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Magnetic/ electric/ gravitational field?

  1. Dec 4, 2014 #1
    I got a question from my exam. We are given the name of three fields; electric field, magnetic field and gravitational field. A charged particle is placed in a field, but no force will act on it if it's stationary or moving in any direction. What field is the particle placed in?
  2. jcsd
  3. Dec 4, 2014 #2
    Presumably you are intended to assume that say, the particle is massive and charged? If that is the case which of those forces would produce such motion.
    Last edited: Dec 4, 2014
  4. Dec 4, 2014 #3
    Yes, the particle is charged, but not exactly massive. Let's assume that it is a point charge. It's not specified whether is + or -

    In electrical and magnetic field, force will act on a charged particle as long as it is not perpendicular to the field. Which leaves only gravitational field. But then gravitational field exerts force too, right? They're pulling everything towards it's center.
  5. Dec 4, 2014 #4
    If the particle is not massive there is no solution.
    If it is the right combination of electric and a gravitational fields can do the job at rest.
    When it moves, its energy increases by a factor 1/sqrt(1-v^2) but so does its charge.
    So the solution is still valid.
  6. Dec 4, 2014 #5
    Ah, now I get it. Thanks for the clarification!
  7. Dec 4, 2014 #6


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    The charge is a Lorentz invariant, and does not change with motion.
    My guess is they wanted you to neglect the effect of GR.
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