Question

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.

• A. $1.19 \times 10^{-8} T$
• B. $1.71 \times 10^{-8} T$
• C. $0.84 \times 10^{-8} T$
• D. $3.36 \times 10^{-8} T$

Answer 1

Answer: (B)

$\frac{\eta P }{4 \pi r ^{2}}=\frac{ cB _{0}^{2}}{2 \mu_{0}}$
$B _{0}=\sqrt{\frac{\mu_{0}}{4 \pi} \frac{\eta P }{ c } \frac{1}{ r }}$
$\Rightarrow B _{0}=\frac{1}{4} \sqrt{\frac{10^{-7} \times 4 \times 3.5}{3 \times 10^{8}}}$
$=1.71 \times 10^{-8} T$