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

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    (Original post by a.a.k)
    Ok you win

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    What do I win? I'm just saying why 12*30 gave the wrong answer.
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    (Original post by Paul Dirac)
    What do I win? I'm just saying why 12*30 gave the wrong answer.
    Yeah i get it now

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    (Original post by CD223)
    Without sounding really basic, what do we need to know about motors and stuff like DC motors in particular? I just wanna make sure my understanding is on a par with the expectation haha.


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    It's not on the spec:http://filestore.aqa.org.uk/subjects...50-W-SP-14.PDF
    Have there been questions about stuff like dc motors in the past? I don't remember any from past papers, they usually only ask about rotating coil, flux linkage=BANcoswt and emf=BANwsinwt (and not to mention transformers/power stations)
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    Question: Which radioactive sources (alpha, beta or gamma) are most suitable for:
    i) To trace the path of an underground stream
    ii) To kill cells ina tumour deep inside a human body
    iii) To act as a smoke detector

    I know iii would be alpha particles but what about the rest? Btw can someone explain why alpha radiation is good for use in smoke alarms? The cgp book is vague on this point ("Because it allows a current to flow but won't travel very far" :s )

    EDIT: Whoops, wrong thread.
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    (Original post by Paul Dirac)
    Question: Which radioactive sources (alpha, beta or gamma) are most suitable for:
    i) To trace the path of an underground stream
    ii) To kill cells ina tumour deep inside a human body
    iii) To act as a smoke detector

    I know iii would be alpha particles but what about the rest? Btw can someone explain why alpha radiation is good for use in smoke alarms? The cgp book is vague on this point ("Because it allows a current to flow but won't travel very far" :s )
    For part iii aplha particle are highly ionisimg which mean the kknock electron so atoms are now charged so charge floews

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    (Original post by a.a.k)
    For part iii aplha particle are highly ionisimg which mean the kknock electron so atoms are now charged so charge floews

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    Ah that makes more sense, thanks. But why would it be useful for a smoke alarm?

    (Btw sorry this should be in the PHYA5 thread, I just forgot to post it there)
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    (Original post by Paul Dirac)
    Ah that makes more sense, thanks. But why would it be useful for a smoke alarm?
    When smoke particles block alpha particles there is no current so smaoke alarms sets off.

    I am not 100% SURE

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    (Original post by a.a.k)
    When smoke particles block alpha particles there is no current so smaoke alarms sets off.

    I am not 100% SURE

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    But aren't the alpha particles the ones causing the alarm? So if they're blocked they can't ionize anything so a charge wouldn't flow (hence no alarm). Yeah it's slightly confusing but I hope they don't ask anything as detailed as that.
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    Do we need to know C ∝ A/d for capacitance? (A=area of plates, d=plate separation)
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    (Original post by Paul Dirac)
    But aren't the alpha particles the ones causing the alarm? So if they're blocked they can't ionize anything so a charge wouldn't flow (hence no alarm). Yeah it's slightly confusing but I hope they don't ask anything as detailed as that.
    You r rihht except that alpha particles dont set alarm off. They are always arrivibg at detector. When smoke partice block alpha particles the detector doesnt detetect any alpha particles so alarms sets off. The detector detects aplha particles by detecting current.
    Hopefully this makes sense. Someone on google might be able explain clearly as i am bot good with words

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    Sorry for the double post, but have there been questions that use C=C_1+C_2 and 1/C=1/C_1+1/C_2 for capacitors in parallel and series (respectively)?
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    (Original post by Paul Dirac)
    Sorry for the double post, but have there been questions that use C=C_1+C_2 and 1/C=1/C_1+1/C_2 for capacitors in parallel and series (respectively)?
    I havent seen any

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    can some one outline the steps of how a mass spectrometer and a cyclotron operate
    thanks
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    (Original post by a.a.k)
    You r rihht except that alpha particles dont set alarm off. They are always arrivibg at detector. When smoke partice block alpha particles the detector doesnt detetect any alpha particles so alarms sets off. The detector detects aplha particles by detecting current.
    Hopefully this makes sense. Someone on google might be able explain clearly as i am bot good with words

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    Thanks, this made it a lot clearer.

    I foud this which looks helpful: http://www.bbc.co.uk/schools/gcsebit...opesrev3.shtml
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    (Original post by Paul Dirac)
    Sorry for the double post, but have there been questions that use C=C_1+C_2 and 1/C=1/C_1+1/C_2 for capacitors in parallel and series (respectively)?
    No, that's required for OCR specification


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    (Original post by Paul Dirac)
    Thanks, this made it a lot clearer.

    I foud this which looks helpful: http://www.bbc.co.uk/schools/gcsebit...opesrev3.shtml
    Cool

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    Name:  ImageUploadedByStudent Room1433630027.879071.jpg
Views: 81
Size:  104.1 KB

    I keep getting B, any help ?


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    (Original post by Ilovemaths96)
    can some one outline the steps of how a mass spectrometer and a cyclotron operate
    thanks
    I can outline cyclotron, bit rusty with mass spectrometer (do we need to know about it? )

    From the top a cyclotron is basically two D-shaped metal "dees" with a small gap between them. When a charged particle enters the cyclotron an alternating voltage across the plates which accelerates the particle across the gap between the dees. You need an alternating voltage because the particle crosses the dees in one direction and after it travels 180 degrees across the circle it crosses the gap again in the other direction. As the particle keeps accelerating across the dees its speed increases, and as r=mv/BQ, this means the radius of its circular path increases with time. If you work it out you'll find that the time the particle spends in each semicircular dee is constant, so the alternating voltage switches regularly no matter what the current radius of the particle.Eventually when the radius is large enough the particle leaves through a tunnel to a detector.
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    (Original post by Mehrdad jafari)
    No, that's required for OCR specification


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    Thanks
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    For a rotating coil in a magnetic field:

    When the coil is vertical the flux linkage is greatest and when the coil is horizontal the induced emf is greatest

    I thought induced emf is the change of flux linkage so why when the coil is at right angle to the magnetic field lines the induced emf us zero? This sounds so unreasonable


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