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Magnetic Interactions

  1. Jun 10, 2009 #1
    1. The problem statement, all variables and given/known data
    Shown at right is cross-sectional view of two long straight wires that are parallel to one another. One wire carries a current out of the page, the other carries an equal current into the page.
    Draw a vector on the diagram to show the direction of the magnetic field, if any, at point P. Explain your reasoning.
    Draw a vector on the diagram to indicate the magnetic force, if any, exerted on the current in the wires.

    I don't have the diagram here, but (using a coordinate system) the current going into the page is at (3,0), P is at (8,0) and the current out of the page is at (8,5).


    2. Relevant equations
    ??


    3. The attempt at a solution
    I really don't know how to do this. There are three questions like his one, so I am not looking for an answer, but more like an expiation of how to solve problems like this one.

    Help I am so confused :(
     
    Last edited: Jun 10, 2009
  2. jcsd
  3. Jun 10, 2009 #2

    drizzle

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    you assume these coordinations or are they illustrated in the diagram? either way you'll need to apply the right hand role
     
  4. Jun 10, 2009 #3
    They are from the diagram.
    So, that is all I do... use the right hand rule?
     
  5. Jun 10, 2009 #4

    drizzle

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    then you should consider the distance of the P point from both wires, however you'll have the net magnetic force directed towards [x,-y]
     
    Last edited: Jun 10, 2009
  6. Jun 10, 2009 #5

    dx

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    Find the directions of the magnetic field at P due to each wire using the right hand rule (you should find that they are perpendicular). Since the point P is equally distant from both wires, they will have the same magnitude.
     
    Last edited: Jun 10, 2009
  7. Jun 10, 2009 #6

    dx

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    Wrong.
     
  8. Jun 10, 2009 #7

    drizzle

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    yah, I've corrected it
     
  9. Jun 10, 2009 #8

    dx

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    Ok, but please don't give away the final answer.
     
  10. Jun 10, 2009 #9

    drizzle

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    I thought it was a hint, I didn't say the vector will make an angle of 45o from the x-axis in the forth quarter:biggrin:
     
  11. Jun 10, 2009 #10
    I am still really confused. If the current is going into the page, then the magnetic field could be at any angle on the page, as long as it was still perpendicular with the current.

    Do I say that the current is moving from the in to the out, thus moving 45 degrees to the left, and thus the magnetic field would be running 45 degrees to the right.

    Is that the correct way of doing it?
     
  12. Jun 10, 2009 #11

    dx

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    You were asked to find the direction of B at the particular point P. Forget about one of the currents for now. With just the current at (3,0), what is the direction of B at P? (Use the right hand rule.)

    No, that doesn't make much sense.
     
  13. Jun 10, 2009 #12
    Just at this point... is it moving straight up? (I point my thumb (which represents the current) out of the page... and my fingers are straight out)
     
  14. Jun 10, 2009 #13

    dx

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    The current is into the page at (3,0), so you point your thumb into the page. B therefore points straight down at this point.

    Also, your fingers should be curled, not straight out. They curl in the direction in which B curls around the wire.
     
  15. Jun 10, 2009 #14
    Oooh I see. Okay, that makes sense. So then at the point where the current goes out of the page B points straight up?

    So, does this mean that the magnetic field at P is zero?
     
  16. Jun 10, 2009 #15

    dx

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    No, the B due to the other current will point to the right. I think you're not applying the right hand rule correctly. Point your right thumb in the direction of the current, and curl your fingers into your palm. The direction in which your fingers curl is the direction in which B circles around the wire.
     
  17. Jun 10, 2009 #16
    That does not make sense to me. When my thumb is down, B points straight down. When my thumb is up, B points to the right?
    Shouldn't B be in the opposite direction, since I is?
     
  18. Jun 10, 2009 #17

    dx

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    You are aware that B circles around the wires right? The position of the wires is important too, to find the field at P. Why dont you draw a picture with both the wires, and the directions in which B circles around each. It is clockwise around the (3,0) wire, and anticlockwise around the (8,5) wire.
     
  19. Jun 10, 2009 #18
    I feel really stupid asking this, but if B circles around the wires, how can you draw a vector and say that B is straight down, or to the right? Isn't it technically moving around the wire, and not in any one direction specifically?
     
  20. Jun 10, 2009 #19

    dx

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    This is what I mean when I say B circles around the wire:

    em_01.gif

    At any given point, it has a direction. (B is a vector field, with a direction and magnitude at each point.)
     
  21. Jun 10, 2009 #20
    But what determines why when the current moves into the page it has a magnetic field that is straight down, and when a current moves out of the page it has a magnetic field that is to the right?
    This is so damn confusing!!!
     
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