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**Electricity and Magnetism questions....Gauss's law, grounding..etc.**

Alright so I am finally starting to understand the material, but I have a few questions.

1. An insulating spherical shell with an inner radius of 0.1cm and outer radius of 0.3 cm carries a total charge of 20nC. Use Gauss's law to find an expression for the electric field at a distance of r=0.08cm, r=0.2cm, r=0.4cm.

So the area of a shell is 4πR^2. EA=Qin/E0, so E=Qin/(4πR^2E0).

Now I know for the r=0.4cm one I can just use Qin=20nC because the whole charge is enclosed, but for the one's where the whole charge isn't enclosed I believe I do Qin=[tex]\sigma[/tex]4πr^2, and so E=[tex]\sigma[/tex]4πr^2/(4πR^2E0)....is this equation correct? and I am a bit confused about what to make r and R and when...like when it is in the inner radius is R=0.1 cm or is it still 0.3cm and why if so? can someone help me out please?

2. An arc with a length of 6cm and a radius of 3cm carries a uniform charge of 10nC. Derive an expression for the magnitude and direction of electric field at the center.

Ok so I am not sure if you are suppose to use Gauss's law or Coulomb's law and integrate for this question...can someone help me get started and point me in the right direction please?

3. A hollow conducting sphere has a radii of 0.8m and 1.2m. It surrounds a charge of +300nC, and the hollow sphere carries a charge of -200nC. Find the charge density on its inner and outer surfaces. B) if the outer surface of the shell is grounded, then the charge on the inner surface is _________ and on the outer surface is _______.

Ok I already found the charge densities, they are -37.3 nC on the inner surface and 5.53 on the outer surface. However I am not sure what happens if the outer surface of the shell is grounded. :S