The current density in a cylindrical wire of radius $4 \; mm$ is $4 \times 10^{6} \; A \cdot m^{-2}$. The current through the outer portion of the wire between radial distance $\frac{R}{2}$ and $R$ is $\dots \; \pi \; A$.

Statement $(I)$: Specific resistance depends on the nature of the material and is independent of the temperature of the material.
Statement $(II)$: $A$ wire of resistance $6 \ \Omega$ is drawn out so that its new length is four times its original length. The resistance of the new wire is $48 \ \Omega$.
Statement $(III)$: Drift velocity is the average constant velocity acquired by free electrons inside a metal by the application of an electric field,which results in current.
Which of the following is correct?

Current density in a cylindrical wire of radius $R$ is given as $J = \begin{cases} J_0 \left( \frac{x}{R} - 1 \right) & 0 \leqslant x < \frac{R}{2} \\ J_0 \frac{x}{R} & \frac{R}{2} \leqslant x \leqslant R \end{cases}$. The current flowing in the wire is:

$A$ current of $5\, A$ passes through a copper conductor (resistivity $= 1.7 \times 10^{-8}\, \Omega \, m$) of radius of cross-section $5\, mm$. Find the mobility of the charges if their drift velocity is $1.1 \times 10^{-3}\, m/s$.

$A$ potential difference $V$ is applied across the ends of a metallic wire of length $l$. The drift velocity will be doubled if:

Assertion: The current density $\vec J$ at any point in an ohmic resistor is in the direction of the electric field $\vec E$ at that point.
Reason: $A$ point charge when released from rest in a region having only an electrostatic field always moves along electric lines of force.