$A$ beam contains $2 \times 10^8$ doubly charged positive ions per cubic centimeter,all of which are moving with a speed of $10^5 \,m/s$. The current density is ............. $A/m^2$.

$A$ wire of non-uniform cross-section is carrying a steady current. Along the wire,

For a cylinder of radius $R$,the current density is given by $J = J_0 \frac{r}{R}$,where $J_0$ is a constant and $r$ is the distance from the axis. Calculate the total current.

Choose the correct option with respect to the statements $A$ and $B$:
$(A)$: When no electric field is applied across a conductor,the path of free electrons between two successive collisions in it is straight.
$(B)$: When an electric field is applied across a conductor,the drift velocity of electrons is independent of time.

$A$ current of $2\,A$ flows through a wire of cross-sectional area $25.0\,mm^2$. The number of free electrons per cubic meter is $2.0 \times 10^{28}$. The drift velocity of the electrons is $...............\times 10^{-6}\,ms^{-1}$ (given,charge on electron $= 1.6 \times 10^{-19}\,C$).

On increasing the temperature of a conductor,its resistance increases because