1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Trying to get this

  1. Jul 18, 2011 #1
    Hi, I'm trying to understand point charges along a uniformly charged line with a charge per unit length. This is a screenshot of what i'm trying to understand. http://i.imgur.com/s1vX7.jpg

    I am not sure where the term r/R is coming from. I realise that the r term in sigma*dx/4pi*enot should change as you go down the x axis but I don't get where the factor r/R comes from. I feel like i'm missing something. This is an example page from the book "Electromagnetism" by John C. Slater and Nathaniel Frank.
     
  2. jcsd
  3. Jul 18, 2011 #2

    Doc Al

    User Avatar

    Staff: Mentor

    They are only considering the component of the field perpendicular to the line of charge (the parallel component will add to zero for an infinite line of charge). r/R is equivalent to the cosine of the angle.
     
  4. Jul 18, 2011 #3
    No, you are not trying to "understand point charges along a uniformly charged line"...you are trying to "understand the electric field intensity at point P which is at a distance r from a charge line".

    You see, you are trying to determine the influence of the entire line charge (which is discretized in a bunch of little dx's) on a given point P.

    If the x-axis is vertical and the y-axis is horizonatl and the charge line pretty much lies on the x-axis and the point P is at a coordinate (x,y)=(0,r)

    The field intensity at P due to a given dx (x not equal 0) will be along the line the joins that dx and the P point, and hence, it can be thought of having two components, one parallel to the x-axis and one parallel to the y-axis...since the charge line is infinite long, the point P "sees" the same thing above it (x>0) as below it (x<0) and hence all the x-components of the field at P cancel out and all the y-components add up.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Trying to get this
Loading...