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Homework Help: Answer didnt check with back of book

  1. Apr 22, 2008 #1
    hey,

    I'm not getting the answer matching the back of the book for these questions, can you show me how to do it with your work?

    A long wire is known to have a radius greater than 4.0 mm and to carry a current uniformly distributed over its cross section. If the magnitude of the magnetic field is 0.285 mT at a point 4.0 mm from the axis of the wire and 0.200 mT at a point 10 mm from the axis, what is the radius of the wire?

    and

    A long solenoid (n = 1200 turns/m, radius = 2.0 cm) has a current of a 0.30 A in its winding. A long wire carrying a current of 20 A is parallel to and 1.0 cm from the axis of the solenoid. What is the magnitude of the resulting magnetic field at a point on the axis of the solenoid?
     
  2. jcsd
  3. Apr 22, 2008 #2
    Can you show us what you tried and what answers you got (and how) ?

    For the first question, Im assuming you calculated the magnetic field using
    [tex] \int \vec{B} \vec{dl} = \mu_0 I_{\text{enclosed}}[/tex]

    Did you keep in mind that, at least for the 4mm radius, the enclosed current is NOT the total current?
     
  4. Apr 22, 2008 #3
    Finally find the right latex code, sorry :p
    [tex]\oint \vec{B} \cdot \vec{dl} = \mu_0 I_{\text{enclosed}}[/tex]

    (Why can't I edit my post??)
     
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