A spherical conductor of radius 10 cm has a charge of $3.2\times 10^{-7}$ C distributed uniformly. What is the magnitude of electric field at a point 15 cm from the centre of the sphere? $\left ( \frac{1}{4\pi \varepsilon _{0}}=9\times 10^{9} Nm^{2}/C^{2} \right )$