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Homework Help: Finding Electric Field from an insulating sphere

  1. Sep 13, 2013 #1
    An insulating sphere with a radius of (3.3E-2) m has a uniform charge density of (6.74E-6) C/m^3 throughout its volume.

    (1)Find the magnitude of the electric field at a point (5.7E-2) m from the center of the sphere.

    (2) Find the magnitude of the electric feild at the surface of the sphere

    (3) find the magnitude of the electric field at a point (2E-2) m from the center of the sphere.


    Relevant Equations:
    Volume of sphere:4/3(pi)r^3
    (D) Charge Density=Charge/volume

    I tried to find Q (charge) using D=Q/(4/3(pi)r^3)

    Now I am Lost. There are many Electrical Field Equations. I tried plugging into EA=Q/ε but it did not work. I also tried E=q/((4/3(pi)r^2)r^3)

    Please help me solve this. Can I see a step by step guide?

    Thank you
  2. jcsd
  3. Sep 13, 2013 #2


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    Homework Helper

    According to the Forum rules, you need to show some attempt for the solution.
    What are those many electrical field equations? Which do you think of them relevant?

  4. Sep 13, 2013 #3
    There is a nice little "trick" one can use for the first two questions - it's a fact about the electric field of a uniformly charged sphere that will surely be mentioned in your textbook.

    There is another useful tool for finding the electric field of charge distributions that are symmetric. This tool will make this problem even easier.

    You're close - you just need to know which tools to use and when. Skim over your text and try to understand the underlying concepts and when you can apply different methods of finding electric fields because "there are many electrical field equations" as you said.
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