# Gauss's Law problem

Here's the homework problem I'm having trouble with:
A solid metal sphere of radius .4 m carries a net charge of .190nC. Find the magnitude of the electric field at a point .130 m outside the surface of the sphere.

I used the equation EA=q/Epsilon_0
E(4*pi)(r^2)=q/Epsilon_0
E(4*pi)(.130+.4)^2=(.190E-9)/(8.854E-12)
E on the right hand side of equation is used to denote scientific notation.
Solving for the electric field I get 75.748 N/C.
Am I doing this correctly?
Any feedback would be appreciated!

chroot
Staff Emeritus
Gold Member
Your work looks correct, but your conclusion does not. I think you must have simply made an arithmetic mistake.

- Warren

Why does my conclusion seem incorrect?

chroot
Staff Emeritus
Gold Member
Because I don't see how you solved the equation for E and got the value you got. It looks to me like you simply made a mistake in your arithmetic. Make sure you use parentheses appropriately.

- Warren

Am I using the correct value for the radius? Should r=.4+.13, r=.4, or r=.13?
I did make an error in calculations, though. I now get 6.079 N/C.

chroot
Staff Emeritus