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Homework Help: Find magnetic field at a distance from wire

  1. Oct 30, 2016 #1
    1. The problem statement, all variables and given/known data
    A wire with radius R= .01 m carries a current with uniform density. At a distance r1 =1.25m from the wire, the magnetic field has a magnitude of 2.55 μT. Calculate the magnitude of the magnetic field r2=.0065m from the wire.

    2. Relevant equations
    B= (μ0I)/ 2πr

    3. The attempt at a solution
    So first I did:
    B1=(μ0I)/( 2πr1)
    Solving for I:
    I = (B12πr1)/μ0
    Plugging it into the B2 equation:
    = (B1r1)/r2
    =4.9 x 10^-4 T

    I'm not sure if I did this right since other people in my class got something different. Can someone corroborate this? Thanks.
  2. jcsd
  3. Oct 30, 2016 #2


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    Hello. Keep in mind that the equation B= (μ0I)/ (2πr) is only valid for points outside the wire.
  4. Oct 30, 2016 #3
    Oh I see, I would have to use the equation for the magnetic field inside the wire for B2:

    B2= (μ0Ir2)/ (2πR2)

    Plugging I into this equation:
    B2= (μ0B12πr1r2)/ (2πR2μ0)
    =(B1r1r2)/ R2
  5. Oct 30, 2016 #4


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    That looks correct to me.
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