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This magnet question confounds me

  1. Mar 19, 2013 #1
    1. The problem statement, all variables and given/known data
    Today, my teacher gave us some practice AP multiple choice and my friend and I were debating between answer choices H and J (changed from A and B to remove any confusion) the question as i remember it:
    blah blah... B-field into the page, constant velocity v, radius R and charge q. which graph shows the relationship between B-field and radius.. something with radius getting smaller as B-field gets larger

    Choice H: this is a straight line coming from +y crossing the +x axis (basically a straight negative slope)

    Choice J: the (1/x) graph with x>0



    2. Relevant equations
    Fc = mv^2/r
    Fb= qvB


    3. The attempt at a solution

    Fc=Fb
    [itex]\frac{mv^2}{r}[/itex]=qvB
    [itex]\frac{mv}{r}[/itex]=qB
    since mv and q are constants, is it ok to treat them as 1? that is what i did so...

    [itex]\frac{1}{r}[/itex]=B
    [itex]\frac{1}{B}[/itex]=r

    since on the graphs r was the y-axis and B was the x-axis

    i chose the (1/x) shaped graph
     
  2. jcsd
  3. Mar 19, 2013 #2

    berkeman

    User Avatar

    Staff: Mentor

    Yes, it is known as the Lamor Radius:

    http://www-fusion.ciemat.es/fusionwiki/index.php/Larmor_radius

    :smile:
     
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