The electric field $E$,current density $J$ and conductivity $\sigma$ of a conductor are related as

$A$ cylindrical conductor of diameter $0.1 \text{ mm}$ carries a current of $90 \text{ mA}$. The current density (in $\text{Am}^{-2}$) is (take $\pi \simeq 3$):

When there is an electric current through a conducting wire along its length,then an electric field must exist

The drift velocity of an electron is $v_d$ in a conductor of area of cross-section $A$ and carries a current $I$. Now, the area of cross-section and current flowing through the conductor are doubled, then the new drift velocity of the electron is . . . . . . .

The number density of free electrons in a copper conductor is estimated to be $8.5 \times 10^{28} \ m^{-3}$. How long does an electron take to drift from one end of a wire $6 \ m$ long to its other end? The area of cross-section of the wire is $1.0 \times 10^{-6} \ m^2$ and it is carrying a current of $1.5 \ A$.

Derive the equation of mobility in terms of electric current.