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

$A$ wire of cross-sectional radius $r$ carries a current $I$ due to free electrons moving with a drift velocity $V$. What will be the current in another wire of the same material having half the radius and a drift velocity of $2V$?

The drift speed of electrons in a material is found to be $0.3 \ m s^{-1}$ when an electric field of $2 \ V m^{-1}$ is applied across it. The electron mobility (in $m^2 \ V^{-1} \ s^{-1}$) in the material is

In a conductor,if the number of conduction electrons per unit volume is $8.5 \times 10^{28} \ m^{-3}$ and the mean free time is $25 \ fs$ (femtosecond),what is its approximate resistivity? (Given: $m_e = 9.1 \times 10^{-31} \ kg$,$e = 1.6 \times 10^{-19} \ C$)

The mechanism of the heat produced in a conductor when an electric current flows through it can be explained on the basis of

The lengths of two wires made of the same material are in the ratio $2:3$ and their radii are in the ratio $1:2$. If the two wires are connected in parallel to a battery,then the ratio of the drift velocities of free electrons in the two wires is