In a wire of radius 1 mm a steady current of 2 A uniformly distributed across the cross-section of the wire is flowing. Then the magnetic field at a point 0.25 mm from the centre of the wire is
The magnetic field at the centre of a current carrying circular coil of radius $R$ is $B_c$ and the magnetic field at a point on its axis at a distance $R$ from its centre is $B_a$. The value of $\frac{B_c}{B_a}$ is
A short bar magnet of magnetic moment $10^4 \mathrm{JT}^{-1}$ is free to rotate in a horizontal plane. The work done in rotating the magnet slowly from the direction parallel to a horizontal magnetic field of $4 \times 10^{-5} \mathrm{~T}$ to a direction $60^{\circ}$ to the direction of the field is
A metallic disc of radius 0.3 m is rotating with a constant angular speed of $60 \mathrm{rad} \mathrm{s}^{-1}$ in a plane perpendicular to a uniform magnetic field of $5 \times 10^{-2} \mathrm{~T}$. The emf induced between a point on the rim and centre of the disc is
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