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Homework Help: Mass spectograph and mass of particles

  1. Nov 14, 2008 #1


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    1. The problem statement, all variables and given/known data
    A mass spectograph is used to identify some element and 12/6 Carbon is used as a reference. 12C has a circular path of radius = 5cm. The particle of the unknown element is travelling at the same velocity as the 12C. What is the mass of one atom of the unknown element if the unknown element has a circular path radius of 10cm?
    (The figures may be off because I made up this question basing on other questions to improve my understanding of mass/charge ratio)

    2. Relevant equations
    Centripetal force = mv^2/r
    Elect. F = Bvq
    Mass/charge ratio

    3. The attempt at a solution
    Firstly I made centripetal force equal Elct. F:
    mv^2/r = Bvq
    Using algebra:
    Bvqr = mv^2
    Bqr/v = mv
    B/v = m/qr

    The mass/charge ratio of 12C = 12/6 = 2

    As the two things are travelling at same velocity in same magnetic field (B),
    m/qr = m/qr
    2/0.05 = m/qr
    40 = m/qr
    40 x 0.08 = m/q
    3.2 = m/q

    This is where I am stuck. I don't fully understand the relationship between m and q and other things that may be involved! I cannot really see a way around this - and this is precisely why I made this question up. I don't understand the basics but can do generic questions because I've practised solving general questions...

    Please help!
  2. jcsd
  3. Nov 15, 2008 #2


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    Right - folks, I have checked with my Physics teacher and he says this question is possible - i.e. it can be solved!!!!

    I am still stuck! He says we have to make a very educated jump from this part to the next! Any ideas??

    And I guess I could ask him this question but I don't understand when he explains things... :(
  4. Nov 18, 2008 #3


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    For any of you who wanted to know, you just had to find the ratio between the two - they were very specific ratios and allowed the lowest one to be 14/6 and then you just had to times 14 by the mass of proton to get mass of proton and neutron and times 6 by the mass of electron.
  5. Nov 18, 2008 #4


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    Science Advisor
    Homework Helper

    m/q = 3.2 you said you got, now that is 16/5.

    I am not sure if that is correct, I got ratio 4 of mass and q.

    you did:

    40 = m/qr
    40 x 0.08 = m/q
    3.2 = m/q

    but the radius of the unknown was 10cm, i,e 0.1m.. you should get 4, the same as I got.
  6. Nov 18, 2008 #5


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    Oh I used different figures - figures that worked well. So yeah, that's why I got a different number.
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