Question

A solid metallic sphere has a charge + 3Q. concentric with this sphere is a conducting spherical shell having charge –Q. The radius of the sphere is a and that of the spherical shell is b. (b > a). What is the electric field at a distance R (a < R < b) from the centre.

• A. $\frac{Q}{2\pi {ϵ}_{0}R}$
• B. $\frac{3Q}{2\pi {ϵ}_{0}R}$
• C. $\frac{3Q}{4\pi {ϵ}_0{R}^2}$
• D. $\frac{4Q}{2\pi {ϵ}_0{R}^2}$

Answer 1

The correct option is C $\frac{3Q}{4\pi {ϵ}_0{R}^2}$

We only need to consider the E contribution due to sphere.
Shell gives zero contribution to E at any point inside it.

Use direct formula.

$E=\frac{1}{4\pi {ϵ}_0}\frac{3Q}{R^2}$
$E=\frac{3Q}{4\pi {ϵ}_0{R}^2}$