1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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: Electron [N] moving in a magnetic field [up]

  1. Apr 26, 2014 #1
    1. The problem statement, all variables and given/known data

    A magnetic field of 0.0200 T (up) is created in a region

    a. Find the initial magnetic force on an electron initially moved at 5.00 x 10^6 m/s [N] in the field

    b. What is the radius of the circular path? Make a sketch showing the path of the electron.

    2. Relevant equations

    Fm = q v b sinΘ

    Fc = (m v^2)/r

    3. The attempt at a solution


    F_M=q v β

    F_M=(1.6*10^(-19) C)(5.00*10^6 m/s)(0.00200 T)

    F_M=1.6*10^(-15) N

    my issue is the direction of the force. how do you apply the right hand rule to a charged particle moving in the same direction as the field? doesn't the particle need to travel perpendicular to the field to interact with it? because it's an electron i know i have to reverse it or use the "left hand rule" but i'm still utterly confused since the particle is moving in the same direction as the field.


    Fnet = Fm

    in circular motion Fnet = Fc

    Fc = Fm

    (m v^2)/r = q v β

    r = m v/q β

    r = (9.1 * 1^-31 kg)(5.00 x 10^6 m/s)/(1.6 x 10^-19 C)(0.00200 T)

    r = 0.142 m

    r = 14.2 cm
  2. jcsd
  3. Apr 26, 2014 #2

    rude man

    User Avatar
    Homework Helper
    Gold Member

    If the B field is in the same direction as the velocity of the electron then there is no force acting on the electron.

    Don't use a 'left-hand' rule. Use the right-hand rule and reverse the direction of the thumb if the scalar is negative, as in this case where F = -q v x B, q = +1.6e-19C.

    I like to set up an xyz coordinate system and use vector notation. That IMO is often a good idea when several vectors are involved.

    Also decide if B = 0.02T or 0.002T, and use B rather than b or β.
  4. Apr 29, 2014 #3
    sorry it's definitely 0.0200 T.

    so if the B field is in the same direction as the velocity of the electron...why are they even asking the question and why is there an follow-up part b to the question? I really don't understand this question. like conceptually i get it but i don't think it's possible and it can't be a trick question since there is a part b to it.

    lastly sorry i've just watched a lot of youtube videos and khang academy videos and they used that "left hand rule.
  5. Apr 29, 2014 #4

    rude man

    User Avatar
    Homework Helper
    Gold Member

    They did not say that the velocity and B fields are pointed in the same direction. They said the B field points up and v points north. That was your assumption I think.
    Well, as I said, it's not necessarily wrong but I would not want to start getting confused as to which hand to use in a given situation. Do what you're comfortable with.
  6. Apr 29, 2014 #5
    ok thank you for your advice. so if the B field points "up" do you think that if we had a x y z co ordinate system, up would refer to the positive z direction and north would be the positive y direction?
  7. Apr 29, 2014 #6

    rude man

    User Avatar
    Homework Helper
    Gold Member

    EDIT: (sorry!)
    I would pick x north, y east and z up.

    You can pick any coordinate system so long as it's right-handed. An example of a left-handed system would be x north, y up and z west.

    In unit vector terms, make sure that i x j = k.
    Last edited: Apr 29, 2014
  8. Apr 29, 2014 #7
    ok thanks. so specifically in relation to that question, they are talking about a 3-dimensional plane correct? (up) being the 3rd dimention or axis, (north) being the 2nd, and (east) for example being the 1st?
  9. Apr 29, 2014 #8

    rude man

    User Avatar
    Homework Helper
    Gold Member

    There is no such thing as a 3-dimensional plane. There is a 3-dimensional coordinate system.

    Any time you're dealing with a moving charge in a B field (F = q v x B) you are automatically dealing with a 3-d system unless v and B are in perfect alignment.
  10. Apr 29, 2014 #9
    ok makes sense now thank you!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted