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: A rather tricky magnetic field problem.

  1. Mar 17, 2008 #1
    1. The problem statement, all variables and given/known data
    A proton moves through a region of space where there is a magnetic field [tex]\vec{B}[/tex]= (0.61[tex]\hat{i}[/tex] + 0.36[tex]\hat{j}[/tex]) T and an electric field
    [tex]\vec{E}[/tex]= (2.8[tex]\hat{i}[/tex] - 4.3[tex]\hat{j}[/tex]) *10[tex]^{3}[/tex] V/m. At a given instant, the proton's velocity is [tex]\vec{v}[/tex]= (6.3[tex]\hat{i}[/tex] + 2.9[tex]\hat{j}[/tex] - 4.8[tex]\hat{k}[/tex]) *10[tex]^{3}[/tex] m/s.

    Determine the components of the total force on the proton.
    F[tex]_{x}[/tex]= ?, F[tex]_{y}[/tex]= ?, F[tex]_{z}[/tex]=?

    2. Relevant equations
    I assume what needs to be used is the "Lorentz Equation" which is:
    [tex]\vec{F}[/tex]= q([tex]\vec{E}[/tex] + [tex]\vec{V}[/tex] * [tex]\vec{B}[/tex])
    and use the q of a proton which is 1.6*10[tex]^{-19}[/tex].

    3. The attempt at a solution
    I attempted to use a cross product of [tex]\vec{V}[/tex] and [tex]\vec{B}[/tex] and it seemed to be on the right track but I ended up getting stuck on what to do with [tex]\vec{E}[/tex] and honestly I ended up getting stuck on the whole problem and not sure at this point where to start and end.
  2. jcsd
  3. Mar 17, 2008 #2


    User Avatar
    Homework Helper
    Gold Member

    Your on the right track.

    HINT: Try starting by working out the cross product and getting it in terms of components. Then, group all the x component terms together, all the y component terms together, etc. (This includes the E field terms as well.) Do you see how you can get your answer from here on out?
  4. Mar 17, 2008 #3
    How'd you get stuck on E? V cross B times the scalar q gives you a vector, and q*E is a vector. Do you remember how to add two vectors?

    Then you'll have the force vector, with its x, y, and z components
  5. Mar 17, 2008 #4
    I would say I am a bit rusty when it comes to vectors, I did attempt to do the basic vector addition and the answer I imputed the online homework application disliked lol.
  6. Mar 17, 2008 #5
    I got the answers


    and the homework program told me to check my signs, and told me I was incorrect. What did I do wrong?
  7. Mar 17, 2008 #6
    I keep trying to do the cross product, then adding the vectors together and the third vector never seems to work out right.
  8. Mar 17, 2008 #7
    I havent been doing vectors much, but isnt it just possible to take each composant at a time? So for example Fi=Ei*Q + Q*Vk*Bj. Then no vector addition is required.
  9. Mar 17, 2008 #8
    Out of curiousity, what then would be the setup for Fj and Fk?
  10. Mar 17, 2008 #9


    User Avatar
    Homework Helper
    Gold Member

    What do you get when you work out v X B?
  11. Mar 18, 2008 #10
    If my theory is correct, it should be like:

    it seems like the the +/- depends on how you define the positive directions for y/x/z in k/i/j, so im not sure on these.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook