If a charged particle enters a uniform magnetic field normally with certain velocity, then the time period of revolution of the particle

A long straight wire of circular cross-section of radius ' $a$ ' is carrying a steady current. The current is distributed uniformly across the cross-section of the wire. The ratio of the magnetic fields at points $0.5 a$ and $1.5 a$ from the centre of the wire is

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