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An electric current passes through a long straight copper wire. At a distance 5 cm from the straight wire, the magnetic field is $B$. The magnetic field at 20 cm from the straight wire would be
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Verified Answer
The correct answer is:
$\frac{B}{4}$
The magnetic field at a distance $r$ from the straight wire
$\begin{aligned}
& B & =\frac{\mu_0 i}{2 \pi r} \text { or } B \propto \frac{1}{r} \text { or } \frac{B_2}{B_1}=\frac{r_1}{r_2} \\
\therefore \quad & \frac{B_2}{B} & =\frac{5}{20}=\frac{1}{4} \text { or } B_2=\frac{B}{4}
\end{aligned}$
$\begin{aligned}
& B & =\frac{\mu_0 i}{2 \pi r} \text { or } B \propto \frac{1}{r} \text { or } \frac{B_2}{B_1}=\frac{r_1}{r_2} \\
\therefore \quad & \frac{B_2}{B} & =\frac{5}{20}=\frac{1}{4} \text { or } B_2=\frac{B}{4}
\end{aligned}$
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