Question

A lamp emits monochromatic green light uniformly in all directions. The lamp is $3\%$ efficient in converting electrical power to electromagnetic waves and consumes $100W$ of power. The amplitude of the electric field associated with the electromagnetic radiation at a distance of $5m$ from the lamp will be nearly :

• A. $1.34V/m$
• B. $2.68V/m$
• C. $4.02V/m$
• D. $5.36V/m$

Answer 1

Answer: (B)

$I=<u>c=\frac{P}{4\pi r^2}$
$\frac{{ϵ}_0{\in }_0^2}{2}C=\frac{P}{4\pi r^2}$
${ϵ}_0^2=\frac{P}{2\pi r^2{ϵ}_{0}C}$
${ϵ}_0=\sqrt{\frac{P}{2\pi r^2{\in }_{0}C}}$
$=\sqrt{\frac{3}{2\pi (5{)}^2\times 8.85\times 10^{-12}\times 3\times 10^8}}$
${ϵ}_0=2.68volt/metre$