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Physics · 2026 · 3 marks
CBSE 2026 · Region 4 · Set 3 · Q28
Two coils, one of radius 0.5 cm having 10 turns and the other of radius 5 cm having 50 turns are placed coaxially in air such that their centres are…
Two coils, one of radius $\displaystyle 0.5$ cm having $\displaystyle 10$ turns and the other of radius $\displaystyle 5$ cm having $\displaystyle 50$ turns are placed coaxially in air such that their centres are coincident. Calculate :
(a)
the magnetic flux through the smaller coil if the larger coil carries a current of $\displaystyle 3$ A, and
(b)
the mutual inductance of the two coils.
Marking-scheme solution
(a)
$\displaystyle \phi_{1}=N_{1} B_{2} A_{1}=N_{1} \times \dfrac{\mu_{0} I_{2}}{2 r_{2}} \times \pi r_{1}^{2} \times N_{2}$
$\displaystyle =\dfrac{10 \times 4 \pi \times 10^{-7} \times 3 \times 3.14 \times 0.5 \times 0.5 \times 10^{-4} \times 50}{2 \times 5 \times 10^{-2}}$
$\displaystyle =14.789 \times 10^{-7} \mathrm{~Wb}$
(b)
$\displaystyle \mathrm{M}_{12}=\dfrac{\phi_{1}}{\mathrm{I}_{2}}=\dfrac{14.789 \times 10^{-7}}{3}$
$\displaystyle =4.92 \times 10^{-7} \mathrm{H}$
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CBSE Class 12 Physics past-paper question from the 2026 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.