Electric Field of a Charged Spherical shell

[SamTsui86]

Homework Statement

A solid conducting sphere of radius 2.00 cm has a charge 6.70 µC. A conducting spherical shell of inner radius 4.00 cm and outer radius 5.00 cm is concentric with the solid sphere and has a charge -3.50 µC. Find the electric field at the folowing radii from the center of this charge configuration.

a) r = 3 cm
b) r = 7 cm

Homework Equations

E = Q(inside) / (4 * pie * Eo * r^2)

Eo = 8.85 * 10^ -12

The Attempt at a Solution

Q(inside) is just 6.7 uc - 3.5 uc = 3.2 uc

I just plug in r to the equation to solve for E

It's wrong, please give me some hints. Thank You

 

[stunner5000pt]

that Q inside you calculated...

would you that value of Q in both questions??


remember enclosed charge

 

[mjsd]

Gauss's Law: $$\int_\text{closed surface} \vec E. d\vec S = q_\text{enclosed}/\epsilon_0$$

 

[SamTsui86]

Ohh I understand it now, thank you very much