Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Magnetic Force Question Really givin me a heart burn

  1. Dec 8, 2004 #1
    1)A beam of electrona travels along an evacuated tube towards a floriscent screen.A coil is placed around the tube producing magnetic field along the axis of the tube.

    a)describe and explain the motion of the beam if:

    1)it is exactly parallel to the axis of the tube
    My working:
    I think it will go in a circular but dunno why

    2)If it makes a slight angle with the axis of the tube

    Any kinda help wud highly be appriciated

    2)A marble toy has a vertical circle of track of diameter D.V1 is the speed of approach which just allows the marble to complete the loop.

    (small marble) o --------->V1 O |Diameter of big marble

    a)state the force producing the centripetal acceleration at the top of the loop in this case.
    ans;I think its the weight,isnt it?

    b)write down an energy conservation equation relating the velocity of approach V1,and the marble's velocity at the top of the loop ,V2 .assume that the track is friction less

    c)write down the centripetal force equationfor the marble at he top of the loop and rearrange it to give an expression for V2.

    Any help wud highly be appriciated
  2. jcsd
  3. Dec 8, 2004 #2
    The equation for magnetic force on a charged particle is proportional to the cross product between the velocity vector and the magnetic field. What are the properties of the cross product ?
  4. Dec 8, 2004 #3
    i figured out what to do when it is 90--the force is acting downwards..due to this force the electron follows the circular path...
    what happens when it is not 90 degree

    thanks in advance
  5. Dec 8, 2004 #4


    User Avatar
    Science Advisor

    then decompose the velocity into a component parallel to the magnetic field and another component perpendicular. You know what each component does, so you should be able to imagine what the sum does.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook