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B-Field at a Perpendicular Distance

  1. Oct 23, 2009 #1
    Two long horizontal straight parallel wires are 28.28 cm apart and each carries a current of 2.2 A in the same direction, namely, due south. What is the B-field at a point that is a perpendicular distance of 23 cm from both wires? Answer in microtesla.

    B= (u0*I)/(2Pi*r)
    u0= 4Pi*10^-7= 1.26*10^-6

    23cm= 0.23m
    2.2A*2= 4.4A
    B= (1.26*10^-6)(4.4)/(2Pi*0.23)= 3.84*10^-6T= 3.84uT

    3.84uT is incorrect. I don't really know what to do here. I'm pretty sure I have the right equation, but the fact that it's two wires throws me off. Do I add the currents or subtract them? Does the 28.28cm come into play? Is my conversion from tesla to microtesla correct?
    Where am I going wrong? Any help would be greatly appreciated.
  2. jcsd
  3. Oct 23, 2009 #2


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    I think you are missing the 3D aspect of this. Draw two dots to represent the currents coming straight at you out of the page. Mark the distance between them 28.28 cm. Now, where is your point where you are calculating the field? Must be a little above or below the halfway point between the currents, right? Sketch arrows to show the direction of the field due to each current at that point. You'll see that you have a vector addition calculation to do in order to get the final answer.
  4. Oct 24, 2009 #3
    I didn't understand that at all. So I should draw something like this? And then the point is 23cm from the middle?

    ________________________<2.2A South______________


    ________________________<2.2A South______________
  5. Oct 24, 2009 #4


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    Find the magnetic field vectors at P due to A and B. Add them.
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