Question

Two charged concentric spheres have radii of 10cm and 18cm.the charge on the inner sphere is 6.0×10^8c and that on the outer sphere is 2.0×10^-8c.find the electric field at r=12cm

Answer 1

The field outside the smaller sphere:

$$E_1=\frac Q{4\pi\epsilon_or^2}=3.74·10^20\text{ N/C}.$$

The field inside the greater sphere caused by its charge is zero. Therefore, E₁ is the field inside.