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

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    (Original post by k9000)
    For the capcitance and 2mj question did you guys get B (the energy taken from the battery was 2mj) that was incorrect and some energy would have been wasted so more energy was actually taken?
    Yes, energy from the battery should be twice the energy stored in the capacitor, i.e. 4mJ. Therefore 2mJ was incorrect.
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    (Original post by k9000)
    For the capcitance and 2mj question did you guys get B (the energy taken from the battery was 2mj) that was incorrect and some energy would have been wasted so more energy was actually taken?
    Yep
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    (Original post by M1llward)
    The Electric field strength should have been negative as the charge was negative. -1560 Vm^-1.

    I got this wrong.

    Also any clues on the damping question? I wrote there would be more damping without the ring as the total energy given to the system with the ring will be greater as GPE=mgh. The work done against air resistance is the same as time period is not affected - because the pendulum system has more energy with the ring it will take longer for the initial energy to be lost through doing work against air resistance. Thus the amplitude will decrease at a slower rate.
    Less damping, mass is increased and so therefore is momentum. Increased momentum results in resistive forces having a lesser effect on the motion of the pendulum, thus less damping.
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    (Original post by DanTheMan358)
    Ahh i think you may be right.
    Do you remember how the questions was worded?
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    The answer was weight. Rate if change of momentum is force.
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    (Original post by joesterman)
    Surely A because the longest length was perpendicular to the field?
    The fact that they wanted the couple means you multiple by the distance between the two vertical wires, which is the width of the rectangle. This introduces a factor of area as I showed in my previous post.
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    yeh but it gave it as half the ke not half the velocity
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    (Original post by TashJacobs)
    well I just lost my uni place for september!
    Ah dont worry theres still Unit 5

    Where are you hoping to go?
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    (Original post by Disney0702)
    Hmmm just out of curiosity why'd ya 55 for A*?

    Last it was 61/75 do you think this exam was significantly harder than last year.

    Coz i dont know if i found harder than last year after reading peoples comments
    Firstly, it seems that this year covered a lot of niche topics, compared to last year covering a bigger amount of the unit.

    I also factored in the 3 marks that will be lost due to the typo in question 3, and the polarizing 'cone' question.
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    (Original post by Humbers)
    A newton is not the same unit as acceleration

    Force = newton = rate of change of momentum mv-mv/t = kgms-2
    Acceleration = rate of change of velocity = ms-2


    for 22. i know its the ratio but if it has half the kinetic energy it has root 2 of the velocity F=BQV so force is proportional to velocity
    The rate of change of momentum is acceleration. A body cannot accelerate if it's momentum doesn't change


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    (Original post by k9000)
    For the capcitance and 2mj question did you guys get B (the energy taken from the battery was 2mj) that was incorrect and some energy would have been wasted so more energy was actually taken?
    Yes because the enrgy taken from the battery is qv not half qv so from the battery would have been 4mj
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    (Original post by k9000)
    For the capcitance and 2mj question did you guys get B (the energy taken from the battery was 2mj) that was incorrect and some energy would have been wasted so more energy was actually taken?
    Isn't E proportional to V^2 so wouldn't A have been incorrect


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    (Original post by NEWT0N)
    Yes, energy from the battery should be twice the energy stored in the capacitor, i.e. 4mJ. Therefore 2mJ was incorrect.
    agree'd with everything you've said so far, good job :)
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    anyone have an mark scheme
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    (Original post by NEWT0N)
    Yes B had the biggest area. The force is only on the vertical pieces of the wire because the horizontal ones are parallel to the field and therefore experience no force. F=BIL with I=5 for all the currents. Then the couple is Fd-BILd=BI*Area of the rectangle. BI is constant so the couple is proportional to area of loop.
    So do you think B is correct then?
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    (Original post by NEWT0N)
    No V=IR so I=V/R is proportional to R since V is constant. R increases so 1/R decreases, hence I is smaller.
    Only the discharge was using a different resistor, i think they wanted you to assume they start at the same current. Then they just increased resistance. I'm not too sure but I don't think they would've made a typo.
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    (Original post by SGBKU)
    The answer was weight. Rate if change of momentum is force.
    I put weight as my answer too.

    Some people are saying acceleration
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    Having done June 2014 last year, and sitting through that torture, this was an average paper in comparison, so I think grade boundaries will be fairly high


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    What did you guys get on the written for the number of degrees the lead ball turned through? I got 131 or something similar?
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    (Original post by DJH12)
    yeh but it gave it as half the ke not half the velocity
    Yes, and KE is proportional to v^2 since the factor of 0.5m is constant since both are electrons and therefore have the same mass m.

    Kinetic energy of x is twice that of y, hence v_x^2=0.5v_y^2 by what I just said. Hence \frac{v_x}{v_y}=\sqrt{0.5}.

    Now, the field exerts a force of F=BQv on a charge Q moving at right angles to the field with velocity v. BQ is the same for both electrons for obvious reasons, so F is proportional to v. Therefore, \frac{F_x}{F_y}=\frac{v_x}{v_y}=  \sqrt{0.5} which is the same as \frac{1}{\sqrt{2}}. (option B I think)
 
 
 
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