Which particles are responsible for the conductivity of metals?

Two cylindrical rods of uniform cross-sectional area $A$ and $2A$,having free electrons per unit volume $2n$ and $n$ respectively,are joined in series. $A$ current $I$ flows through them in a steady state. Then the ratio of the drift velocity of free electrons in the left rod to the drift velocity of electrons in the right rod is $\left( \frac{v_L}{v_R} \right)$.

$A$ $1\,m$ long copper wire carries a current of $1\,A$. If the cross-section of the wire is $2.0\,mm^{2}$ and the resistivity of copper is $1.7 \times 10^{-8}\,\Omega\,m$,the force experienced by a moving electron in the wire is $x \times 10^{-23}\,N$. Find the value of $x$. (Charge on electron $= 1.6 \times 10^{-19}\,C$)

What is current density? Derive Ohm's law in the form of current density.

$A$ potential difference $V$ is applied across the ends of a copper wire of diameter $d$ and length $L$. If only the diameter $d$ is doubled,the drift velocity becomes:

$A$ charged particle having a drift velocity of $7.5 \times 10^{-4} \, m s^{-1}$ in an electric field of $3 \times 10^{-10} \, V m^{-1}$ has a mobility in $m^{2} V^{-1} s^{-1}$ of: