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OCR Physics B G495 Field and Particle Pictures June 21st 2011 Exam Thread watch

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    (Original post by JoeCarr)
    I know right, big love to whoever produced it at whatever school it was, it's perfect but we now just need to work through it with answers, I have an English Lit exam tomorrow but can start putting my answers up in the afternoon.
    I am going to focus on the section C side of things today, so I most likely will post my answers sometime today. Best of luck with your English exam
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    The pre-release was actually quite an interesting read. Sort of lost track of it all on the last page though with the krypton and caesium.

    Random question, eddy currents, what exactly are they? Never truly understood them. I understand they're produced when there is a change in magnetic flux, but what causes energy to be lost through them in the form of heat? Also, why does insulating the core reduce the effect?
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    (Original post by Summerdays)
    I am going to focus on the section C side of things today, so I most likely will post my answers sometime today. Best of luck with your English exam
    Thanks, I am actually a little worried about revising too much for it (something that can't be done for physics) because after reading examiner reports it seems like they really do put a considerable amount of emphasis on original interpretations of texts and contrasts with your core one. They feel that going in with rigid essays and 'trophy' critical quotes as they call them, which you just want to regurgitate in the exam, can be damaging to your marks. However unlike physics, I can afford to mess up the exam and still get an A overall, whereas I need a mid to high B on the 21st so all the help I can get is greatly appreciated, including your answers to that question paper to see if they match mine
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    (Original post by Swindan)
    The pre-release was actually quite an interesting read. Sort of lost track of it all on the last page though with the krypton and caesium.

    Random question, eddy currents, what exactly are they? Never truly understood them. I understand they're produced when there is a change in magnetic flux, but what causes energy to be lost through them in the form of heat? Also, why does insulating the core reduce the effect?
    In a transformer, an alternating emf causes an alternating current to travel through the prinary coils. This alternating current produces an alternating magnetic field/magnetic flux linkage in the iron core of the transformer. Because Iron is a good conductor, has a very high Permeance (how well a material alows flux to be established - this is analogous to the conductance of a material when we are referring to electricity) and permeability (how well a medium can set up a magnetic field - this is analogous to conductivity.) Because of this, an alternating emf is induced in the iron core. This alternating emf produces an alternating current in the iron core - this is the 'eddy currents' we are referring to. The eddy currents cause heat loss (I^2R) and also the eddy currents produce a magnetic field that opposes the original magnetic field that induced the eddy currents (Lenz's law.) Not only do eddy currents waste energy but they reduce the magnitude of the emf that can be induced in the secondary coils.

    The iron core is laminated, each lamina is insulated from its neighbour. The iron core still has a high permeance and permeability, but the resitance at right angles to the changing/alternating magnetic flux linkage/magnetic field is very high, which means that the eddy currents produced are very small.
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    After the monumental ****-up that was my chemistry exam I'll be spending a lot of time here.

    Good luck revising people, gonna start going over the course now fully before checking out section C/past papers.
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    (Original post by JoeCarr)
    Thanks, I am actually a little worried about revising too much for it (something that can't be done for physics) because after reading examiner reports it seems like they really do put a considerable amount of emphasis on original interpretations of texts and contrasts with your core one. They feel that going in with rigid essays and 'trophy' critical quotes as they call them, which you just want to regurgitate in the exam, can be damaging to your marks. However unlike physics, I can afford to mess up the exam and still get an A overall, whereas I need a mid to high B on the 21st so all the help I can get is greatly appreciated, including your answers to that question paper to see if they match mine
    Here are the answers I came up with

    Spoiler:
    Show


    1 a. d= k xm^2 1 = k(7.5x10^-3)^2 so k = 1.78x10^4
    2 = k(10.8x10^-3)^2 so k = 1.71x10^4
    3 = k(13x10^-3)^2 so k = 1.77x10^4
    4 = k(15.2x10^-3)^2 so k = 1.75x10^4

    k is constant (2x10^4, to one sig fig.)

    b. 0.1/70 = 1/700 minutes. 1/700 x 60 = 0.09 seconds

    c. s = 6, t = 2.64, u = 0 s = 0.5at^2 so 17/(2.64)^2 = 1.72

    d. Percentage uncertainty (time) = (0.09/2.64) x 100 = 3.4%
    u/1.72 = 3.4% so u = (+-)0.06

    e. 1.72 (+-)0.06

    f. An arrow that is perpendicular to the surface of the ramp, representing the normal force on the ball. A downward arrow representing the weight of the ball.

    g. mgsinth = F = ma so a = gsinth = 1.7. Thi is the same as the answer above (to two sig figs.)

    h. Balls spin, which means that the work done by gravity is used as rotational kinetic energy as well as translational kinetic energy. Different sizes and masses of balls causes different balls to spin by different amounts. The acceleration is never the same as gsinth because there's always SOME energy that is used to cause the ball to spin.

    i. 78/10 = 7.8 s

    j. 7.8 = 2piroot(l/g) so 1.24 = root(l/g) so l = 15.1

    k. (0.09/7.8)100 = 1.15%

    l. Uncertainy = half resolution = 0.005. 0.005/9.81 = 0.005%

    m. The emasurement of time has the highest percentage uncertainty.
    u/15.1 = 0.0115 so u = 0.2

    15.1 (+-)0.2

    2a. 30x0.001% = 0.03%

    b. 40x0.03% = 0.012. 0.012 x 36 = 0.432 inches. 0.432 x 2.54 = 1.1cm

    3. (1/60 / 360) x 6371km = 0.295km

    0.295km x 2pi = 1.853km

    4a. 3959.2 x 0.5 = 1979 oscillations
    b. 30x60/1929 = 0.9
    c. T = 2iroot(l/g) 9.81x(0.9/2pi)^2 = 0.2m
    d 2m
    e. The unit of length is now more universal and more constant than previously.

    5a. 2 = 2piroot(l/g) so 9.8(1/pi)^2 = l = 0.99

    ((1-0.99)/1)100 = 0.7%

    b. 9.819(1/pi)^2 = 0.995 and 9.779(1/pi)^2 = 0.991

    0.995 - 0.991 = 4x10^-3

    6a. (1/10x10^6)x6371x10^3 = 0.6371m

    b. If the earth wasn't rotating the gravitational field should be the same in every country, but that isn't the case in reality. An obsolete sphere however wouldn't produve a gravitational field that is constant in every country, which is the case. The earth must be an obsolete sphere.

    c. root((6.672x10^-11x5.9763x10^24)/9.8322)) = r = 6.3682x10^6

    7a. 10%x22420 + 90% x 21430 = 21547

    b. ((21547 - 21539)/21539)100 = 0.04%

    c. p = m/v so v = 1.2 x 4x10^-4 = 10.3

    d. 10.3x9.81x0.5 = 50.7

    e. y = (kgm^-3 x Nkg^-1 x m^4)/(Nm^-2 x m^2) = m

    f. y = (5 x 9.81 x 21538 x 1.2^4)/(32 x 185x10^9 4x10^-4) = 9.25x10^-4

    g. If the ball naturall falls under its own weight then the distance between the two marks would lengthen.

    8a. The path difference is an integer wavelength, thus constructive interference occur s, producing the maxima.

    b. The path difference has changed from an integer wavelength to an odd half-integer wavelength. Destructive interference occurs, as a result.

    c. 0.5 lambda is the distance from one maxima to another. So 36/2 x 640nm = 1.15x10^-5

    d. The lasers no longer have a constant phase difference, thus total cancellation can't occur, which means that no maxima can be observed. The degree of constructive superposition is now minimal.

    e. 10x10^-2/640x10^-9 = 1.56250x10^5

    f. 86 maximas = 43 wavelengths. 1.5625x10^5 - 43 = 1.56207x10^5 (I am not sure about this answer though.)

    g. Lambda for air = 10x10^-2/(1.5625x10^5 + 43) = 6.398x10^-7 (I am also not sure about this answer...)

    h. n = c/v = lambda for vacuum/ lambda for air = 6.4x10^-7/6.39810^-7 = 1.00028

    9a. kT = (270 + 273)x1.38x10^024 - 7.49x10^-21
    b. 0.5mc^2 = 1.5kT = 1.1x10^-20 so mc^2 = 2.248x10^-20 so c = 769

    c. change in lambda = (769/3x10^8)x589x10^-9 = 1.51x10^-12

    d. Change of wavelength wouldn't be the same for all spectral lines which means that the electron transition we are intrested in can be confused (* I am not sure about this answer)

    e. Calibration of results is no longer possible.

    f. 620nm - 570nm = 5x10^-8m This spread isn't as large as the spread in the emitted wavelength about 750nm and 805nm.

    10a. d =ct/2 = (3x10^8x2.31)/2 = 3.77x10^8

    b. V= d/t so 3x10^8 x 2x10^-12 = 6x10^-4m

    c. E = hc/lambda = (6.6310^-34 3x10^8)/552x10^-9 = 3.6x10^-19

    d. 115x10^-3/3.6x10^-19 = 3.210^17

    e. Detected/3.2x10^17 = 1.7x10^-17 so Detected = 5.44 so Around 5.

    f. v = d/t so d = 3x10^8x100x10^-12 = 0.03m

    g. (2/100)x100 = 2%

    u/0.03 = 2% so u 6x10^-4

    h. 20 pulses per second = 100 photon detected per second.

    1000/100 = 10 seconds

    i. 300ps

    j. Uncertainbty in time = half spread = 150ps
    150x10^-12/10 = 1.5x10^-9%

    u/6x10^-4 = 1.5x10^-9% so u = 9.6x10^-15

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    Hey, only just found this thread...

    Just want to clarify, am I the only one who absolutely hates this exam board and exam papers?? Or are there others?

    I can tell you, the day I've just had doing revision has been one of the worst days of my life. Goodbye University and Astrophysics... :'(
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    (Original post by Helsy)
    Hey, only just found this thread...

    Just want to clarify, am I the only one who absolutely hates this exam board and exam papers?? Or are there others?

    I can tell you, the day I've just had doing revision has been one of the worst days of my life. Goodbye University and Astrophysics... :'(
    It's a disgraceful course that doesn't even prepare you for the damn questions they pull out of their you-know-what! Is there anything you need help with?
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    hey just thought i'd glance at this before going to sleep, haven't studied your answers thoroughly as i'm looking over english essays but you look like you've done a damn good job overall, just noticed for number 3 though where you say you couldn't get the value, you simply needed to times the radius by 2pi, as you are calculating one minute of arc along the circumference, that should give you 1.85km or so which is approximate to the given 2km

    will post my answers hopefully tomorrow afternoon, if not then during friday, i'm definitely glad I found the questions!
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    (Original post by JoeCarr)
    hey just thought i'd glance at this before going to sleep, haven't studied your answers thoroughly as i'm looking over english essays but you look like you've done a damn good job overall, just noticed for number 3 though where you say you couldn't get the value, you simply needed to times the radius by 2pi, as you are calculating one minute of arc along the circumference, that should give you 1.85km or so which is approximate to the given 2km
    OH YEAAAAAAAAAAAAAAAHHHHHHHHHHHHH.. . thanks
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    (Original post by Helsy)
    Hey, only just found this thread...

    Just want to clarify, am I the only one who absolutely hates this exam board and exam papers?? Or are there others?

    I can tell you, the day I've just had doing revision has been one of the worst days of my life. Goodbye University and Astrophysics... :'(
    Don't think there's anyone who thinks it's a good course. Even my teachers at college think it's crap. Which begs the question why they're even making us do it... :mad:
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    (Original post by Summerdays)
    OH YEAAAAAAAAAAAAAAAHHHHHHHHHHHHH.. . thanks
    haha thought you'd kick yourself for not spotting it, it's the easy stuff like that which can just be missed in an exam so its undoubtedly useful to go through this all now.
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    (Original post by Swindan)
    Don't think there's anyone who thinks it's a good course. Even my teachers at college think it's crap. Which begs the question why they're even making us do it... :mad:
    Originally my college said they chose OCR B because they thought it was the best physics course (lol, clearly not!) This year they gave the lower sixths the choice to pick between this and AQA A. AQA A looks a LOT better.
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    (Original post by JoeCarr)
    haha thought you'd kick yourself for not spotting it, it's the easy stuff like that which can just be missed in an exam so its undoubtedly useful to go through this all now.
    Definitely. The more section C stuff I can get through the better I will feel come exam day
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    (Original post by Summerdays)
    It's a disgraceful course that doesn't even prepare you for the damn questions they pull out of their you-know-what! Is there anything you need help with?

    (Original post by Swindan)
    Don't think there's anyone who thinks it's a good course. Even my teachers at college think it's crap. Which begs the question why they're even making us do it... :mad:
    Ok, thanks you two. Makes me feel a little better. But doesn't help the fact that I need an A in this stupid exam board to get to St Andrews.
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    Just looking in the spec it says we need to know how to graph electrical potential energy against distance, am I being thick or is it just the same as potential?? seen as F and E graphs are the same? help!!
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    (Original post by Helsy)
    Ok, thanks you two. Makes me feel a little better. But doesn't help the fact that I need an A in this stupid exam board to get to St Andrews.
    I need an A for Durham blerghhhhh good luck!!
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    (Original post by Birkatron)
    Just looking in the spec it says we need to know how to graph electrical potential energy against distance, am I being thick or is it just the same as potential?? seen as F and E graphs are the same? help!!

    They're similar but an Electric field falls by 1/r^2 but an electric potential falls by 1/r.

    By the way, I think you mean V and E graphs.
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    (Original post by Summerdays)
    They're similar but an Electric field falls by 1/r^2 but an electric potential falls by 1/r.

    By the way, I think you mean V and E graphs.
    oops yes i did haha sorry, do graphs of W against r and F against r fall in the same ways as V and E graphs?? Also it says something about relation of electrical field to tangent to graph??
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    (Original post by Birkatron)
    oops yes i did haha sorry, do graphs of W against r and F against r fall in the same ways as V and E graphs?? Also it says something about relation of electrical field to tangent to graph??
    Work done on an object falls as 1/r (or becomes less negative by 1/r if the object has the opposite charge to the point charge) as the distance increases. The force on an object falls as 1/r^2 (or becomes less negative by 1/r^2 if the object has the opposite charge to the point charge) as distance increases. V falls by 1/r and E falls by 1/r^2

    The gradiant of the tangent of a point, in an the electrical potential against r curve, gives the electric field strength at that point. The electric field strength is the gradiant of the potential. The closer together the equipotential lines are, the stronger the electric field (and vice versa.)
 
 
 
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