400 micro coulomb charge is uniformly spread over the surface of a spherical shell and surface density is 0.314 C/m-2.

⇒** Charge = 400 × 10^{-6} C**

**⇒ Surface charge density = 0.314 C/m-2.**

**Surface area of spherical shell = 4πr²**

We use the formula,

**Surface charge density = Charge / Surface area**

0.314 = 400 × 10^{-6} / 4πr²

0.314 = 400 × 10^{-6} / (4 × 3.14 × r²)

⇒ r² = (400 × 10^{-6} ) / (0.314 × 4 × 3.14)

⇒ r² = 1 × 10^{-4}

∴ **r = 0.001 m**.