An insulator in the shape of a spherical shell is shown in cross-section above. (see attached .gif) The insulator is defined by an inner radius a = 4 cm and an outer radius b = 6 cm and carries a total charge of Q = + 9 mC (1 mC = 10-6 C). (You may assume that the charge is distributed uniformly throughout the volume of the insulator).
What is Ey, the y-component of the electric field at point P which is located at (x,y) = (0, -5 cm) as shown in the diagram?
I cant do this I am totally stuck.
The Attempt at a Solution
OK so obviously gauss law.
I chose sphere inside the sphere, of radius 4cm for my gauss surface
i need to calc the charge in this enclosed sphere.
So charge in the sphere should be:
Qin = (r^3 / (R^3 - r^3))*Q
this much is wrong but dont know why
please help --thanks
Heres the drawing on flickr: