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Homework Help: Calculate R from Electric field lines. Help Please

  1. Sep 21, 2010 #1
    1. The problem statement
    The electric field E (arrows) is produced by a point charge. Find R in meters

    2. Relevant equations
    E(due to pt charge) = kq/R^2
    # of field lines/unit area represents relative field strength (only statement in my notes that seems to apply to problem)

    3. The attempt at a solution
    I assumed the two areas are equal, and the relative strengths are 5Q and 2Q, respectively. Then, k5Q/(1.5)^2 = k2Q/R^2 I solved and got .77, and neither .77 or 1.5+.77 are answer choices so I am obviously wrong. This problem is obviously supposed to be very easy since it is the first one on my practice test but it is nothing like anything in the book or examples done is class so I am clueless.
  2. jcsd
  3. Sep 21, 2010 #2
    If anyone knows where to start on this problem or can point me in the right direction, it would be greatly appreciated. I have access to online tutoring through cengage and two tutors were unable to help me with it so I am getting desperate.
  4. Sep 21, 2010 #3


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    Homework Helper

    Q is the same for both distances. It is the electric field, E=kQ/r^2 which is E=5 for r=1.5 and E=2 for r=R.

    Last edited: Sep 21, 2010
  5. Sep 21, 2010 #4
    Thank you! My notes have Q varying with the amount of electric field lines, which is where I was mistaken. The problem makes sense now
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