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

If a wheel with 24 metallic spokes each 40 cm long is rotated with a speed of $180 \mathrm{rev} / \mathrm{min}$ in a plane normal to the horizontal component of Earth's magnetic field, the emf induced between the axle and the rim of the wheel is $E$. If the number of spokes is made 12 and the wheel is rotated with a speed of $90 \mathrm{rev} / \mathrm{min}$ in the same field, the induced emf is