An electric bulb is rated as 200 W. What will be the peak magnetic field at 4 m distance produced by the radiations coming from this bulb? Consider this bulb as a point source with 3.5% efficiency.

The total power (P_T) of the light bulb is given as 200 W, but only 3.5% of this power is actually emitted as radiation, which we will call P. <br/><br/>So, Effective power output of the bulb <br/><br/>$\mathrm{P}=\frac{3 \cdot 5}{100} \times 200=7 \mathrm{~W}$ <br/><br/>$$ \begin{aligned} \text { Intensity } \mathrm{I} & =\frac{\mathrm{P}}{4 \pi r^2}=\frac{7}{4 \pi(4)^2} \mathrm{~W} / \mathrm{m}^2 \\\\ \text { Intensity } \mathrm{I} & =\text { Average energy density } \times c \\\\ & =\frac{1}{2} \in_0 \mathrm{E}_0^2 c \\\\ & =\frac{1}{2} \frac{\mathrm{B}_0^2}{\mu_0} c \end{aligned} $$ <br/><br/>$$ \begin{aligned} \therefore \quad \mathrm{B}_0 & =\sqrt{\frac{2 \mu_0 \mathrm{I}}{c}} \\\\ & =\sqrt{\frac{2 \times 4 \pi \times 10^{-7} \times 7}{3 \times 10^8 \times 4 \pi \times 16}} \\\\ & =1.71 \times 10^{-8} \mathrm{~T} \end{aligned} $$