A sphere of radius $2 a$ is made of a nonconducting material that has a uniform volume charge density $
ho$. (Assume the material does not affect the electric field.) A spherical cavity of radius $a$ is now removed from the sphere as shown in Figure $mathrm{P} 19.78$.Show that the electric field within the cavity is uniform and is given by $E_{x}=0$ and $E_{y}=
ho a / 3 epsilon_{0}$ (Suggestion: The field within the cavity is the superposition of the field due to the original uncut sphere plus the field due to a sphere the size of the cavity with a uniform negative charge density $-
ho .$ )