Due to what is current formed in conductors?

An electron revolves $6 \times 10^{15}$ times per second in a circular loop. The current in the loop is:

$A$ steady current flows in a metallic conductor of non-uniform cross-section. The quantity/quantities constant along the length of the conductor is/are:

The relation between the current $i$ (in ampere) in a conductor and the time $t$ (in second) is $i=12 t+9 t^2$. The charge passing through the conductor between the times $t=2 \,s$ and $t=10 \,s$ is (in $C$)

Explain the current formed in solid conductors in the presence of an external electric field.

$A$ copper rod of cross-sectional area $A$ carries a uniform current $I$ through it. At temperature $T$,if the volume charge density of the rod is $\rho$,how long will the charges take to travel a distance $d$?