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Homework Help: Find a point where this magnetic field is zero

  1. Mar 31, 2005 #1
    Two insulated wires, each 2.4 m long are taped together to form a two wire unit that is 2.4 m long. One wire carries a current of 7.00 A; the other carries a smaller current I in the opposite direction. The two wire unit is placed at an angle of 65.0 degrees relative to a magnetic field whose magnitude is 0.360 T. The magnitude of the net magnetic force experience by the two wire unit is 3.13 N. What is the current I?

    A square coil and a rectangular coil are each made from the same length of wire. Each contains a single turn. The long sides of the rectangle are twice as long as the short sides. Find the ratio of torque of the square over the torque of the rectangle of the maximum torques that these coils experience in the same magnetic field when they contain the same current

    Two long, straight parallel wires are separated by a distance of one meter. They carry currents in opposite directions, and the current in wire A is one-third of that in wire B. On a line drawn perpendicular to the wires, find the point where the net magnetic field is zero. Determine this point relative to wire A.

    I have no clue at all how to do these questions or what unit they're from, i'm doing some studying and reviewing and these were on our last assignment but i didnt do them cuz i didnt know how... can someone give me some forumlas or something i could use to approach it?
  2. jcsd
  3. Mar 31, 2005 #2
    First question: the force on a current-carrying is F = LIxB, where L is the length of the wire, I is the current it carries, and B is the magnetic field. Note that you are taking the cross product of the I and B (in this order), so the direction of I has meaning. Also, two current-carrying wires exert force on each other (because each is creating a magnetic field around itself, and by that causing the other wire to experience a force), but in this case the two wires are considered one unit, so the internal forces can be ignored (I think, but I may be wrong).
  4. Mar 31, 2005 #3
    thnx F=ILxB makes good sense.

    now for the other two.. for the second and third one, they're just ratios? no numbers?
  5. Mar 31, 2005 #4
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