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    (Original post by JF-17)
    In a demonstration, electrons with 200eV of kinetic energy are going round in a 12cm diameter circle. Calculate the magnetic flux density. Give your answer to 2 significant figures.
    For this question you literally just need to plug in some numbers but lets derive the answer.
    The Lorentz force is given by
     F=qvB
    The centripetal force for the electrons is
    F=mv^2/r
    Equating these two forces since it is the magnetic field which provides the force on the electrons so that they follow the circular path.

     qvB=mv^2/r
     B= mv/qr

    So you use this formula to calculate the magnetic field, using the mass and charge of an electron
 
 
 
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