Q 12-01-008NEETNEET 2020Top questionEasy
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 ? $\left(\dfrac{1}{4\pi\epsilon_0} = 9 \times 10^9\ \text{N m}^2/\text{C}^2\right)$
Answer: (B) $1.28 \times 10^5$ N/C
Concept. Outside a charged sphere the charge acts as if concentrated at the centre: $E = \dfrac{kq}{r^2}$.
Step 1. Substitute. $r = 0.15$ m:
$$E = \frac{9 \times 10^9 \times 3.2 \times 10^{-7}}{0.0225} = 1.28 \times 10^5\ \text{N/C}$$
Answer. $1.28 \times 10^5$ N/C (Option B).
Solution by Sreeraj P, M.Sc Physics