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Homework Help: T or F: Magnetism and Magnetic Field

  1. Mar 10, 2010 #1
    1. The problem statement, all variables and given/known data
    A 2 kg particle carrying a charge of 97 uC enters a uniform 7 T magnetic field at a speed of 26 m/s and with an angle of 52o with respect to the field lines, as shown in the figure. Answer the following questions:
    10fnpxd.gif
    A) The work done by the field on the particle is zero as the force is normal to the displacement.
    B) The y-component of the particle's velocity is unchanged as it passes through the B-Field.
    C) The Force on the particle is in the -z direction.
    D) The particle follows a circular path as the force is normal to the velocity.
    E) The particle's speed is unchanged as it passes through the B-Field.

    3. The attempt at a solution
    A) The work done by the field on the particle is zero as the force is normal to the displacement.T but I'm not sure.
    B) The y-component of the particle's velocity is unchanged as it passes through the B-Field.F because the field lines are perpendicular to the y-component. Parallel components remain unchanged.
    C) The Force on the particle is in the -z direction.No idea
    D) The particle follows a circular path as the force is normal to the velocity.No idea
    E) The particle's speed is unchanged as it passes through the B-Field.T because B can only change the direction of the velocity.

    Any help is much appreciated!
     
  2. jcsd
  3. Mar 11, 2010 #2
    How do you find the direction of the force, if you know the direction of the field and the direction of the velocity of the particle? You need this for A and C

    for D, look at your answer for B (wich is correct)
    How can you answer E be not sure about A?
     
  4. Mar 11, 2010 #3
    I don't know. I assume I have to use the right hand rule, but I don't get how that works. The particle is moving through the field at 52 degrees, but work is zero if the displacement and force are perpendicular to each other. I don't see how work can be zero because isn't the displacement going to be 52 degrees on every line? I am so confused.
     
  5. Mar 12, 2010 #4
    Whatever the angle of the field and the velocity, it's always possible to have a
    force that's perpendicular to both.
     
  6. Mar 12, 2010 #5
    So I figured out the answers:
    A) True, because of what you just stated.
    B) False, because the direction is always changing. The B-field is constantly moving.
    C) True, because of the right hand rule. -z is pointing into the page.
    D) False, because the B-field is always moving, so the particle would follow a spiral-like shape instead of circular.
    E) True, because if work is zero then speed can not change.

    Thanks for the help!
     
  7. Mar 12, 2010 #6
    The B field isn't moving
    but the direction of the force on the particle is.

    The particle follows a spiral and not a circle, because the x component of its velocity is in the direction of the B field and the force on the particle can't have a component in the x direction, so the x component of the velocity can't change.
     
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