An arrangement of three parallel straight wires placed perpendicular to plane of paper carrying same current $$'I'$$ along the same direction as shown in fogure. Magnitude of force per unit length on the middle wire $$'B'$$ is given by

A long wire carrying a steady current is bent into a circular loop of one turn. The magnetic field at the centre of the loop is B. It is then bent into a circular coil of n turns. The magnetic field at the centre of this coil of n turns will be

An electron is moving in a circular path under the influence of a transverse magnetic field of 3.57 $$ \times $$ 10^-2 T. If the value of e/m is 1.76 $$ \times $$ 10¹¹ C kg^$$-$$1, the frequency of revoluation of the electron is

A long straight wire of radius $$a$$ carries a steady current $$I$$. The current is uniformly distributed over its cross-section. The ratio of the magnetic fields B and B', at radial distance $${a \over 2}$$ and 2$$a$$ respectively, from the axis of the wire is