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Homework Help: Group of partcles in a magnetic field

  1. May 22, 2012 #1
    1. The problem statement, all variables and given/known data

    A group of particles is traveling in a magnetic field of unknown magnitude and direction. You observe that a proton moving at 1.60 km/s in the + x-direction experiences a force of 2.10×10−16 N in the + y-direction, and an electron moving at 4.30 km/s in the - z-direction experiences a force of 8.50×10−16 N in the +y-direction.

    A) What is the magnitude of the magnetic field?

    B) What is the direction of the magnetic field? (in the xz-plane) (from the -ve z direction)

    C) What is the magnitude of the magnetic force on an electron moving in the - y-direction at 3.40 km/s?

    D) What is the direction of this the magnetic force? (in the xz-plane) (from the -ve x direction)

    2. Relevant equations


    3. The attempt at a solution

    I tried using the above formula to no success
  2. jcsd
  3. May 22, 2012 #2


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    You tried; okay, can you show us your attempt?
  4. May 23, 2012 #3


    F/q=1600i x B

    F/q=-1600j + 1600k

    (2.1x10-16/1.6*10-19)/1600 = B

    Then used Pythagoras to calculate the magnitude of B
  5. May 23, 2012 #4


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    Where does this 2nd equation come from? You don't know what [itex]\mathbf{B}[/itex] is; that's what you're supposed to calculate.

    What you do know is that [tex]\mathbf{F}=q_{\text{proton}}(1600\text{km/s})\mathbf{i}\times \mathbf{B}=(-2.1\times 10^{-16}\text{N})\mathbf{j}[/tex] as well as a similar equation for the force that the electron experiences.

    I'd suggest that you let [itex]\mathbf{B}=B_x\mathbf{i}+B_y\mathbf{j}+B_z\mathbf{k}[/itex] and carry out the cross product in both your equations and compare terms to solve for the components of [itex]\mathbf{B}[/itex]
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