Posted by **J** on Sunday, January 27, 2008 at 6:27pm.

(a) How many excess electrons must be distributed uniformly within the volume of an isolated plastic sphere 30.0 cm in diameter to produce an electric field of 1150 N/C just outside the surface of the sphere?

(b) what is the electric field at a point 10.0 cm outside the surface?

ok would I use Guass's law for this one? I am lost... and with the 10 cm..where do I put that in the problem?

- physics -
**drwls**, Sunday, January 27, 2008 at 6:37pm
a) Use Gauss' law.

4 pi R^2 E = N*e/(epsilono)

N is the number of electrons and e is the electron charge in Coulombs.

E is the electric field strangth in N/C

"epsilon zero" = 8.8554*10^-12 C^2/N m^2

Make sure R is in meters

b) Use the same equation, but use R + 0.1 m in place of R since you are 10 cm farther away.

- physics -
**J**, Sunday, January 27, 2008 at 6:44pm
ok..thank you very much. I thought that is what you would do for part b but I wasn't sure! Thank you!

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