The dominant force experienced by an electron moving in a wire is

An electric current flows along an insulated strip $PQ$ of a metallic conductor. The current density in the strip varies as shown in the graph. Which one of the following statements could explain this variation?

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

Electric current is due to the drift of electrons in

Define mobility and write its $SI$ unit.

$A$ copper wire of length $1\, m$ and radius $1\, mm$ is joined in series with an iron wire of length $2\, m$ and radius $3\, mm$ and a current is passed through the wires. The ratio of the current density in the copper and iron wires is