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OCR Physics A AS Forces+Motion/Electrons & Photons revision thread watch

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    Kirchoffs laws which are

    1st: The sum of the currents flowing into any point in a circuit is equal to the sum of the currents flowing out of that point.

    2nd: The sum of the e.m.f’s around any loop in a circuit is equal to the sum of the P.d’s around that loop.
    perhaps add:
    1st. -> conservation of charge.

    2nd. -> conservation of energy.


    I've seen questions come up regarding that
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    I have forces and motion past papers jan03, jun03, jan04, jun04 so i might type up some hard questions nearer the exam date.
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    Oh and IIRC for Electrons&P, you need to know the wavelengths and corresponding frequencies of the electromagnetic spectrum. But only bother learning either frequencies OR wavelengths and use v \ = \ f\lambda to calculate the other.

    Tip:
    If you have to convert between eV and joules, remember:
    V=W/Q
    => W=VQ
    or W = eV
    ie W=(1.6*10^-19)V




    Great thread btw Chats
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    (Original post by Widowmaker)
    I have forces and motion past papers jan03, jun03, jan04, jun04 so i might type up some hard questions nearer the exam date.
    Mate, send them to oxymoron and he'll upload 'em.
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    (Original post by endeavour)
    perhaps add:
    1st. -> conservation of charge.

    2nd. -> conservation of energy.
    I've seen questions come up regarding that
    I've added the other bit you said, but cant add anymore, the post has reached its 10,000 word limit :eek: I wonder if a mod can make the first post longer, or put in a second post so I can add future stuff there.

    (Original post by Widowmaker)
    I have forces and motion past papers jan03, jun03, jan04, jun04 so i might type up some hard questions nearer the exam date.
    yeh that would be good, thanks
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    (Original post by endeavour)
    Great thread btw Chats
    Thanks :cool: just hope it helps everyone!
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    (Original post by endeavour)
    Mate, send them to oxymoron and he'll upload 'em.
    yeah I would but I don't have a scanner.
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    (Original post by chats)
    I've added the other bit you said, but cant add anymore, the post has reached its 10,000 word limit :eek: I wonder if a mod can make the first post longer, or put in a second post so I can add future stuff there.



    yeh that would be good, thanks
    just copy and paste the stuff and just make a new thread, with a module per post.
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    (Original post by chats)
    *bump*

    if anyone needs help
    if uve done the jan 05 paper of electrons and photons....please can u tell me as many questions that u remember are in that paper. Cheers mate,
    Vinnie
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    (Original post by vinnie11)
    if uve done the jan 05 paper of electrons and photons....please can u tell me as many questions that u remember are in that paper. Cheers mate,
    Vinnie
    No sorry I didnt do that paper
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    Thanks lol
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    You also need to know the equation;
    hf = work function + Ekmax
    work function - the minimum amount of energy required to remove an electron from the surface of a material.
    hf - the energy of the photon.
    Ekmax - the maximum kinetic energy of the electron that has left the material (0.5m(vmax)^2)
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    (Original post by Widowmaker)
    You also need to know the equation;
    hf = work function + Ekmax
    work function - the minimum amount of energy required to remove an electron from the surface of a material.
    hf - the energy of the photon.
    Ekmax - the maximum kinetic energy of the electron that has left the material (0.5m(vmax)^2)
    Thanks
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    I also have all mark schemes, 2001 - 2005 if anyone needs them.
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    Great thread, i'm doing Forces, Electrons and Waves on the same day
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    Does anyone know the boundaries to get an A/B/C and stuff? Or is it always 80%
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    (Original post by marthafocker)
    Does anyone know the boundaries to get an A/B/C and stuff? Or is it always 80%
    Can you help with answers this from my mock exam

    Monochromatic light of wavelenth 450nm falls on cathode of photocell at a rate of 25microwatts. Only 10% of photons produice an electron and all these electrons produce a current I in an external circuit.
    a) calculate
    i) energy of one photon in J
    ii) number of photons falling on cathode per second
    iii) the current, I
    b) same amount of energy per second falls on cathode in form of light of wavelength 600nm. the Work functon of the material of cathode is 3x10^-19J. SHOW that a photoelectric current will also flow.
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    Monochromatic light of wavelenth 450nm falls on cathode of photocell at a rate of 25microwatts. Only 10% of photons produice an electron and all these electrons produce a current I in an external circuit.

    a) calculate
    i) energy of one photon in J

    E = hf = h[c/λ]
    E = 6.63 x 10-34 x (3 x 108/450 x 10-9)
    E = 4.42 x 10-19J
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    ii) number of photons falling on cathode per second
    Power = 25 x 10-6W
    Energy = Pt = 25 x 10-6 x 1 = 25 x 10-6J

    Number of photons
    = 25 x 10-6J/4.42 x 10-19J
    = 5.66 x 1013 photons per second

    iii) the current, I
    Number of electrons = 0.1 x 5.66 x 1013 = 5.66 x 1012

    Charge = 5.66 x 1012 x 1.6 x 10-19 = 9.05 x 10-7C

    I = Q/t = [9.05 x 107C]/[1s] = 9.05 x 10-7A = 905nA

    b) same amount of energy per second falls on cathode in form of light of wavelength 600nm. the work functon of the material of cathode is 3x10-19J. SHOW that a photoelectric current will also flow.

    E = hc/λ
    E = 6.63 x 10-34 x (3 x 108/600 x 10-9)
    E = 3.32 x 10-19J

    Energy of photon > work function therefore electrons will be removed and a current will flow.
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    This is a great revision thread ...
    Though im dreading all my exams
 
 
 
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