Topic: Electromagnetics
Electric displacement density D at any point on spherical surface of radius R with a charge Q at the centre in a medium with dielectric constant $\epsilon$ is
- A. $Q/(4\pi\epsilon~R^{2})$
- B. $Q/(4\pi R^{2})$
- C. $(Q/4\pi\epsilon R)^{2}$
- D. $Q/(4\pi~R)^{2}$
Answer: B. $Q/(4\pi R^{2})$
Explanation: Electric field intensity, E at any spherical surface of radius R is $E = \frac{Q}{4\pi\epsilon R^{2}}$. Also, $D=\epsilon\vec{E}$, so $D=\frac{Q}{4\pi R^{2}}$.