Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Question about magnitude positive y direction

  1. Apr 3, 2015 #1
    Hello everybody.

    If I choose the positive y direction to be vertically downwards, and the positive x direction to be to the right, and take the cross product y cross x, then the direction of the resultant is out of the page (if I draw x and y as lines on paper). The magnitude is yx sin(φ), where φ is the angle between them. I do understand that, but it's been put in a context where I can't find φ. An infinite wire carrying current I in the positive y direction generates a field at P, which is a distance a along the x axis. I want to cross y, which is infinite, with x, which is finite but changing. x is the position vector of point P, relative to the infinite wire. I don't know what to use as the angle between them. Is φ=90°? Is sinφ x/(x2+y2)1/2, by Pythagoras theorem and the geometry of the situation, or should that be a y on the numerator? My textbook explains this poorly, and I think it's still more maths than physics, but I'm sorry if I posted in the wrong place.
    Thank's a lot!
     
  2. jcsd
  3. Apr 3, 2015 #2

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Both angle and displacement depend on the point on the conductor (and depending on what you want to calculate, you probably want to use an integral), but if you calculate the cross-product, you'll see that those effects cancel.

    What do you want to do?
     
  4. Apr 4, 2015 #3

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    You seem to be using "x" and "y" in two different ways here. "If I choose the positive y direction to be vertically downwards, and the positive x direction to be to the right, and take the cross product y cross x". If you mean "x" to indicate the x-axis and "y" to indicate the y-axis then the angle between x and y is [itex]\pi/2[/itex] radians.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook