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Homework Help: Crossed fields

  1. Jun 29, 2017 #1
    1. The problem statement, all variables and given/known data
    A proton moves with constant velocity in a certain region of gravity-free space.Which one of the following conclusions is valid?
    A)There must be no electric field or magnetic fields in the region.
    B)There could be an electric field without any magnetic field.
    C)There could be a magnetic field without any electric field.
    D)There can be an electric field as well as magnetic field.
    2. Relevant equations
    Force on a charged particle=(charge on particle)× (Vector cross product of it's velocity vector and Magnetic field vector)
    F=qE,for a particle in an electric field.
    If the 2 fields(magnetic and electric) are perpendicular to each other,then they are said to be crossed fields,and their effects cancel out each other.
    3. The attempt at a solution
    I know D is correct for sure,that's what crossed fields is all about.
    If we apply an electric and a magnetic field at right angles to each other,their effects cancel out each other and the velocity of the particle will remain unaffected.
    But,the answer is given (C), (D)

    How is C valid?
    A magnetic field,on it's own should change the direction of the particle(hence changing it's velocity).
    Hence,my problem.. (does it have something to do with gravity free space??)
    Help appreciated..
  2. jcsd
  3. Jun 29, 2017 #2


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    Staff: Mentor

    Consider the formula you stated for the force due to the charge moving through a magnetic field. Why are there vectors involved? :wink:
  4. Jun 29, 2017 #3
  5. Jun 29, 2017 #4


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    OK. What is your overall conclusion ?
  6. Jun 30, 2017 #5
    Well,I didn't notice that if you place a magnetic field in the direction of the particle's velocity,the net force on it becomes zero(Cross product of two parallel vectors is zero),and there is obviously no electric field involved.So,C is also correct,hadn't thought of that before
  7. Jun 30, 2017 #6


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    Excellent !
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