A square wire of each side l carries a current $$I$$. The magnetic field at the mid-point of the square

A magnetic field does not interact with:

A metallic rod of $$10 \mathrm{~cm}$$ is rotated with a frequency 100 revolution per second about an axis perpendicular to its length and passing through its one end in uniform transverse magnetic field of strength $$1 \mathrm{~T}$$. The emf developed across its ends is:

The radius of the current carrying circular coil is doubled keeping the current passing through it the same. Then the ratio of the magnetic field produced at the centre of the coil before the doubling of the radius to the magnetic field after doubling of the radius.