Topic: Electromagnetics
For an isotropic radiator, electric field intensity at a distance R is measured as $3~\text{V/m}$. What will be the electric field intensity at a distance 3 R?
- A. $1~\text{V/m}$
- B. $\frac{1}{3}~\text{V/m}$
- C. $\frac{1}{9}~\text{V/m}$
- D. $3~\text{V/m}$
Answer: A. $1~\text{V/m}$
Explanation: Electric field intensity $E$ is inversely proportional to the distance $r$ ($E \propto \frac{1}{r}$).
At distance R, $E = 3\text{ V/m} = \frac{K}{R} \Rightarrow K = 3R$.
At distance 3R, $E' = \frac{K}{3R} = \frac{3R}{3R} = 1\text{ V/m}$.