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Homework Help: Calculate magnetic field intensity

  1. Sep 23, 2013 #1
    1. The problem statement, all variables and given/known data
    Two infinitely long filaments are placed parallel to the x-axis as shown in Figure 1.
    a)Find H at the origin
    b)Find H at (-1,2,2)


    2. Relevant equations
    ∫Hdl=Ienclosed
    H=H1+H2

    3. The attempt at a solution
    Well following the equation above I get Hl= Ienc then H1=Ienc/(2piRy) and H2=Ienc/(2piRz) then Ry=(0,0,0)-(0,4,0)=(0,-4,0) then length of Ry=4 and Rz=(0,0,0)-(0,0,4)=(0,0,-4) then Rz=4 thus H1=Ienc/(8pi) and H2=Ienc/(8pi) then H=H1+H2=10/(8pi)+10/(8pi)=20/(8pi)=.795 A/m that is for part a) I only listed this as I used the same method for part b) just with (-1,2,2) instead of (0,0,0)
     
  2. jcsd
  3. Sep 24, 2013 #2

    rude man

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    Figure 1?
     
  4. Sep 24, 2013 #3
    whoops I was tired and forgot to add it
     

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  5. Sep 24, 2013 #4

    rude man

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    Use Ampere's law: ∫H*ds = i. Which you did, sort of.

    What you failed to do was to
    (1) pay attention to the signs of Ienc y and Ienc z. The two currents are in opposite directions.

    (2) compute Ry and Rz correctly. Given two points (x1 y1 z1) and (x2 y2 z2) what is the distance between them?
     
    Last edited: Sep 24, 2013
  6. Sep 24, 2013 #5
    (x1,y1,z1)-(x2,y2,z2)=(x1-x2,y1-y2,z1-z2) then Ry=(0,0,0)-(0,4,0)=(0,-4,0)
     
  7. Sep 24, 2013 #6
    right or no? I think that is what I did earlier uggg
     
  8. Sep 24, 2013 #7

    rude man

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    That's what you did, and it's wrong.

    Check your analytic geometry text or the Web.
     
  9. Sep 24, 2013 #8

    rude man

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    That's wrong. Furthermore, Ry is a distance and cannot be described by (x,y,z).
     
  10. Sep 24, 2013 #9
    well I found a thing on euclidean distance Ry=sqrt((0-0)^2+(0-4)^2+(0-0)^2)=4
     
  11. Sep 25, 2013 #10

    rude man

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    Much better.
     
  12. Sep 25, 2013 #11
    Ok so then Rz=sqrt((0,0,0)^2-(0,0,4)^2)=4 right?
     
  13. Sep 25, 2013 #12
    Ah the directions are different one in into the page and one is out. I can't remember does x mean into the page or out of the page?
     
  14. Sep 25, 2013 #13

    rude man

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    x is into the page and . is out of the page.
     
  15. Sep 25, 2013 #14
    Ok so on the current is clockwise and on the dot current is counter clockwise so in the z direction it is negative and y it is positive
     
  16. Sep 25, 2013 #15
    H=H1+H2=(10/2(pi)4)-(10/2(pi)4)=0
     
  17. Sep 25, 2013 #16
    That doesn't seem right?
     
  18. Sep 25, 2013 #17

    rude man

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    Right.
     
  19. Sep 25, 2013 #18
    Ok well now that my distances are confirmed correct is my new answer correct?
     
  20. Sep 25, 2013 #19

    rude man

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    Currents don't run in circles. Currents run in the wires which are straight.
     
  21. Sep 25, 2013 #20

    rude man

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    No.
    H is a vector, that's why I use bold type for it.

    Take one wire at a time and determine the direction of H for it using your pix. You have the magnitudes right.
     
  22. Sep 25, 2013 #21
    well it seems to be flowing into the page on the y axis and out of the page on the z axis. So on the y-axis the H vector would go left. Then I am guessing on the z axis since they are opposite the other H field must be to the right.
     
  23. Sep 25, 2013 #22

    rude man

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    Why left on the z axis? Do you know the right-hand rule?
    Same comment for the z wire.
    Draw circles around each wire intersecting with the origin, then use the rt-hand rule.
     
  24. Sep 25, 2013 #23
    Well for the one into the page the B field will rotate clockwise and out of the page the B field will go counterclockwise. I said left and right because you have said something about the wires being straight and not circular so I guess I did not understand what you meant?
     
  25. Sep 25, 2013 #24

    rude man

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    That is correct. So, taking the one into the page first, what direction does the H field point at the origin?
     
  26. Sep 25, 2013 #25
    well since H is the magnetic field strength then it should turn in the same direction as the B field right so wouldnt it point in the same direction as the B field im confused?
     
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