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

A square loop ABCD carrying a current $$i$$, is placed near and coplanar with a long straight conductor XY carrying a current $$I$$, the net force on the loop will be

A proton and an alpha particle both enter a region of uniform magnetic field B, moving at right angles to the field B. If the radius of circular orbits for both the particles is equal and the kinetic energy acquired by proton is 1 MeV, the energy acquired by the alpha particle will be

A conducting square frame of side 'a' and a long straight wire carrying current $$I$$ are located in the same plane as shown in the figure. The frame moves to the right with a constant velocity 'V'. The emf induced in the frame will be proportional to