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Confused on Magnetic fields

  1. Nov 12, 2005 #1
    confused on Magnetic fields!!

    Hello everyone, we just started a chapter on magentic fields due to currents and i'm pretty lost. I'm looking at the homework and its making no sense to me..
    I read the section and yet its not helping either. It gives lots of formulas but doesn't really explain how to interpret them. For instance:
    Figure 29-25 shows four arrangements in which long parallel wires carry equal currents directly into or out of the page at the corners of identical squares. Rank the arrangements according to the magnitude of the net magnetic field at the center of the square, greatest first. (Use only the symbols > or =, for example a=b>c>d.)
    I said:
    which was wrong

    How could u apparoach this? I understand if a current is going into the page and another current is going out of the page, u get a repulisve force, and if they are both going in the same direction, in or out of the page, u get an attractive force, but whats that mean about magntiude?

    i'm suppose to figure out:
    f the net magnetic field due to the two currents is to be zero at point P, describe the current i2 in the right hand wire. (Select all that apply.)
    into the page
    out of the page
    i2 > i1
    i2 < i1
    i2 = i1

    I don't want the answers, I would like someoen to epxlain to me how to intrepert this, or can any of you direct me to a webpage that explains the rules? Thanks!
  2. jcsd
  3. Nov 12, 2005 #2


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    Gold Member

    If the current goes out of the page then the field goes counter-clockwise around it, if it goes into the page the it goes around clockwise. So if we have a wire with a current going out. To it's right it would make a field going up and to it's left going down.
    Now if there're two wires with currents in the same direction then the field in the middle of them is zero, since ,for example, the left wire makes a field that is going up in that position(to the right of the left one), and the right one makes a field going down and they cancel out. If their currents are in oppisite directions then the add to each other instead of cancelling out cause now they're going in the same direction.
    With this you can easily solve the first question. It is obvious that the total field in the middle of a and b is 0 cause the bottom left will cancel out with the top right and the bottom right will cancel out with the top left.
  4. Nov 13, 2005 #3
    sorry i got lost... I understand the first part, when u said,
    but i'm lost when u said to its right it would make a feild going up and to its left going down? U mean if you are going around in a counter clockwise position? to the right of the circle it would make a field going up and to its left going down? like 3 o'clock and 9?
  5. Nov 13, 2005 #4


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    Homework Helper

    If the current comes out of the page, then at the 3 o'clock position
    the B-Field is pointing from the bottom of the 3 toward the top of the 3.
    At the 9 o'clock position, the B-Field points from the top of the 9 toward its bottom.
    Usually we picture the B-Field as continuous loops. These "encircle" the current.

    Electric current (moving electric charge) produces Magnetic Field.
    the B-Field encircles the moving charge according to the right-hand-rule.
    If you have a long straight wire, the strength of B decreases with distance
    as 1/r ... specifically, B dot 2 pi r = mu I_enclosed .
    (Different geometries of current-carrying wire have slightly different forms.)

    Don't confuse Magnetic Field with the Force on moving charge BY the B-Field.
    Last edited: Nov 13, 2005
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