Assume that a lamp radiates power P uniformly in all directions. What is the magnitude of electric field strength at a distance r from the lamp?
Answer & explanation
Correct answer: option 3
Intensity $ I = \frac{pressure}{area}=\frac{p}{4\pi r^2}$
=average energy density × velocity
$=\frac{1}{2}ε_0E^2_0c$
$∴ E_0 = \sqrt{\frac{P}{4\pi ε_0r^2c}}=\sqrt{\frac{P}{2\pi ε_0r^2c}}$