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Homework Help: Direction of Magnetic Field

  1. Jan 10, 2013 #1
    1. The problem statement, all variables and given/known data
    https://dl.dropbox.com/u/63664351/Physics/ELEC1.PNG [Broken]

    2. Relevant equations
    https://dl.dropbox.com/u/63664351/Physics/Magnetioc%20Force%20Between%20Wires.PNG [Broken]

    3. The attempt at a solution
    We want the tension to be zero.
    But since the force of gravity going downward, then magnetic force in wire 2 must goes up.
    But magnetic force in wire 2 goes up only if current going to the right and magnetic field goes entering the screen . My question. How do I know for sure that the magnetic field is entering the screen, not out of the screen?

    Also from the relevant equation image there is something I don't understand in wire 1.
    Isn't by right hand rule, if thumb is thrust and middle finger the current, then B must be out of the screen? but why in wire one the direction is looping?
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Jan 10, 2013 #2


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    2017 Award

    Staff: Mentor

    You can determine the direction of magnetic field with the right-hand-rule (or just look at your illustration).
    If it goes out of the screen, you need the opposite direction of current flow.

    There is a shorter way to find the direction: Currents are attractive if they flow in the same direction (in parallel wires), which you can see in your image as well.
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