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Homework Help: Electric field created in a bar by electrons deflected in a magnetic field

  1. Apr 21, 2010 #1
    Hi everyone, first post here. Not sure if I'm posting in the right place but here goes. This question is from a sample AP Physics C E&M exam. It's kind of hard to explain without a diagram so I'll link to the exam:

    Here is the link: http://apcentral.collegeboard.com/apc/public/repository/ap09_frq_physics_cem.pdf"
    Answer Key: http://apcentral.collegeboard.com/apc/public/repository/ap09_physics_c_e_and_m_sgs.pdf"

    1. The problem statement, all variables and given/known data
    It's E&M #2 parts e and f. Starting with this: "The electrons moving through the bar are initially deflected by the external magnetic field. On the diagram below, indicate the direction of the additional electric field that is created in the bar by the deflected electrons." See the link. I understand up to part d but I'm stuck here.

    2. Relevant equations
    For part e, its more of a qualitative analysis using the right hand rule (I think). For part f, they use F = qE and F = qvB in the answer key but I'm not entirely sure why that is or how they get that solution.

    3. The attempt at a solution
    For part e, looking at the diagram, conventional current flows from a to b. Thus, electrons should flow from b to a. From the right hand rule, with the magnetic field running into the page, the force on the electrons should push them upwards. Thus, I get the electric field to be pointing downwards since electrons are negatively charged. However, the answer key says upwards. I have a feeling this has something to do with the electrons not actually causing the field or something like that.

    For part f, well, I'm not quite sure where to begin. Why does the electric force balance the magnetic force as stated in the answer key?

    It would be great if you could provide a link where I could read up on this.

    Thanks in advance!
    Last edited by a moderator: Apr 25, 2017
  2. jcsd
  3. Apr 22, 2010 #2
    Never mind I figured it out. Since electrons are pushed to the top of the bar, the electric field pulls upwards. After they reach equilibrium, new flowing electrons will have to balance out the magnetic field and the electric field thus causing no net vertical force or movement (on average).
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