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Homework Help: Current from stream of particles

  1. Feb 19, 2012 #1
    1. The problem statement, all variables and given/known data
    An α (Alpha) particle consists of two neutrons and two protons bound together into a single particle with a charge of +2e. We normally measure the kinetic energy of such particles in millions "electron volts" or MeV. (1 eV = 1.6 X 10-19J).

    Assume a steady stream of α particles travels with a constant energy of 20 MeV and carries a current of 2 μA.

    a) If the beam is directed perpendicular to a plane target, how many α's will strike this target in 5 seconds?
    b) At any given instant, how many α's are in a 15 cm length of the beam?
    c) Through what electrical potential diffence [sic] is it necessary to accelerate each α from rest to bring it to the energy of 20 MeV?

    2. Relevant equations

    v = ir
    r = p*l/a
    E = .5*mv^2

    3. The attempt at a solution

    I have the velocity of the particles.
    But I don't know how many particles go through a point in 1 second.

    if I can get a, b falls in to place.

    but I have no idea how to start c
  2. jcsd
  3. Feb 19, 2012 #2


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    That is what the current gives you. 1 Ampere = 1 Coulomb/second
  4. Feb 19, 2012 #3
    *facepalm* duh!

    the current on the stream is 2 uA, so we have 2*10^-6 *1/(2e) := N1 particles going through a point
    the unit is Particles/S
    multiply that by 5, to get the number of particles passing through a point in 5 seconds

    the velocity is about 30000000 m/s. so, it takes .15/3e7 seconds to cover 15 cm
    in .15/3e7 seconds, there are (.15/3e6)*(N1) = 30287 particles going through a point

    lastly, to bring a particle to V = U/q, so V = 20 MeV / (2eV) = 10000000
  5. Feb 20, 2012 #4


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    Happens to me all the time ;)
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