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Homework Help: Magnetic Field / Force problem

  1. Mar 4, 2012 #1
    1. The problem statement, all variables and given/known data
    a particle with a charge of -1.24 x 10^-8 c is moving with instantaneous velocity v= 4.19 x 10^4 i + -3.85 x 10^4 j (both in m/s). what is the force exerted on this particle by a magnetic field a) 1.4 T i and b) 1.4 T k ?

    2. Relevant equations

    F = qv x B (cross product not multiplication)

    3. The attempt at a solution

    I tried using a determinate for part a where you put i j k and the velocity vector multiplied by q and then the magnetic field vector underneath that but i got -6.68 x 10^-4 i, however the books answers says that it is in the -k direction.

    I don't understand the way my physics teacher does the cross multiplication that's why i tried using a determinate like in calculus....please help?!?!
  2. jcsd
  3. Mar 4, 2012 #2


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    Can you show your work ?
  4. Mar 4, 2012 #3
    sure, this is what i was trying to do: for part a)

    i , j , k
    q(4.19x10^4) , q(-3.85x10^4) , 0
    1.4 , 0 , 0

    (i added commas in-between to not confuse)

    i did the cross product using this determinate thinking it would give me the right answer and i got the right number just the wrong direction....

    q(-3.85x10^4)(1.4) in the i direction, the book says -k direction....
  5. Mar 4, 2012 #4


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    Use the code icon, https://www.physicsforums.com/Nexus/editor/code.png [Broken] , to display items with lots of spaces, like your determinant ... or simply use [CODE ] [/CODE ] tags.

    Code (Text):

    i          ,                j            ,        k

    q(4.19x10^4)  ,  q(-3.85x10^4)  ,    0

    1.4           ,            0              ,     0
    You are calculating your determinant incorrectly.

    what you multiply i by,
    what you multiply j by,
    what you multiply k by;​
    including zeros, & prior to simplifying anything.
    Last edited by a moderator: May 5, 2017
  6. Mar 5, 2012 #5
    I just figured out what i did wrong....i crossed out the columns and rows backwards....i didnt realize until you told me to write everything out lol thanks ....stupid mistake on my part
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