A current $I$ flows along the length of an infinitely long, straight and thin-walled pipe. Then
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(b) Applying Ampere’s law $\oint {B.dl = {\mu _0}i} $ to any closed path inside the pipe we find no current is enclosed. Hence $B = 0$.
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At what distance on the axis, from the centre of a circular current carrying coil of radius $r$, the magnetic field becomes $1 / 8$ th of the magnetic field at centre?
A circular coil having $200$ turns, $2.5 \times 10^{-4} \mathrm{~m}^2$ area and carrying $100 \mu \mathrm{A}$ current is placed in a uniform magnetic field of $1 \mathrm{~T}$. Initially the magnetic dipole moment $(\vec{M})$ was directed along $\vec{B}$. Amount of work, required to rotate the coil through $90^{\circ}$ from its initial orientation such that $\overrightarrow{\mathrm{M}}$ becomes perpendicular to $\vec{B}$, is. . . . $\mu \mathrm{J}$.
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