^{-6}F


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    (Original post by Jimmy20002012)
    Can someone help me on this question. Nor lamely really good at capacitance but this one has thrown me, I know it has something to do with E is proportional with V^2 but cant seem to figure it out for some reason. Do you have to set up two simultaneous equations one with V^2 and one with (V-2)^2. Have no clue. Thanks

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    Alternative (and longer!)

    We are given information about V and we need to find C, so the equation to use is E=\frac{1}{2}CV^2. Initially, E_0=\frac{1}{2}CV^2=1600\mu implying that CV^2=3200\mu. Then the voltage decreases by 2V and the energy stored becomes 400\mu, which we can express as the equation E_1=\frac{1}{2}C(V-2)^2=400\mu, or C(V-2)^2=800\mu.

    Now square rooting both sides of each equation we obtain V\sqrt{C}=\sqrt{3200\mu} and (V-2)\sqrt{C}=\sqrt{800\mu}. Thus V=\frac{\sqrt{3200\mu}}{\sqrt{C}  }=\frac{\sqrt{800\mu}}{\sqrt{C}}  +2, or equivalently \sqrt{3200\mu}-\sqrt{800\mu}=2\sqrt{C}. Now we just square both sides and divide by 4 to get C=\frac{(\sqrt{3200\mu}-\sqrt{800\mu})^2}{4}=200\mu F
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    (Original post by CD223)
    It's the hardest Capacitance question I've seen. June 2014, right?

    Essentially, as E \propto V^2, this means that, if we set E = 1600\mu J, then when V has decreased by 2.0V, E has become \frac{E}{4} = 400\mu J.

    As E has decreased by a factor of 4 and E \propto V^2, V must have decreased by a factor of 2 \left(\sqrt{\frac{V^2}{4}}\right  ).

    As V has halved by decreasing by 2.0V, the initial pd must have been 4.0V.

    This follows that, as E = \frac{1}{2}CV^2,

    C = \frac{2 \times 1600 \times 10^{-6}}{4^2}

    

\therefore C = 200 \times 10^{-6}F


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    Yes it is June 2014, has some pretty tough multi choice questions in there. Thanks for that makes much more sense.


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    (Original post by PotterPhysics)
    Alternative (and longer!)

    We are given information about V and we need to find C, so the equation to use is E=\frac{1}{2}CV^2. Initially, E_0=\frac{1}{2}CV^2=1600\mu implying that CV^2=3200\mu. Then the voltage decreases by 2V and the energy stored becomes 400\mu, which we can express as the equation E_1=\frac{1}{2}C(V-2)^2=400\mu, or C(V-2)^2=800\mu.

    Now square rooting both sides of each equation we obtain V\sqrt{C}=\sqrt{3200\mu} and (V-2)\sqrt{C}=\sqrt{800\mu}. Thus V=\frac{\sqrt{3200\mu}}{\sqrt{C}  }=\frac{\sqrt{800\mu}}{\sqrt{C}}  +2, or equivalently \sqrt{3200\mu}-\sqrt{800\mu}=2\sqrt{C}. Now we just square both sides and divide by 4 to get C=\frac{(\sqrt{3200\mu}-\sqrt{800\mu})^2}{4}=200\mu F
    That is a nice maths based solution, might keep this in mind when doing similar types of questions. Thanks


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    (Original post by Jimmy20002012)
    That is a nice maths based solution, might keep this in mind when doing similar types of questions. Thanks


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    No problem! This is exactly how I would do it in an exam because it's the first approach that came to mind and takes at most a few seconds to carry out if you know what you're doing, but CD's one is definitely nicer. (What I did would best be described as a "bash".)
 
 
 
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