CBSE 2023 · Region 3 · Set 1 · Q19 · 2 marks
A wire of length $\displaystyle l$ is in the form of a circular loop A of one turn. This loop is reshaped into loop B of three turns. Find the ratio of the magnetic fields at the centres of loop A and loop B for the same current through them.
Marking-scheme solution
\[\begin{aligned}
& B=\frac{\mu_{o} N I}{2 r} \\
& l=2 \pi r ; \quad 2 \pi r_{A}=3\left(2 \pi r_{B}\right) \\
& r_{B}=\frac{r_{A}}{3} \\
& \frac{B_{A}}{B_{B}}=\frac{\mu_{o} N_{A} I}{2 r_{A}} \times \frac{2 r_{B}}{\mu_{0} N_{B} I}
\end{aligned}
\] &
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CBSE Class 12 Physics past-paper question from the 2023board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.