$A$ battery is connected to a uniform resistance wire $AB$ and $B$ is earthed. Which one of the graphs below shows how the current density $J$ varies along $AB$?

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

Assertion : The electric bulb glows immediately when the switch is on.
Reason : The drift velocity of electrons in a metallic wire is very high.

$A$ 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 $2V$?

$A$ 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

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