Q 12-04-025NEETJEE MainMedium
Two concentric circular loops of radii $R$ and $2R$ lie in the same plane and carry equal currents $I$ in opposite directions. The magnetic field at their common centre is
Answer: (B) $\dfrac{\mu_0 I}{4R}$
Inner loop: $\dfrac{\mu_0 I}{2R}$. Outer loop: $\dfrac{\mu_0 I}{2(2R)} = \dfrac{\mu_0 I}{4R}$, in the opposite direction.
Net: $\dfrac{\mu_0 I}{2R} - \dfrac{\mu_0 I}{4R} = \dfrac{\mu_0 I}{4R}$.
Solution by Sreeraj P, M.Sc Physics