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A long current carrying wire produces a magnetic field of 1 $\mathrm{T}$ at a distance of $r$. The magnetic field (a) $\frac{\mathrm{r}}{2}$ (b) $2 \mathrm{r}$ and (c) $3 \mathrm{r}$ is
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(a) $2 \mathrm{~T},(\mathrm{~b})=\frac{1}{2} \mathrm{~T},(\mathrm{c})=\frac{1}{3} \mathrm{~T}$
For a long straight wire
$$
B=\frac{\mu_0 \mathrm{i}}{2 \pi \mathrm{r}}
$$
$\mathrm{B} \propto \frac{1}{\mathrm{r}} \Rightarrow \frac{\mathrm{B}_{\mathrm{r} / 2}}{\mathrm{~B}_{\mathrm{r}}}=\frac{\mathrm{r}}{\frac{\mathrm{r}}{2}} \Rightarrow \frac{\mathrm{B}_{\mathrm{r} / 2}}{\mathrm{~B}_{\mathrm{r}}}=2 \Rightarrow \mathrm{B}_{\mathrm{r} / 2}=2 \times 1=2 \mathrm{~T}$
Similarly, $\mathrm{B}_{2 \mathrm{r}}=\frac{1}{2} \mathrm{~T}$ and $\mathrm{B}_{3 \mathrm{r}}=\frac{1}{3} \mathrm{~T}$
$$
B=\frac{\mu_0 \mathrm{i}}{2 \pi \mathrm{r}}
$$
$\mathrm{B} \propto \frac{1}{\mathrm{r}} \Rightarrow \frac{\mathrm{B}_{\mathrm{r} / 2}}{\mathrm{~B}_{\mathrm{r}}}=\frac{\mathrm{r}}{\frac{\mathrm{r}}{2}} \Rightarrow \frac{\mathrm{B}_{\mathrm{r} / 2}}{\mathrm{~B}_{\mathrm{r}}}=2 \Rightarrow \mathrm{B}_{\mathrm{r} / 2}=2 \times 1=2 \mathrm{~T}$
Similarly, $\mathrm{B}_{2 \mathrm{r}}=\frac{1}{2} \mathrm{~T}$ and $\mathrm{B}_{3 \mathrm{r}}=\frac{1}{3} \mathrm{~T}$
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