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Homework Help: Magnetic Fields problem set

  1. Jul 30, 2006 #1
    I'm having trouble with 4 questions and not sure how to go about starting to solve them. If anyone can give me help on how to approach these questions, it'd be really appreciated. I just put them all down at once rather then making seperate threads. Thank you.

    1) An electrical transmission line that carries a DC of 100 A West is suspended between two towers 50m apart. The dip angle is 45 degrees and the magnetic field strength is 3.0x10^-5

    a) How far from the high voltage power lines do you have to be in order for the artificial magnetic field to balance Earth's magnetic field?

    b) If the transmission lines are 25m above the ground and all the physical parameters remain the same, calculate the exact moment of this cancellation.

    2) The two wires in a typical household extension cord are 2.4mm apart. What force per meter pushes them apart when 13.0 A. The rubber insulation has the same permeability as the air with 4pi x 10^-7

    3) A bullet travelling at 400m/s picks up a charge of 20 C. What is the maximum force exerted on the bullet by Earth's magnetic field (4.5x10^-5 T)?

    4) What is the magnitude and direction of the magnetic force on the proton moving vertically upward at 4.3x10^4 m/s in a 1.5-T magnetic field pointing horizontally to the west?
  2. jcsd
  3. Aug 1, 2006 #2


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    Staff: Mentor

    -1- I don't know why they try to give you a magnetic field strength of 3*10^-5 [units?] -- you need to calculate that based on the current in the wire(s). It's also a poorly stated problem, because they only refer to one wire and no return wire at some spacing away. Whatever. How do you calculate the magnetic field around a current-carrying conductor?

    (a) Do you know the magnitude of the earth's B field?

    (b) What is the exact wording of this question? What is meant by "moment"?

    -2- What is the equation for the force between two current-carrying wires?

    -3- Hint -- use the Lorentz force.

    -4- Same hint.
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