Two long conductors, separated by a distance $$d$$ carry current $${I_1}$$ and $${I_2}$$ in the same direction. They exert a force $$F$$ on each other. Now the current in one of them is increased to two times and its direction is reversed. The distance is also increased to $$3d$$. The new value of the force between them is

Alternating current can not be measured by $$D.C.$$ ammeter because

A current $$i$$ ampere flows along an infinitely long straight thin walled tube, then the magnetic induction at any point inside the tube is

An electric current is passed through a circuit containing two wires of the same material, connected in parallel. If the lengths and radii are in the ratio of $${4 \over 3}$$ and $${2 \over 3}$$, then the ratio of the current passing through the wires will be