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Magnetic Field Homework Question

  1. Mar 1, 2006 #1
    Two long, straight wires are oriented perpendicular to the computer screen, as shown in Figure 22-43, in which L = 6.0 cm. The current in one wire is I1 = 3.7 A, pointing into the screen, and the current in the other wire is I2 = 4.0 A, pointing out of the screen. Find the magnitude and direction of the net magnetic field at point P.


    [​IMG]

    ok so first off i used pythagorean theorem to find the straight line distance from point to the wire coming out of the page and used that as [tex]r_{2}[/tex]. then, i used the formual for magnetic force to find the force on point P from the two wires [tex] B = \mu_{0}I_{1}/2 \pi r_{1}[/tex] and [tex] B= \mu_{0}I_{2}/2 \pi r_{2}[/tex] and added the results. The answer i'm getting is wrong.. any suggestions?? i know the current in each of the wires are going in opposite directions so the forces repel but how does that relate to a point??
     
    Last edited: Mar 1, 2006
  2. jcsd
  3. Mar 1, 2006 #2
    for a more detailed description of what i did:

    [tex]B= (4 \pi x 10^-7)(3.7A)/(2 \pi (.06m) + (4 \pi x 10^-7)(4.0A)/(2 \pi (.0848m))[/tex] which gave [tex]2.1767x10^-5 T[/tex]
     
    Last edited: Mar 1, 2006
  4. Mar 1, 2006 #3

    Meir Achuz

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    You have to add the two B's vectorially. B1 is horizontal.
    B2 makes and angle of 45 degrees.
     
  5. Mar 1, 2006 #4
    like this??:

    [tex]I_{1}_{y}[/tex]= (4[tex]\pi[/tex]x 10[tex]^-7[/tex])(3.7)/(2[tex]\pi[/tex](.06)) which gives 1.233x10[tex]^-5[/tex]

    [tex]I_{1}_{x}[/tex]= 0

    then, [tex]I_{2}_{y}[/tex]= (4[tex]\pi[/tex]x 10 [tex]^-7[/tex])(4.0)/(2[tex]\pi[/tex](.0848)(sin 45[tex]^0[/tex])

    [tex]I_{2}_{x}[/tex]= (4[tex]\pi[/tex]^-7[/tex](4.0)/(2[tex]\pi[/tex](.0848)(cos 45[tex]^0[/tex])

    then use pythagorean theorem again to find the total force??
     
    Last edited: Mar 1, 2006
  6. Mar 2, 2006 #5

    Meir Achuz

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    I_1y=0. I_1x=what you have for I_1y.
    I_2x and I_2y are both negative.
    Then use Phyth for magniktude of B.
    You should know the right hand rule to give you the correct direction of
    B from each wire.
     
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