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Homework Help: Not sure why this is true

  1. Apr 7, 2008 #1
    In a uniformly charged sphere, why inside the sphere is [tex] \bold{E} = \frac{1}{4 \pi \epsilon_{0}} \frac{q}{R^3} r \bold{\hat{r}} [/tex]?

    I don't see how this follows from Gauss's law.
  2. jcsd
  3. Apr 7, 2008 #2
    The ratio of the charges between the inner sphere (Q') and the outer sphere (Q) is equal to the ratio of the volume between the inner and outer spheres.

    [tex]\frac{\frac{4}{3}\pi r^3}{\frac{4}{3}\pi R^3}=\frac{Q'}{Q}[/tex]

    Thus: [tex]Q' = Q\frac{r^3}{R^3}[/tex]

    Now use Gauss's Law to find the electric field inside the inner sphere.
  4. Apr 7, 2008 #3
    Just to clarify what Snazzy said, the inner sphere is your Gaussian Surface.
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