Question

A point source of electromagnetic radiation has an average power output of $1500\, W$. The maximum value of electric field at a distance of $3\, m$ from this source in $V\, m ^{-1}$ is

• A. 500
• B. 100
• C. $\frac{500}{3}$
• D. $\frac{250}{3}$

Answer 1

Answer: (B)

Average intensity of electromagnetic waves is
$I=\frac{P}{4 \pi r^{2}}=\frac{1}{2} \varepsilon_{0} E_{0}^{2} c \,\,\,\,\,$ or $\,\,\,\,E_{0}=\left[\frac{P}{2 \pi r^{2} \varepsilon_{0} c}\right]^{1 / 2}$
Substituting the given values, we get
$E_{0}=\left[\frac{1500}{2 \pi(3)^{2} \times\left[\left(1 / 4 \pi \times 9 \times 10^{9}\right)\right] \times 3 \times 10^{8}}\right]^{1 / 2}=100\, V\, m ^{-1}$