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Magnetic Field required to seprate to ions

  1. Feb 17, 2012 #1
    Magnetic Field required two seprate to ions

    1. The problem statement, all variables and given/known data

    Consider a mass spectrometer used to separate the two isotopes of uranium, 238U3+ and 235U3+. Assume the ions enter the magnetic field region with a speed of 5.30X10^5 m/s. What value of B is required to give a separation of 0.6 mm when the ions leave the spectrometer?

    2. Relevant equations

    L = 6*10^(-4) m and B = ? T v = 5.30*10^5
    e = 1.602*10^(-19)
    F = qvXB

    3. The attempt at a solution

    I know that F = qvXB q = 92e since e = 1.602*10^(-19) so F/qv = B ,
    q = 92(1.602*10^(-19)), v = 5.30*10^5

    I am not sure how to calculate the force

    but I get confused because they are both ions they would have a positive charge and want to repel so why should they attract and I do not know where to take it from here.
    Last edited: Feb 17, 2012
  2. jcsd
  3. Feb 17, 2012 #2
    Just treat the ions as going through at different times, and get the separation if you could have a switch that shot out one ion or the other.
  4. Feb 18, 2012 #3
    Do you realise that the ions will be travelling in a circular arc and the force you have calculated is the value of the centripetal force?
    In a mass spectrometer the ions travel in a semi circle to reach the detector.
    Because of the slight difference in mass the radius of each path will be different ( the centripetal force is the same)
    This should enable you to calculate the difference in radius and therefore the separation over the semi circle.
    Hope this helps
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