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Homework Help: Direction of Magnetic Field when given velocity and force

  1. Feb 26, 2010 #1
    1. The problem statement, all variables and given/known data
    An electron, moving west, enters a magnetic field of a certain strength. Because of this field the electron curves upward. What is the direction of the magnetic field?

    2. Relevant equations
    Right Hand Rule

    3. The attempt at a solution
    I don't understand what the question means by the electron curving "upward" - does that mean curves north, or curves out of the page?
    If you start with west velocity (fingers) and want to end up with a force directed north, your magnetic field would go out of the page (then the thumb would point up). But the answer claims it is "magnetic field is towards the north". How can this be true? The magnetic field and the force can't be in the same direction? And if the magnetic field is north, and the velocity is west, the force would push the electron into the page, which since it is negative would really be out of the page. So, I guess when it says the electron curves upward, they actually mean out of the page?

  2. jcsd
  3. Feb 26, 2010 #2


    User Avatar
    Homework Helper
    Gold Member

    I'm pretty sure it means out of the page. The question already mentioned the direction "West." So I think it's reasonable to assume that if the question meant North, it would have said "North," not upward. So we can safely assume that the force is upward, out of the page (if the page is sitting flat [horizontally] on the desk).

    Right! There ya go. :approve:

    Btw., judging by your answer assuming that the force was North, you might be forgetting that electrons are negatively charged. Don't forget about the sign. :wink:
  4. Feb 26, 2010 #3
    Yeah I realized that I forgot about the sign. But the whole "upward" thing confused me, it's easier when they stick to in and out of the page. Thanks a ton for your help!!
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