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    Does anyone know if the spectral classes might come up tomorrow? It's never come up in past papers so I haven't particularly revised it but wondering if I should learn it now?
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    (Original post by candycake)
    Does anyone know if the spectral classes might come up tomorrow? It's never come up in past papers so I haven't particularly revised it but wondering if I should learn it now?
    as in OBAFGKM ? not on our spec
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    (Original post by SuruthiG)
    as in OBAFGKM ? not on our spec
    Brilliant, thanks! I just saw it in the edexcel text book and started to panic!
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    (Original post by candycake)
    From the examiner's report for the IAL paper:
    The question refers to simple harmonic motion, which may or may not be damped. If undamped, total energy would remain constant (no answer key), but this is not a condition of simple harmonic motion. In general, none of the energies stated must remain constant, and so the correct response is B.
    Thank you, but the mark scheme says the answer is D?
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    (Original post by demotivated)
    Thank you, but the mark scheme says the answer is D?
    That's odd - possibly a typo in the examiner's report because what they've written seems to correlate with D. Can't help other than that - sorry!
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    (Original post by candycake)
    That's odd - possibly a typo in the examiner's report because what they've written seems to correlate with D. Can't help other than that - sorry!
    Okay, I was thinking the same thing as well. I'm having trouble understanding this quesiton as well if anyone could help out


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    (Original post by demotivated)
    Okay, I was thinking the same thing as well. I'm having trouble understanding this quesiton as well if anyone could help out
    I'm working on this paper at the moment too and my (nowhere near perfect!) answer for this question is:

    When the can is heated, the internal energy of the gas increases. The molecules gain kinetic energy, so there are more frequent collisions with the walls of the container, increasing the rate of change of momentum. This leads to an increased force on the container and an increased pressure. The liquid inside the can would also evaporate, increasing the pressure further. The can would explode before it reached 900K as its maximum temperature (excluding the evaporated liquid) is 870K, and the increased number of gas molecules would further decrease its maximum temperature before it explodes.
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    Don't know how to go about this question.
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    (Original post by target21859)
    Don't know how to go about this question.
    ooh I just did this question today and my teacher had to explain it to me! Imagine a spring system being set into an oscillation, at maximum amplitude all the total energy is PE while all the total energy at 0 amplitude is KE. Consider the spring then at 0 amplitude in both scenarios. KE = 1/2 mv^2. Since it is SHM, v max = Amplitude x omega. When the amplitude has halved, v^2 = (1/2 amplitude)^2 x omega^2, hence the maximum KE has decreased by a factor of 4 and thus the total energy of the system.
    Since it is asking for a ratio, you do 1/(1/4) to get 4
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    (Original post by candycake)
    I was thinking along the same lines as you - do you know why the answer can't be D? Thanks!
    I'm not entirely sure but I reckon it's because that answer doesn't explain why it is expanding at a faster rate? It could be younger but still slowing down I suppose...
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    (Original post by candycake)
    I'm working on this paper at the moment too and my (nowhere near perfect!) answer for this question is:

    When the can is heated, the internal energy of the gas increases. The molecules gain kinetic energy, so there are more frequent collisions with the walls of the container, increasing the rate of change of momentum. This leads to an increased force on the container and an increased pressure. The liquid inside the can would also evaporate, increasing the pressure further. The can would explode before it reached 900K as its maximum temperature (excluding the evaporated liquid) is 870K, and the increased number of gas molecules would further decrease its maximum temperature before it explodes.
    Hmm, I would put the momentum stuff too (even though it isn't mentioned in the MS) but now I understand the latter part of the explanation. Thank you once again !
    (Original post by target21859)
    Don't know how to go about this question.

    Basically the formula for total energy is 0.5.k.A^2, so use that to find the initial Et and the final Et, and divide them (ignore k as it's constant)

    Initial = 0.5 * 1^2 = 0.5
    final = 0.5* (0.5)^2 = 0.125

    so 0.5/0.125 = 4 Is D the right answer?
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    (Original post by demotivated)
    Hmm, I would put the momentum stuff too (even though it isn't mentioned in the MS) but now I understand the latter part of the explanation. Thank you once again !



    Basically the formula for total energy is 0.5.k.A^2, so use that to find the initial Et and the final Et, and divide them (ignore k as it's constant)

    Initial = 0.5 * 1^2 = 0.5
    final = 0.5* (0.5)^2 = 0.125

    so 0.5/0.125 = 4 Is D the right answer?
    Ah I didn't know that formula. Where does the k come from? And yes D is correct
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    (Original post by target21859)
    Ah I didn't know that formula. Where does the k come from? And yes D is correct
    We know that Ek = 0.5mv^2
    and maximum velocity is Awsinwt

    so Ek = 0.5m(Aw)^2 = 0.5mA^2w^2sin^2wt

    Same with PE, using Ep=0.5kx^2
    use x=Acoswt which gives Ep=0.5kA^2cos^2wt

    Adding them together woudl give you the total energy = 0.5kA^2. Sorry this is such a mess
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    (Original post by demotivated)
    We know that Ek = 0.5mv^2
    and maximum velocity is Awsinwt

    so Ek = 0.5m(Aw)^2 = 0.5mA^2w^2sin^2wt

    Same with PE, using Ep=0.5kx^2
    use x=Acoswt which gives Ep=0.5kA^2cos^2wt

    Adding them together woudl give you the total energy = 0.5kA^2. Sorry this is such a mess
    Nah that's good thanks you!
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    (Original post by demotivated)
    We know that Ek = 0.5mv^2
    and maximum velocity is Awsinwt

    so Ek = 0.5m(Aw)^2 = 0.5mA^2w^2sin^2wt

    Same with PE, using Ep=0.5kx^2
    use x=Acoswt which gives Ep=0.5kA^2cos^2wt

    Adding them together woudl give you the total energy = 0.5kA^2. Sorry this is such a mess
    If you were using the formula E= 0.5kA^2, what value should be used for k?
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    (Original post by candycake)
    If you were using the formula E= 0.5kA^2, what value should be used for k?
    Any value (I use 1 for convinience) as long as you use the same value in both the initial and final energies because it's a constant so it's effect will be cancelled out
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    (Original post by demotivated)
    Any value (I use 1 for convinience) as long as you use the same value in both the initial and final energies because it's a constant so it's effect will be cancelled out
    Would that apply to all questions? What if we were given the amplitude and asked to calculate total energy?
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    (Original post by candycake)
    Would that apply to all questions? What if we were given the amplitude and asked to calculate total energy?
    Unless it was a ratio of energies, then they would give you the spring constant as well.
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    Could anyone help me with a calculation in the Jan 2013 paper?

    http://qualifications.pearson.com/co...e_20130123.pdf

    Question 18 (d) asks you to calculated efficiency, I don't understand why the total possible output power (denominator) is 2200+3100 instead of just 3100?

    2200MW is given as a total output power and the value we calculate, 3100, is the "rate at which energy is removed from the reactors"?
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    A general question: Do you loose mark for giving your answers to greater significant figures than required?
 
 
 
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