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Homework Help: What is the magnitude of the electric field (rod)?

  1. Apr 16, 2017 #1
    1. The problem statement, all variables and given/known data
    Two thin glass rods are placed side by side 4.70 cm apart as shown in the diagram below. They are each 11.3 cm long and are uniformly charged to +10.4 nC .

    Answer the questions below regarding the electric field along a line which is drawn through the middle of the two glass rods (labeled Bisector in the diagram).

    (Figure 1)

    2. Relevant equations
    *See attached file for relevant equations*

    3. The attempt at a solution

    Length = 0.113 m
    λ = 9.20×10^−8 C/m
    Eo = 8.85*10^-12
    r1 = 0.037m
    r2 = 0.01m

    Using the attached equation... I got -1.21*10^5 N/C ... the magnitude of the field is 1.21*10^5 N/C

    I know this is wrong but I am confused as to why?

    Attached Files:

  2. jcsd
  3. Apr 16, 2017 #2


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    Are you allowed to treat the rods as infinitely long?
  4. Apr 16, 2017 #3
    Honestly, I have no idea. I am confused about how to set this problem up but found this equation in my book.
  5. Apr 16, 2017 #4
    I would think not though as they gave you the length of the rods ?
  6. Apr 16, 2017 #5


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    I tend to agree with you, that they do not want you to approximate the rods as infinitely long. But the formula for E that you are using is for an infinitely long rod.

    Do you have an idea of how to handle the case of a rod of finite length? Are you using calculus in your course?
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