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Homework Help: Figuring out how to approach an electrostatics questioni

  1. Oct 21, 2016 #1
    1. The problem statement, all variables and given/known data

    My problem is in figuring out how to solve number 22 through 23, particularly getting the set-up done.

    2. Relevant equations

    Newton's law of gravitation and coulomb's law of acceleration

    3. The attempt at a solution

    I initially misread the question and thought that there was "a neutron and a proton initially at the same distance from a relatively large nucleus moving at a constant velocity."

    OK. The nucleus moving at the constant velocity will not have any coulombic forces exerted on it because it has a neutral charge on it, so the only way it can be moving at a constant velocity is if the sum of the gravitational forces applied on the nucleus is zero. This fits perfectly into my scenario with just a proton and neutron: having a proton and a neutron of equal masses at the same distance on the opposite sides of the moving nucleus

    PROTON <------------------ distance "D" -----------> Nucleus moving upwards <--------------------distance "D"-----------> Stationary neutron.

    This would ensure that the gravitational forces that are applied onto the moving neutron would cancel each other out laterally and the neutron would move at a constant velocity. However, how can I add in an electron into this mix such that the gravitational force that it exerts will also cancel out with the other gravitational forces so that the nucleus would be able to move at a constant velocity?

    And how would I conceptualize and draw a set-up for this problem so as to get started in solving the rest of the problems?

    Thanks in advance for the help. It is greatly appreciated
  2. jcsd
  3. Oct 21, 2016 #2


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    Staff: Mentor

    The question's setup is flawed as it claims that the nucleus is both stationary and moving. We cannot ascribe the constant velocity to all three of the elementary particles because the Coulombic force interactions will accelerate at least two of them. So we have to conclude that the question author meant for the relatively large nucleus to be moving at a constant velocity and massive enough that any acceleration it feels as a result of interacting with the elementary particles of interest will be negligible.

    You might want to rethink that. The nuclei of atoms are all positively charged, being comprised of (essentially) protons and neutrons; It's this positive charge that is responsible for attracting and holding electrons to the atom. I think we can completely ignore external gravitational forces here as no outside masses or gravitational fields were given. A large nucleus (like that of a gold atom for example) might have the mass of a couple of hundred protons or neutrons.

    So you have a large positively charged nucleus coasting along at a constant velocity and three comparatively light particles all at some same unspecified initial distance. The object of the question is for you to determine the relative effects of any forces acting between the nucleus and the individual particles.
  4. Oct 22, 2016 #3
    We will have a force of repulsion between the proton and the nucleus and an equal force of attraction between the nucleus and the electron. No electrostatic charge between the nucleus and the neutron exists.

    Thus we have the set up with the nucleus being pushed towards the electron as we have a spatial orientation of the particles being (I am trying to use the text to "draw" my visualization of this")

    nucleus ----------- neutron

    with the nucleus going downwards towards the electron.

    Thus for 22, the answer is C as the electron be the first particle the nucleus hits.

    23) will be D as both the proton and the neutron have the same charges, so the different coulombic forces will be the same.

    24) the electron will experience the greatest acceleration as it has the same of the coulombic forces and the least mass.

    Am I right in my answers/approaches? Thanks!
  5. Oct 22, 2016 #4


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    Staff: Mentor

    Your results are fine (although you mixed up electron and neutron in your description for #23).
  6. Oct 23, 2016 #5
    All righty! Thanks for the help!!!
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