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?

$\frac{P}{\pi cε_0r^2}$ $\frac{P}{2\pi cεr^2}$ $\sqrt{\frac{P}{2\pi ε_0r^2c}}$ $\sqrt{\frac{P}{\pi ε_0cr^2}}$

$\sqrt{\frac{P}{2\pi ε_0r^2c}}$

Intensity $ I = \frac{pressure}{area}=\frac{p}{4\pi r^2}$ =average energy density × velocity $=\frac{1}{2}ε_0E^2_0c$ $∴ E_0 = \sqrt{\frac{P}{4\pi ε_0r^2c}}=\sqrt{\frac{P}{2\pi ε_0r^2c}}$